Learning Objectives
Chapter Overview
Key skills you will develop by the end of Chapter 6: Chemical Reactions.
Explain what a chemical reaction is and state the Law of Conservation of Mass
Balance chemical equations and use the state symbols (s), (l), (g) and (aq)
Identify synthesis, decomposition, displacement, double displacement and combustion reactions
Tell endothermic (takes in heat) from exothermic (gives out heat) reactions
Explain how ionic bonds form and why ionic compounds have high melting points and conduct when molten
Explain covalent bonding and why water is a polar molecule and a good solvent
Answer board-style MCQs, true/false, balancing, bonding and investigation questions accurately
General Science: Chemical Reactions
Complete chapter notes: conservation of mass, balancing equations, reaction types, endothermic and exothermic reactions, ionic and covalent bonds, plus the full board exercise — PDF format
How to use these notes: Every concept is turned into exam questions below, each written in several ways (the different phrasings your board or teacher might use). Tap a question, try it in your own words, then check the answer. If you can answer every phrasing, you have learned the concept. Download the PDF for a printable copy.
Core Notes · Part 1
What Is a Chemical Reaction?
Reactants, products, and why the number of atoms never changes.
What Is a Chemical Reaction?
Note: In a reaction the starting substances are the REACTANTS and the new substances made are the PRODUCTS. The number of atoms is always the same before and after — none are lost or gained.
1. What is a chemical reaction?
• Define a chemical reaction.• What happens during a chemical reaction?
A chemical reaction happens when two or more substances (the reactants) interact to produce one or more new substances (the products). The products can be completely different from the reactants and have different properties.
2. What are reactants?
• What do we call the starting substances in a reaction?• Define the term reactant.
Reactants are the starting substances that interact (react) together in a chemical reaction — they are the substances that get used up.
3. What are products?
• What do we call the new substances made in a reaction?• Define the term product.
Products are the new substances that are formed (made) in a chemical reaction.
4. How do the products of a reaction compare with the reactants?
• Are the products the same as the reactants?• Why can products be very different from the reactants?
The products can be completely different from the reactants and may have many different properties, because the atoms have been rearranged into new substances.
5. What happens to the number of atoms during a chemical reaction?
• Are atoms lost or gained in a reaction?• Is the number of atoms the same before and after a reaction?
The number of atoms is always the same before and after a chemical reaction — none are lost and none are gained; the atoms are only rearranged.
6. Where do chemical reactions take place?
• Where are chemical reactions happening around us?• Give places where chemical reactions occur.
Chemical reactions are happening all the time — in our environment, inside our bodies, and out in our Solar System and beyond.
7. Can a reaction have more than one reactant or product?
• How many reactants and products can a reaction have?• Does a reaction always have just one product?
Yes — a reaction can involve two or more reactants and produce one or more products.
8. Give an everyday example of a chemical reaction.
• Name a chemical reaction that happens in the body.• What is one common example of a chemical reaction?
Digestion of food inside the body is a chemical reaction; so is burning a fuel and photosynthesis in plants.
9. Why is a chemical reaction different from simply mixing substances?
• What makes a reaction “chemical” rather than physical?• How can you tell a chemical reaction has happened?
In a chemical reaction new substances (products) are formed with different properties, whereas simply mixing or a physical change (like melting) makes no new substance.
Core Notes · Part 2
Conservation of Mass
Why total mass stays the same — even when a gas escapes.
Conservation of Mass
Common Mistake: When a gas is given off, the reaction only APPEARS to lose mass. If you trap the gas (e.g. with a balloon), the total mass stays the same — mass is conserved.
Tip: Antoine Lavoisier discovered the Law of Conservation of Mass by weighing materials before and after experiments.
10. What is the Law of Conservation of Mass?
• State the Law of Conservation of Mass.• What does the Law of Conservation of Mass tell us?
It states that during any chemical reaction (or change of state) no atoms are created or destroyed, so the total mass of the atoms before and after the reaction stays the same. The total mass of the reactants always equals the total mass of the products.
11. During a chemical reaction, are atoms created or destroyed?
• Can atoms be made or lost in a reaction?• What happens to atoms in terms of being created or destroyed?
No atoms are created or destroyed during a chemical reaction — they are only rearranged, so mass is conserved.
12. What happens to the total mass before and after a reaction?
• How does the mass of reactants compare with the mass of products?• Is mass gained or lost during a reaction?
The total mass stays the same: the total mass of the reactants is always equal to the total mass of the products. Mass is neither created nor destroyed.
13. If 10 g of sugar is stirred into 250 g of hot water, what is the mass of the solution?
• What is the mass of the sugar solution made from 10 g sugar and 250 g water?• Explain the sugar-and-water example of conservation of mass.
The mass of the sugar solution is 260 g (10 g + 250 g). Although the sugar seems to “disappear,” all the sugar particles and water particles are still present, so no mass is lost.
14. Does sugar really disappear when it dissolves in water?
• Where does the sugar go when it dissolves?• Is mass lost when sugar dissolves?
No — the sugar only appears to disappear. All the sugar particles are still there mixed among the water particles, so the mass is unchanged.
15. Why do some substances appear to lose mass during a reaction?
• Why does a reaction sometimes seem to lose mass?• What causes an apparent loss of mass in a reaction?
Because a gas is produced and given off (escapes) very quickly, so the mass of what is left behind decreases — but the gas still has mass, so no mass is really lost.
16. What gas is produced when marble (calcium carbonate) reacts with hydrochloric acid?
• Marble reacts with hydrochloric acid — which gas is given off?• Which gas escapes in the marble + acid reaction?
Carbon dioxide gas is produced. The equation is CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
17. How can you show that mass is conserved when a gas is produced?
• How is the escaping gas collected to prove mass is conserved?• What is done with a balloon in the marble + acid experiment?
By fitting a balloon to the flask, the gas is collected instead of escaping. All the products are then in the flask and the balloon, so the total mass stays the same — proving mass is conserved.
18. Who discovered the Law of Conservation of Mass?
• Which scientist discovered conservation of mass?• Name the scientist linked with the Law of Conservation of Mass.
Antoine Lavoisier (1743–1794), a French scientist. By weighing materials before and after an experiment, he found that the mass remained the same.
19. Does conservation of mass apply to changes of state as well as reactions?
• Is mass conserved when a substance changes state?• Does melting or freezing change the total mass?
Yes — during any chemical reaction OR change of state, no atoms are created or destroyed, so the total mass is conserved (for example, water has the same mass as the ice it melted from).
20. Explain, using an example, the Law of Conservation of Mass.
• Give a worked example of conservation of mass.• How would you demonstrate conservation of mass with numbers?
When 10 g of sugar is dissolved in 250 g of water, the solution weighs 260 g — the mass of the products equals the mass of the reactants because no atoms are lost or gained. This shows the Law of Conservation of Mass.
Core Notes · Part 3
Balancing Equations & State Symbols
Making atoms equal on both sides, and the symbols (s), (l), (g), (aq).
Balancing Chemical Equations & State Symbols
Tip: A balanced equation has the SAME number of atoms of each element on both sides. You balance by putting numbers in front of formulae — never by changing a formula.
Common Mistake: “Balanced” means equal ATOMS of each element on each side, NOT the same number of molecules.
21. What is a chemical equation?
• What does a chemical equation represent?• How are chemical reactions written by chemists?
A chemical equation uses symbols and formulae to represent the reactants and products of a reaction, showing what reacts and what is made.
22. What do chemists use to represent elements and compounds?
• How are elements and compounds shown in an equation?• What are formulae used for in equations?
Chemists use symbols (for elements) and formulae (for compounds) to represent them in chemical equations.
23. What is a balanced chemical equation?
• Define a balanced equation.• When is a chemical equation balanced?
A balanced equation is one that has the same number of atoms of each element on one side as on the other (reactants side = products side). This must be true because no atoms are gained or lost.
24. Why might an equation written with formulae not be balanced?
• Why is an equation sometimes unbalanced when first written?• Why do we need to balance equations?
When you first write an equation using formulae instead of names, the number of atoms of each element may not be equal on both sides, so it must be balanced to obey the Law of Conservation of Mass.
25. What forms when copper is burnt in oxygen?
• Copper is heated in oxygen — what black compound is made?• What is the product of burning copper in oxygen?
A black compound called copper oxide is made: copper (solid) + oxygen (gas) → copper oxide (solid).
26. Why is the equation Cu + O₂ → CuO not balanced?
• What is wrong with Cu + O₂ → CuO?• Explain why Cu + O₂ → CuO needs balancing.
It is not balanced because there are 2 oxygen atoms on the reactant side but only 1 on the product side — an oxygen atom is “missing” on the product side.
27. Write the balanced equation for copper burning in oxygen.
• Balance: Cu + O₂ → CuO.• What is the balanced equation for copper + oxygen?
2Cu(s) + O₂(g) → 2CuO(s). Now there are 2 copper and 2 oxygen atoms on each side (4 atoms each side).
28. How was the copper + oxygen equation balanced?
• What was added to balance Cu + O₂ → CuO?• How do you make the copper equation balance?
By adding another copper atom to the reactant side (and adjusting the number of CuO), giving 2Cu + O₂ → 2CuO, so both sides have equal atoms.
29. What is a state symbol?
• What does a state symbol tell you?• Why are state symbols used in equations?
A state symbol is a small letter in brackets that shows what physical state a reactant or product is in during the reaction.
30. What is the state symbol for a solid?
• How is a solid shown in an equation?• Which symbol means solid?
(s) is the state symbol for a solid.
31. What is the state symbol for a liquid?
• How is a liquid shown in an equation?• Which symbol means liquid?
(l) is the state symbol for a liquid.
32. What is the state symbol for a gas?
• How is a gas shown in an equation?• Which symbol means gas?
(g) is the state symbol for a gas.
33. What is the state symbol for a substance dissolved in water?
• What does (aq) mean?• Which symbol shows an aqueous solution?
(aq) means aqueous — it is used when a substance is dissolved in water to form an aqueous solution.
34. What three states can water be written as, with symbols?
• Give the state symbols for solid, liquid and gaseous water.• How is water shown in its three states?
Water can be a solid H₂O(s), a liquid H₂O(l), or a gas H₂O(g).
35. Write a balanced equation for hydrogen and oxygen forming water.
• Balance: hydrogen (H₂) + oxygen (O₂) → water (H₂O).• What is the balanced equation for making water?
2H₂ + O₂ → 2H₂O. This gives 4 hydrogen atoms and 2 oxygen atoms on each side.
36. Write a balanced equation for nitrogen and hydrogen forming ammonia.
• Balance: nitrogen (N₂) + hydrogen (H₂) → ammonia (NH₃).• What is the balanced equation for making ammonia?
N₂ + 3H₂ → 2NH₃. This gives 2 nitrogen atoms and 6 hydrogen atoms on each side.
37. Silver chloride decomposes in light into silver and chlorine — write a balanced equation with state symbols.
• Balance (with state symbols): silver chloride → silver + chlorine.• Write the decomposition of silver chloride including state symbols.
2AgCl(s) → 2Ag(s) + Cl₂(g). This is balanced (2 silver and 2 chlorine atoms on each side) with correct state symbols.
Core Notes · Part 4
Synthesis (Combination)
Two substances joining to make one new substance.
Types of Reaction — Synthesis (Combination)
Note: Synthesis pattern: A + B → AB. Two substances join to make ONE new single substance.
38. What is a synthesis (combination) reaction?
• Define a synthesis reaction.• What happens in a combination reaction?
In a synthesis (combination) reaction, two different substances combine to make one new single substance.
39. Give an example of a synthesis reaction using iron and sulphur.
• What forms when iron is heated with sulphur?• Describe how iron sulphide is made.
When iron is heated with sulphur, the two elements combine to make iron(II) sulphide: Fe(s) + S(s) → FeS(s). This is a synthesis reaction.
40. Write the equation for iron + sulphur → iron sulphide.
• Give the equation for making iron(II) sulphide.• How is the iron + sulphur reaction written?
Fe(s) + S(s) → FeS(s) — iron metal combines with sulphur to form iron(II) sulphide.
41. Give another example of a synthesis reaction (burning magnesium).
• What is made when magnesium burns in oxygen?• Describe the burning of magnesium as a synthesis reaction.
Burning magnesium in oxygen is a synthesis reaction — magnesium combines with oxygen to make magnesium oxide: 2Mg(s) + O₂(g) → 2MgO(s).
42. Write the balanced equation for magnesium burning in oxygen.
• Balance: magnesium + oxygen → magnesium oxide.• What is the equation for making magnesium oxide?
2Mg(s) + O₂(g) → 2MgO(s).
43. In a synthesis reaction, how many new substances are formed?
• How many products does a synthesis reaction make?• What is the result of a synthesis reaction?
One new single substance is formed by combining two (or more) substances.
44. What is the general pattern of a synthesis reaction?
• Write the general form of a combination reaction.• How can you represent synthesis using letters?
A + B → AB — two substances combine to form one new compound.
45. What type of reaction is the burning of magnesium in oxygen?
• Is making magnesium oxide a synthesis or decomposition reaction?• Classify the magnesium + oxygen reaction.
It is a synthesis (combination) reaction, because two substances (magnesium and oxygen) combine to make one new substance (magnesium oxide).
Core Notes · Part 5
Decomposition & Thermal Decomposition
Breaking one compound into simpler products, using heat.
Decomposition & Thermal Decomposition
Tip: Decomposition pattern: AB → A + B. Energy (often heat) is needed to break the compound apart.
Note: Thermal decomposition of copper carbonate: green powder → black copper oxide + carbon dioxide.
46. What is a decomposition reaction?
• Define decomposition.• What happens in a decomposition reaction?
Decomposition is the breaking up of a single compound into two or more simpler products.
47. What is needed to break the bonds in a decomposition reaction?
• Why does decomposition need energy?• Does decomposition require or release energy to break bonds?
Energy is needed to break the bonds so the compound can split into simpler products.
48. What is thermal decomposition?
• Define thermal decomposition.• What kind of decomposition uses heat?
Thermal decomposition is the use of heat to break down a compound into simpler products.
49. What happens when copper carbonate is heated?
• Describe the colour change when copper carbonate is heated.• What does heating copper carbonate produce?
When copper carbonate is heated it changes from a green powder to a black powder, forming copper oxide and giving off carbon dioxide gas.
50. Write the thermal decomposition of copper carbonate.
• What are the products of heating copper carbonate?• Give the equation for copper carbonate decomposing.
copper carbonate → copper oxide + carbon dioxide (CuCO₃ → CuO + CO₂). The green copper carbonate becomes black copper oxide.
51. Give an example of thermal decomposition used in industry (making lime).
• How is lime (calcium oxide) manufactured?• What is heated to make lime?
In the manufacture of lime, limestone (calcium carbonate) is heated to form calcium oxide (lime) and carbon dioxide gas.
52. What is limestone chemically?
• What compound is limestone?• Name the chemical in limestone.
Limestone is calcium carbonate.
53. What are the products when limestone is heated?
• What does heating limestone produce?• Give the products of the thermal decomposition of limestone.
Heating limestone (calcium carbonate) produces calcium oxide (lime) and carbon dioxide gas.
54. How does decomposition differ from synthesis?
• What is the opposite of a synthesis reaction?• Compare synthesis and decomposition.
Synthesis joins substances together (A + B → AB), while decomposition breaks one compound apart into simpler products (AB → A + B) — they are opposites.
55. What is the general pattern of a decomposition reaction?
• Write the general form of decomposition.• How can decomposition be represented with letters?
AB → A + B — one compound breaks down into two or more simpler substances.
56. Why is heat used in thermal decomposition?
• What role does heat play in breaking down a compound?• Why must copper carbonate be heated to decompose it?
Heat supplies the energy needed to break the bonds holding the compound together, so it can split into simpler products.
Core Notes · Part 6
Test for Carbon Dioxide
The limewater test and what a milky result means.
Test for Carbon Dioxide
Tip: CO₂ test: bubble the gas through limewater — if it turns milky (cloudy white), carbon dioxide is present.
57. What is the chemical test for carbon dioxide?
• How do you test for carbon dioxide gas?• Describe the test for CO₂.
Bubble the gas through limewater. If carbon dioxide is present, the colourless limewater turns cloudy (milky) white.
58. What is limewater?
• What solution is used to test for carbon dioxide?• What is limewater made of?
Limewater is a calcium hydroxide solution — it is used to test for carbon dioxide.
59. What happens to limewater when carbon dioxide is bubbled through it?
• What colour change shows carbon dioxide is present?• How does limewater change in the CO₂ test?
The limewater turns from a colourless solution to a cloudy (milky) white solution when carbon dioxide is bubbled through it.
60. Why can carbon dioxide be poured from one test tube to another?
• What property lets you pour CO₂ between test tubes?• Is carbon dioxide heavier or lighter than air?
Carbon dioxide is heavier than air, so it can be poured from one test tube into another (and will sink into a test tube of limewater, turning it milky).
61. What does it mean if limewater turns milky?
• What is shown when limewater goes cloudy?• What does a milky limewater result tell you?
It shows that carbon dioxide gas is present — the milky/cloudy appearance is a positive test for CO₂.
62. Why is the limewater test useful when studying a reaction?
• How can the CO₂ test help identify a reaction?• Why is testing for carbon dioxide helpful?
It lets you detect whether carbon dioxide is being produced, which helps identify the type of reaction (for example combustion or the decomposition of a carbonate).
Core Notes · Part 7
Displacement Reactions
A more reactive element pushing out a less reactive one.
Displacement Reactions
Common Mistake: In displacement, a MORE reactive element pushes out (displaces) a LESS reactive one. Don’t confuse this with double displacement, where TWO compounds swap parts.
63. What is a displacement reaction?
• Define a displacement reaction.• What happens during a displacement reaction?
During a displacement reaction, one substance pushes out (displaces) another substance to take its place. A more reactive element displaces a less reactive element.
64. In a displacement reaction, which element does the displacing?
• Does the more or less reactive element push the other out?• What decides which element is displaced?
The more reactive element displaces (pushes out) the less reactive element.
65. Are many metals more or less reactive than hydrogen?
• How does the reactivity of many metals compare with hydrogen?• Why can many metals displace hydrogen from acids?
Many metals are more reactive than hydrogen, so they can displace the hydrogen from an acid.
66. What happens when zinc is added to sulphuric acid?
• Describe the reaction of zinc with sulphuric acid.• What does zinc do in sulphuric acid?
Zinc (being more reactive than hydrogen) displaces the hydrogen in the acid, forming zinc sulphate, and hydrogen gas is given off.
67. What gas is given off when zinc reacts with sulphuric acid?
• Which gas is produced when a metal reacts with an acid?• What gas bubbles off in the zinc + acid reaction?
Hydrogen gas is given off when a reactive metal such as zinc reacts with an acid.
68. Write the equation for zinc reacting with sulphuric acid.
• Give the equation for zinc + sulphuric acid.• What are the products of zinc + sulphuric acid?
zinc + sulphuric acid → zinc sulphate + hydrogen: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).
69. What is produced when calcium carbonate reacts with hydrochloric acid?
• What are the products of calcium carbonate + hydrochloric acid?• Which substances form when calcium carbonate reacts with acid?
Calcium chloride and carbon dioxide are produced (along with water) when calcium carbonate reacts with hydrochloric acid.
70. Why does a more reactive metal push out a less reactive element?
• Why does zinc displace hydrogen but not the other way round?• What is the rule about reactivity in displacement?
Because a more reactive element takes the place of a less reactive one — so zinc (more reactive) displaces hydrogen (less reactive) from the acid, but hydrogen cannot displace zinc.
71. Give the reactants and products of the zinc + acid displacement.
• List what goes in and what comes out of zinc + sulphuric acid.• Summarise the zinc + sulphuric acid reaction.
Reactants: zinc and sulphuric acid. Products: zinc sulphate (stays in solution) and hydrogen gas (given off).
Core Notes · Part 8
Double Displacement
Two compounds swapping partners — and precipitates.
Double Displacement Reactions
Note: Double displacement: AB + CD → AD + CB. Two reactants swap partners to make two new compounds; an insoluble product is a PRECIPITATE.
72. What is a double displacement reaction?
• Define a double displacement reaction.• What happens in a double displacement reaction?
In a double displacement reaction, two reactants change places (swap partners) to make two new compounds.
73. How many new compounds are formed in a double displacement reaction?
• How many products does double displacement make?• What is the result of a double displacement reaction?
Two new compounds are formed.
74. Give an example of a double displacement reaction.
• Which reaction is a good example of double displacement?• Name a double displacement example from the chapter.
The reaction between barium chloride and magnesium sulphate is a good example of double displacement.
75. Write the equation for barium chloride + magnesium sulphate.
• Give the products of barium chloride + magnesium sulphate.• What forms when barium chloride reacts with magnesium sulphate?
BaCl₂(aq) + MgSO₄(aq) → BaSO₄(s) + MgCl₂(aq) — barium sulphate (a white solid) and magnesium chloride (which stays in solution).
76. What is a precipitate?
• Define the term precipitate.• What do we call an insoluble solid that forms in a reaction?
A precipitate is an insoluble solid that forms in a reaction and settles out (for example on the bottom of the container).
77. Which product is the precipitate in the barium chloride + magnesium sulphate reaction?
• What is the insoluble product of that double displacement?• Which substance forms the precipitate?
Barium sulphate is the precipitate — it is insoluble and forms a white solid on the bottom of the container.
78. Is barium sulphate soluble or insoluble?
• Does barium sulphate dissolve or form a solid?• What is the state of the barium sulphate produced?
Barium sulphate is insoluble — it forms a white solid precipitate.
79. Which product stays in solution in the barium chloride + magnesium sulphate reaction?
• What is the soluble product of that reaction?• Which compound remains dissolved?
Magnesium chloride stays in solution (it is the aqueous product).
80. How does double displacement differ from a normal (single) displacement reaction?
• What is the difference between displacement and double displacement?• Compare single and double displacement.
In (single) displacement one element pushes out another element; in double displacement two compounds swap parts to form two new compounds.
Core Notes · Part 9
Combustion & Fuels
Burning fuels, hydrocarbons, and the products of combustion.
Combustion & Fuels
Tip: Combustion of a hydrocarbon in oxygen → carbon dioxide + water. It gives out heat and light (exothermic).
81. What is combustion?
• Define combustion.• What is another name for burning?
Combustion is the name for burning — a fuel reacts with oxygen in the air and produces heat and light energy.
82. What does a fuel react with during combustion?
• What does a fuel need to burn?• What gas is used up when a fuel burns?
The fuel reacts with oxygen in the air.
83. What does combustion produce (in terms of energy)?
• What forms of energy are given out when a fuel burns?• What energy is released during burning?
Combustion produces heat and light energy.
84. What happens to the fuel during combustion?
• What is the fuel turned into when it burns?• What does "oxidized" mean in combustion?
The fuel becomes oxidized, forming an oxide (it combines with oxygen).
85. What are hydrocarbons?
• Define hydrocarbons.• What kind of substances are wood, coal, oil and methane?
Hydrocarbons are fuels that contain the elements carbon and hydrogen. Fuels such as wood, coal, oil and methane (natural gas) are hydrocarbons.
86. Which two elements do hydrocarbons contain?
• What elements make up a hydrocarbon?• Name the elements found in fuels like methane.
Hydrocarbons contain carbon and hydrogen.
87. What do the carbon and hydrogen in a fuel form when it burns?
• What products form when a hydrocarbon burns in oxygen?• What is produced during the combustion of a hydrocarbon?
During burning, the carbon and hydrogen combine with oxygen to form carbon dioxide and water.
88. Write the equation for the combustion of methane.
• Give the balanced equation for burning methane in oxygen.• What is the equation when methane burns?
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) — methane + oxygen → carbon dioxide + water vapour.
89. What are the products when methane burns in oxygen?
• What is made when natural gas burns?• Name the products of methane combustion.
Carbon dioxide and water (water vapour) are the products.
90. Why do fuels like wood, coal and methane burn well?
• Why are hydrocarbons good fuels?• What makes these fuels burn well?
They burn well because they contain the elements carbon and hydrogen, which combine with oxygen and release a lot of heat and light energy.
91. Is respiration a kind of combustion?
• How is cellular respiration similar to combustion?• Why is respiration described as a combustion reaction?
Yes — cellular respiration is a combustion reaction in which carbohydrates (the “fuel”) are burnt up very slowly, without any flames, inside the cells of the body.
Core Notes · Part 10
Endothermic Reactions
Reactions that take in heat — so the surroundings get colder.
Endothermic Reactions
Tip: ENDOthermic = takes IN heat → surroundings get COLDER. (Photosynthesis, cooking, melting ice.)
92. What is an endothermic reaction?
• Define an endothermic reaction.• What happens to energy in an endothermic reaction?
An endothermic reaction is one that takes in energy (usually heat) from the surroundings. This makes the reactants and their surroundings become colder.
93. What kind of energy is usually taken in during an endothermic reaction?
• What form of energy is absorbed in endothermic reactions?• What energy do endothermic reactions take in?
Usually heat energy is taken in from the surroundings.
94. What happens to the temperature of the surroundings in an endothermic reaction?
• Do the surroundings get hotter or colder in an endothermic reaction?• Why do endothermic reactions feel cold?
The surroundings become colder, because energy (heat) is taken in from them.
95. Why does sucking a mint give a cooling sensation?
• Explain the cooling feeling from a mint using endothermic reactions.• Why does your mouth feel cold when you suck a mint?
Energy (heat) is required for the saliva in your mouth to break down the mint. This loss of heat from your mouth makes it feel colder — an endothermic process.
96. Is photosynthesis endothermic or exothermic?
• Why is photosynthesis an endothermic reaction?• What type of reaction is photosynthesis in terms of energy?
Photosynthesis is endothermic, because light energy from the Sun is taken in (used) by plants to make sugar.
97. Is cooking endothermic or exothermic?
• Why is cooking an endothermic reaction?• What kind of reaction is cooking food?
Cooking is endothermic — heat from the stove provides the energy to change the molecular structure of the food (for example, cooking eggs).
98. Is melting ice endothermic or exothermic?
• Why does melting ice take in heat?• What kind of change (energy-wise) is melting ice?
Melting ice is endothermic — it takes in heat from the surroundings in order to change state from a solid to a liquid.
99. Give two examples of endothermic reactions.
• Name some endothermic reactions.• What are examples of reactions that take in heat?
Photosynthesis, cooking and melting ice are all endothermic (they take in energy).
100. In an endothermic reaction, do the reactants get warmer or colder?
• What happens to the reactants in an endothermic reaction?• Why do endothermic reactions lower the temperature?
The reactants (and surroundings) become colder, because energy is taken in from them.
101. Sodium carbonate is added to vinegar and the temperature falls — is this endothermic or exothermic?
• A mixture gets colder during a reaction — which type is it?• If a reaction makes the temperature drop, what kind is it?
It is endothermic — the temperature falls because the reaction takes in heat energy from the surroundings.
Core Notes · Part 11
Exothermic Reactions
Reactions that give out heat — so the surroundings get hotter.
Exothermic Reactions
Tip: EXOthermic = gives OUT heat → surroundings get HOTTER. (Burning, freezing water, rusting, neutralisation.)
Common Mistake: Burning (combustion), rusting and neutralisation all GIVE OUT heat — they are exothermic, not endothermic.
102. What is an exothermic reaction?
• Define an exothermic reaction.• What happens to energy in an exothermic reaction?
An exothermic reaction is one that gives out energy to the surroundings, usually as heat or light. This makes the reactants and their surroundings become hotter.
103. What kind of energy is given out in an exothermic reaction?
• What form of energy is released in exothermic reactions?• What energy do exothermic reactions release?
Energy is given out, usually in the form of heat or light.
104. What happens to the temperature of the surroundings in an exothermic reaction?
• Do the surroundings get hotter or colder in an exothermic reaction?• Why do exothermic reactions feel warm?
The surroundings become hotter, because energy (heat) is given out to them.
105. Why does burning a fuel warm the surroundings?
• How does burning fuel show it is exothermic?• Why do people light fires to keep warm?
As the fuel burns, energy is given off to the surroundings in the form of heat and light — which is why fires keep us warm and cook food. Burning is exothermic.
106. Is water freezing into ice endothermic or exothermic?
• What type of reaction is water turning into ice?• Does freezing give out or take in heat?
Water freezing is exothermic — it gives out heat (this heat is usually released from the back of a fridge or freezer).
107. Is rusting (corrosion) endothermic or exothermic?
• Does rusting give out or take in heat?• What kind of reaction is rusting?
Rusting (corrosion) is exothermic — it does give off heat, but very slowly.
108. What is neutralisation, and is it endothermic or exothermic?
• How does toothpaste show an exothermic reaction?• Is neutralising acid exothermic?
Neutralisation is when a base cancels out an acid (for example, toothpaste neutralises acid on your teeth). It is exothermic, though the amount of heat given off is quite small.
109. Give two examples of exothermic reactions.
• Name some exothermic reactions.• What are examples of reactions that give out heat?
Burning fuels, water freezing, rusting and neutralisation are all exothermic (they give out energy).
110. Compare endothermic and exothermic reactions.
• What is the difference between endothermic and exothermic reactions?• How do endothermic and exothermic reactions differ?
Endothermic reactions take IN energy (surroundings get colder); exothermic reactions give OUT energy (surroundings get hotter).
111. Is respiration endothermic or exothermic, and why?
• What type of reaction is respiration in terms of energy?• Explain whether respiration takes in or gives out energy.
Respiration is exothermic — it releases (gives out) energy from food inside the cells of living things, which the body uses.
Core Notes · Part 12
Formation of Ionic Bonds
Electrons transferred, ions formed, and why ionic compounds behave as they do.
Formation of Ionic Bonds
Tip: Metal + non-metal → ionic compound. Electrons are TRANSFERRED: the metal loses electrons (forms a cation), the non-metal gains them (forms an anion).
Common Mistake: A solid ionic compound does NOT conduct electricity — it only conducts when molten or dissolved, because then the ions are free to move.
112. What happens when an atom loses an electron?
• How does losing an electron change an atom?• What does an atom become if it loses an electron?
Losing one electron leaves the atom with all its protons but one fewer electron, so it now has a positive charge — it becomes a positive ion (a cation).
113. What is a cation?
• Define a cation.• What do we call a positive ion?
A cation is a positive ion — an atom that has lost one or more electrons, leaving it with a positive charge.
114. What is an anion?
• Define an anion.• What do we call a negative ion?
An anion is a negative ion — an atom that has gained one or more electrons, giving it a negative charge.
115. What is an ionic bond?
• Define an ionic bond.• How is an ionic bond formed?
An ionic bond is the strong force of attraction between positive ions and negative ions. In its formation, positive and negative ions are pulled close together by electrostatic attraction.
116. What pulls the ions together in an ionic bond?
• What force holds an ionic bond together?• What is electrostatic attraction in ionic bonding?
Electrostatic attraction — the pull between opposite charges — draws the positive and negative ions close together.
117. What happens to electrons when metals react with non-metals?
• How are ions formed when a metal reacts with a non-metal?• Where do the electrons go when metal reacts with non-metal?
Electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.
118. What is an ionic compound?
• Define an ionic compound.• What compound forms when a metal reacts with a non-metal?
An ionic compound is the compound formed when electrons are transferred from metal atoms to non-metal atoms, creating positive and negative ions held together by ionic bonds.
119. How strong are ionic bonds?
• Are ionic bonds weak or strong?• How much energy is needed to break ionic bonds?
Ionic bonds are very strong — a lot of energy is needed to break them.
120. Why do ionic compounds have high melting and boiling points?
• Explain why ionic compounds melt and boil at high temperatures.• What causes the high melting points of ionic compounds?
Because ionic bonds are very strong, a lot of energy is needed to break them, so ionic compounds have high melting and boiling points.
121. When can ionic compounds conduct electricity?
• Under what conditions do ionic compounds conduct?• Do ionic compounds always conduct electricity?
Ionic compounds conduct electricity when molten (melted) or dissolved in solution, because then their electrically charged ions are free to move.
122. Why can molten or dissolved ionic compounds conduct electricity?
• Why do ionic compounds conduct when melted or in solution?• What makes molten ionic compounds conduct?
Because when molten or in solution, the electrically charged ions are free to move and carry the electric current.
123. Why can a solid ionic compound not conduct electricity?
• Why do ionic compounds not conduct when solid?• Explain why solid ionic compounds do not conduct.
In a solid the ions are locked in place and cannot move, so there are no free-moving charged particles to carry a current — so it does not conduct.
124. How is sodium chloride formed?
• Describe the formation of sodium chloride.• What happens when sodium reacts with chlorine?
Sodium chloride forms when sodium and chlorine react. Electrons are lost by sodium atoms and gained by chlorine atoms. The reaction is very vigorous and white crystals of sodium chloride (common salt) are formed.
125. What ions form when sodium chloride is made?
• Which ions are present in sodium chloride?• What charges do the sodium and chlorine ions have?
The sodium atom loses an electron to form a positive sodium ion (Na⁺), and the chlorine atom gains an electron to form a negative chloride ion (Cl⁻).
126. What is the formula of sodium chloride?
• Give the chemical formula of common salt.• How is sodium chloride written as a formula?
Sodium chloride has the formula NaCl.
127. How is magnesium oxide formed?
• Describe the formation of magnesium oxide.• What happens when magnesium burns in oxygen (ionic)?
Magnesium oxide forms when magnesium burns in oxygen with a bright, white flame. Two electrons are lost by each magnesium atom and gained by each oxygen atom.
128. What ions form when magnesium oxide is made?
• Which ions are present in magnesium oxide?• What charges do the magnesium and oxide ions have?
The magnesium atom loses two electrons to form Mg²⁺, and the oxygen atom gains two electrons to form the oxide ion O²⁻.
129. What is the formula of magnesium oxide?
• Give the chemical formula of magnesium oxide.• How is magnesium oxide written as a formula?
Magnesium oxide has the formula MgO.
130. Why are the ionic bonds in magnesium oxide stronger than those in sodium chloride?
• Explain why magnesium oxide has stronger ionic bonds than sodium chloride.• Why is MgO held together more strongly than NaCl?
In magnesium oxide the ions carry double charges (Mg²⁺ and O²⁻), so the electrostatic attraction between them is stronger than between the single-charged ions (Na⁺ and Cl⁻) in sodium chloride.
Core Notes · Part 13
Formation of Covalent Bonds
Sharing electrons, and why covalent compounds have low melting points.
Formation of Covalent Bonds
Tip: Covalent bonding = SHARING electrons (between non-metals). Bonds inside a molecule are strong, but forces BETWEEN molecules are weak → low melting points, often gases or liquids.
131. What is covalent bonding?
• Define covalent bonding.• How does an atom complete its outer shell by sharing?
Covalent bonding is when atoms share electrons to complete their outer shells. The compound formed is called a covalent compound.
132. What is a covalent compound?
• Define a covalent compound.• What compound forms when atoms share electrons?
A covalent compound is a compound whose atoms are held together by shared electrons (covalent bonds), for example water and methane.
133. Are the bonds inside a covalent molecule strong or weak?
• How strongly are the atoms held together within a molecule?• How strong are the covalent bonds inside a molecule?
The atoms within each molecule are held strongly together by the covalent bonds.
134. Are the forces between covalent molecules strong or weak?
• How strong are the forces holding molecules to each other?• What kind of forces exist between covalent molecules?
There are only weak forces holding the molecules together (between separate molecules).
135. Why does candle wax melt easily?
• Explain why wax molecules can flow and escape when heated.• Why can wax turn to vapour easily?
Candle wax is a covalent compound of long carbon and hydrogen chains. The bonds within the molecules are strong, but the forces between the molecules are weak, so when heated the molecules can flow and escape from the surface as a vapour.
136. Why do covalent compounds have low melting points?
• Why are the melting points of covalent compounds low?• What causes the low melting points of covalent substances?
Because the forces holding the molecules together are weak, only a little energy is needed to separate them — so covalent compounds have low melting points.
137. Are most covalent compounds solids, liquids or gases at room temperature?
• What state are most covalent compounds at room temperature?• Why are covalent compounds often liquids or gases?
Most covalent compounds are liquids or gases at room temperature, because the weak forces between their molecules give them low melting and boiling points.
138. Can covalent compounds conduct electricity? Why or why not?
• Why do covalent compounds not conduct electricity?• Do covalent compounds carry an electric current?
No — covalent compounds cannot conduct electricity, because they have no electrically charged particles (no free ions or electrons) to carry a current.
139. How is the covalent bonding in methane formed?
• Describe how a methane molecule is held together.• How does carbon bond with hydrogen in methane?
In methane, the carbon atom shares electrons with four hydrogen atoms. This gives each hydrogen atom two electrons and gives the carbon atom eight electrons in its outer shell.
140. How many covalent bonds are in a methane molecule?
• How many bonds does each methane molecule have?• How many shared pairs are in methane?
Each molecule of methane has four covalent bonds (one to each of the four hydrogen atoms).
141. Why is a methane molecule electrically neutral?
• Why does methane have no overall charge?• What makes methane electrically neutral?
Because the total number of protons in the molecule equals the total number of electrons, so a methane molecule is electrically neutral.
142. Why is methane such a good fuel?
• Explain why methane releases a lot of energy when it burns.• Why does burning methane give out lots of energy?
Because the covalent bonds in methane molecules are very strong, a lot of energy is released when they break during burning — so methane is a good fuel.
143. How is a water molecule formed by covalent bonding?
• How do hydrogen and oxygen share electrons to make water?• Describe the covalent bonding in water.
Water is formed when two hydrogen atoms share their electrons with one oxygen atom: 2H + O → H₂O. The shared electrons form the covalent bonds.
144. What is the main difference between a covalent bond and an ionic bond?
• How does a covalent bond differ from an ionic bond?• Compare covalent and ionic bonding.
In a covalent bond electrons are SHARED between atoms; in an ionic bond electrons are TRANSFERRED (lost by one atom and gained by another) to form ions.
Core Notes · Part 14
Water as a Polar Molecule
Why water is slightly charged at each end, and why that matters.
Water as a Polar Molecule
Note: Water is a POLAR molecule — slightly positive at the hydrogen end and slightly negative at the oxygen end. This is why salt and other ionic substances dissolve so easily in water.
145. Why is the hydrogen end of a water molecule slightly positive?
• Why does one end of a water molecule become slightly positive?• What makes the hydrogen side of water positive?
The oxygen nucleus attracts the shared electrons strongly and pulls them away from the hydrogen atoms, making the hydrogen end of the molecule slightly positive.
146. Why is the oxygen end of a water molecule slightly negative?
• Why does the oxygen side of water become slightly negative?• What makes the oxygen end of water negative?
At the oxygen end there are two pairs of electrons pulled towards it, which makes that end of the molecule slightly negative.
147. What is a polar molecule?
• Define a polar molecule.• What does it mean for a molecule to be polar?
A polar molecule is one that is slightly positive at one end and slightly negative at the other end.
148. Why is water called a polar molecule?
• What makes water a polar molecule?• Explain why water is described as polar.
Water is polar because it is slightly positive at the hydrogen end and slightly negative at the oxygen end.
149. Why do water molecules attract each other?
• What causes water molecules to be attracted to one another?• Why do water molecules stick together?
Because water is polar, the slightly positive end of one molecule attracts the slightly negative end of another — these weak electrostatic attractions pull the molecules together.
150. Why is water a liquid at room temperature and not a gas like methane?
• Why is water liquid while methane is a gas?• What keeps water liquid at room temperature?
The weak electrostatic attractions between the polar water molecules hold them together, which is what makes water a liquid at room temperature, unlike methane (a gas).
151. Why do ionic substances like salt dissolve so easily in water?
• Why is water a good solvent for salt?• How does water’s polarity help it dissolve salt?
Because water is polar (slightly + at one end, slightly − at the other), it attracts and surrounds the charged ions of substances like salt, so many ionic substances dissolve easily in water.
152. Suggest why water is one of the most important substances on Earth.
• Why is water so important for living things?• What makes water so vital?
Because water is a polar molecule, it can dissolve many substances, which makes it an excellent solvent — vital for reactions in living things and in the environment, so it is one of the most important substances on Earth.
Board Exercise
Exercise: Multiple Choice
Board-style MCQs with the correct option and full reasoning.
Exercise — Multiple Choice Questions
153. The substances used up in a chemical reaction are called the… (a) produce (b) products (c) reactants (d) reagents
• What do we call the starting substances that get used up?• Choose the correct term for the starting substances.
(c) reactants — the reactants are the starting substances that are used up in a reaction (the products are what is made).
154. Which of the following is an example of a chemical reaction? (a) Digestion of food (b) Melting of ice (c) Photosynthesis (d) All of the above
• Which option is a chemical reaction?• Pick the chemical reaction from the list.
(a) Digestion of food is a chemical reaction. Note: photosynthesis (c) is also a chemical reaction, but melting of ice (b) is only a physical change — so “all of the above” is not correct because melting is not a chemical reaction.
155. What kind of reaction involves a substance combining with oxygen to produce water and carbon dioxide? (a) combustion (b) decomposition (c) displacement (d) synthesis
• Which reaction produces water and carbon dioxide by combining with oxygen?• Choose the reaction type for burning a hydrocarbon.
(a) combustion — burning a hydrocarbon in oxygen produces carbon dioxide and water.
156. What is a positive ion? (a) an atom that has gained a proton (b) an atom that has gained an electron (c) an atom that has lost a proton (d) an atom that has lost an electron
• Which describes a positive ion (cation)?• Choose the correct description of a positive ion.
(d) an atom that has lost an electron — losing an electron leaves the atom with a positive charge (a cation).
157. What kind of chemical reaction can be represented by the equation AB + CD → AD + BC? (a) Decomposition (b) Double displacement (c) Single displacement (d) synthesis
• Which reaction type matches AB + CD → AD + BC?• Choose the reaction type shown by AB + CD → AD + BC.
(b) Double displacement — the two compounds swap partners to form two new compounds.
Board Exercise
Exercise: True or False
Each statement judged true/false with the reason.
Exercise — True or False
158. True or false: The Law of Conservation of Mass states that more atoms are involved before than after a chemical reaction.
• Is it true that more atoms exist before a reaction than after?• State whether true or false: atoms are more before than after a reaction.
False — the number of atoms (and the total mass) is the SAME before and after; no atoms are gained or lost.
159. True or false: In a balanced equation, there are the same number of molecules on the left as on the right.
• Is a balanced equation about equal numbers of molecules?• State whether true or false: balanced means equal molecules on each side.
False — a balanced equation has the same number of ATOMS of each element on each side, not necessarily the same number of molecules.
160. True or false: State symbols indicate what state the reactants and products of a reaction are in.
• Do state symbols show the state of reactants and products?• State whether true or false: state symbols show the physical state.
True — state symbols (s), (l), (g) and (aq) show the physical state of each reactant and product.
161. True or false: No energy is required in a decomposition reaction.
• Is it true that decomposition needs no energy?• State whether true or false: decomposition requires no energy.
False — energy (often heat) IS needed in a decomposition reaction to break the bonds of the compound.
162. True or false: Ionic compounds in solution cannot conduct electricity because their ions are unable to move.
• Is it true that dissolved ionic compounds cannot conduct?• State whether true or false: ions in solution cannot move, so no conduction.
False — in solution (or when molten) the charged ions ARE free to move, so ionic compounds DO conduct electricity then.
Board Exercise
Exercise: Endothermic or Exothermic?
Classifying everyday changes as endothermic or exothermic.
Exercise — Endothermic or Exothermic?
Tip: Takes IN heat / gets colder = endothermic. Gives OUT heat / gets hotter = exothermic.
163. Is baking bread endothermic or exothermic?
• Baking bread — does it take in or give out heat?• Classify baking bread as endo- or exothermic.
Endothermic — heat must be taken in (from the oven) to bake the bread.
164. Is burning toast endothermic or exothermic?
• Burning toast — does it take in or give out heat?• Classify burning toast as endo- or exothermic.
Exothermic — burning is a combustion reaction, which gives out heat and light.
165. Is dissolving sugar in tea endothermic or exothermic?
• Dissolving sugar — does it take in or give out heat?• Classify dissolving sugar as endo- or exothermic.
Endothermic — dissolving sugar takes in a small amount of heat from the surroundings.
166. Is melting chocolate endothermic or exothermic?
• Melting chocolate — does it take in or give out heat?• Classify melting chocolate as endo- or exothermic.
Endothermic — melting takes in heat to change the solid chocolate into a liquid.
167. Is photosynthesis endothermic or exothermic?
• Photosynthesis — does it take in or give out energy?• Classify photosynthesis as endo- or exothermic.
Endothermic — it takes in light energy from the Sun to make sugar.
168. Is respiration in body cells endothermic or exothermic?
• Respiration — does it take in or give out energy?• Classify respiration as endo- or exothermic.
Exothermic — respiration releases (gives out) energy from food inside the cells.
169. Is rusting of metal railings endothermic or exothermic?
• Rusting — does it take in or give out heat?• Classify rusting as endo- or exothermic.
Exothermic — rusting gives out heat, although very slowly.
170. Is using antacid tablets to neutralise stomach acid endothermic or exothermic?
• Neutralising stomach acid — does it take in or give out heat?• Classify neutralisation (antacid) as endo- or exothermic.
Exothermic — neutralisation gives out heat (the amount is usually small).
171. Is water evaporating from a puddle endothermic or exothermic?
• Water evaporating — does it take in or give out heat?• Classify evaporation as endo- or exothermic.
Endothermic — evaporation takes in heat from the surroundings to change liquid water into vapour.
Board Exercise
Exercise: Balance the Equations
Balancing carbon, methane and glucose combustion equations.
Exercise — Balance the Chemical Equations
Tip: Balance by putting numbers in front of formulae so each element has equal atoms on both sides. Never change a formula.
172. Balance: carbon + oxygen → carbon dioxide.
• Write the balanced equation for carbon burning in oxygen.• Balance C + O₂ → CO₂.
C + O₂ → CO₂ (already balanced: 1 carbon and 2 oxygen atoms on each side).
173. Balance: carbon + hydrogen → methane.
• Write the balanced equation for making methane from carbon and hydrogen.• Balance C + H₂ → CH₄.
C + 2H₂ → CH₄ (1 carbon and 4 hydrogen atoms on each side).
174. Balance: methane + oxygen → carbon dioxide + water.
• Write the balanced equation for burning methane.• Balance CH₄ + O₂ → CO₂ + H₂O.
CH₄ + 2O₂ → CO₂ + 2H₂O (1 C, 4 H and 4 O atoms on each side).
175. Balance: glucose + oxygen → carbon dioxide + water.
• Write the balanced equation for the combustion of glucose.• Balance C₆H₁₂O₆ + O₂ → CO₂ + H₂O.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (6 C, 12 H and 18 O atoms on each side).
Board Exercise
Exercise: Ionic Bonding
Using shell diagrams to explain ionic bond formation.
Exercise — Ionic Bonding (Lithium + Fluorine)
176. Use the shell diagrams to explain how an ionic bond forms between lithium and fluorine.
• How is lithium fluoride formed by ionic bonding?• Explain the formation of the ionic bond in lithium fluoride.
Lithium (2,1) loses its single outer electron to become a positive ion, Li⁺. Fluorine (2,7) gains that electron to complete its outer shell and becomes a negative ion, F⁻. The oppositely charged ions are then pulled together by electrostatic attraction, forming the ionic bond in lithium fluoride (LiF).
177. How strong are ionic bonds, and why?
• Are the ionic bonds in lithium fluoride strong or weak? Give a reason.• Explain the strength of ionic bonds.
Ionic bonds are very strong, because a lot of energy is needed to break them — which is why ionic compounds have high melting and boiling points.
178. Suggest why ionic compounds must be molten or in solution to conduct electricity.
• Why can lithium fluoride only conduct when melted or dissolved?• Explain why solid ionic compounds do not conduct but molten ones do.
In a solid the ions are locked in place and cannot move. Only when the compound is molten or dissolved are the charged ions free to move and carry the current, so it conducts electricity then.
Board Exercise
Exercise: Covalent Bonding & Water
Covalent bonds, and why water is a polar molecule.
Exercise — Covalent Bonding & Water
179. Using the diagram of a water molecule, explain what a covalent bond is.
• What is a covalent bond, using water as an example?• Explain covalent bonding with the water molecule.
A covalent bond is a shared pair of electrons between atoms. In water, the oxygen atom shares electrons with two hydrogen atoms so that each atom completes its outer shell — these shared electrons are the covalent bonds.
180. What is the main difference between a covalent bond and an ionic bond?
• How does a covalent bond differ from an ionic bond?• Compare covalent and ionic bonds.
In a covalent bond electrons are SHARED between atoms; in an ionic bond electrons are TRANSFERRED (lost by one atom and gained by another), forming ions.
181. Explain why most covalent compounds are liquids or gases at room temperature.
• Why are covalent compounds usually liquids or gases?• Why do covalent compounds have low melting and boiling points?
Because the forces holding the molecules together are weak, only a little energy is needed to separate them — so covalent compounds have low melting and boiling points and are usually liquids or gases at room temperature.
182. Water is a polar molecule — what does this mean?
• What is meant by describing water as polar?• Explain the term polar molecule for water.
It means the water molecule is slightly positive at one end (the hydrogen end) and slightly negative at the other end (the oxygen end).
183. Suggest how being polar makes water one of the most important substances on Earth.
• How does water’s polarity make it so important?• Why does being a polar molecule make water so useful?
Because it is polar, water can attract and dissolve many substances (such as salt), making it an excellent solvent. This is essential for reactions in living things and in the environment, so water is one of the most important substances on Earth.
Board Exercise
Exercise: Baking-Soda Car
The carbon-dioxide-powered car practical and its questions.
Exercise — The Baking-Soda Car Investigation
Note: Baking soda (sodium bicarbonate) + vinegar → carbon dioxide gas. The escaping gas pushes the car forwards.
184. Which gas is produced when baking powder (sodium bicarbonate) reacts with vinegar?
• What gas is made in the baking-soda car reaction?• Baking soda and vinegar react to give off which gas?
Carbon dioxide gas is produced.
185. Why does the bottle car move forwards?
• Explain how the carbon dioxide makes the car move.• What pushes the car forwards in the experiment?
The carbon dioxide gas is produced quickly and rushes out of the bottle in one direction; this push (thrust) forces the car to move forwards in the opposite direction.
186. What type of chemical reaction takes place between sodium bicarbonate and vinegar?
• Classify the baking soda + vinegar reaction.• What kind of reaction happens in the car?
It is a reaction that produces a gas (carbon dioxide). It behaves like an acid reacting with a carbonate — and it is endothermic (the mixture gets colder, similar to sodium carbonate + vinegar).
187. Is the baking soda + vinegar reaction endothermic or exothermic?
• Does the baking-soda car reaction take in or give out heat?• Classify the reaction as endo- or exothermic.
It is endothermic — the temperature of the mixture falls, showing that heat is taken in from the surroundings.
188. Suggest changes you could make to help the car travel further.
• How could you make the baking-soda car go further?• What would improve the distance the car travels?
You could use more vinegar and baking powder to make more gas, make the bottle lighter, reduce friction on the wheels/axles, or make the gas escape through a smaller nozzle so it rushes out faster — all of which increase the push on the car.
189. What safety precautions should you take in this investigation?
• List the safety notes for the baking-soda car experiment.• Why must you be careful with the glue gun?
Wear safety goggles, be careful with the hot glue gun (it gets very hot), and test the car outside in a safe place away from traffic.
Key Terms
Chapter Vocabulary
The important words in Chapter 6 and what each one means.
| Term | Meaning |
|---|---|
| Chemical reaction | A change in which reactants interact to form one or more new substances (products). |
| Reactant | A starting substance that is used up in a chemical reaction. |
| Product | A new substance that is formed in a chemical reaction. |
| Law of Conservation of Mass | No atoms are created or destroyed in a reaction, so total mass of reactants = total mass of products. |
| Chemical equation | A way of representing a reaction using the symbols and formulae of the reactants and products. |
| Balanced equation | An equation with the same number of atoms of each element on both sides. |
| State symbol | A letter in brackets showing the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. |
| Synthesis (combination) | A reaction in which two substances combine to make one new single substance (A + B → AB). |
| Decomposition | A reaction in which one compound breaks down into two or more simpler products (AB → A + B). |
| Thermal decomposition | Using heat to break down a compound, e.g. green copper carbonate → black copper oxide + CO₂. |
| Displacement | A reaction in which a more reactive element pushes out (displaces) a less reactive one. |
| Double displacement | A reaction in which two compounds swap partners to form two new compounds. |
| Precipitate | An insoluble solid that forms in a reaction, e.g. barium sulphate. |
| Combustion | Burning — a fuel reacts with oxygen and gives out heat and light. |
| Hydrocarbon | A fuel that contains only carbon and hydrogen, e.g. methane. |
| Endothermic reaction | A reaction that takes IN heat from the surroundings, so they get colder. |
| Exothermic reaction | A reaction that gives OUT heat to the surroundings, so they get hotter. |
| Ion | A charged atom formed by losing or gaining electrons. |
| Cation | A positive ion — an atom that has lost one or more electrons. |
| Anion | A negative ion — an atom that has gained one or more electrons. |
| Ionic bond | The strong electrostatic attraction between positive and negative ions. |
| Ionic compound | A compound formed when electrons are transferred from a metal to a non-metal. |
| Electrostatic attraction | The pull between opposite charges that holds ions together. |
| Covalent bond | A shared pair of electrons between atoms. |
| Covalent compound | A compound whose atoms are held together by shared electrons, e.g. water, methane. |
| Polar molecule | A molecule that is slightly positive at one end and slightly negative at the other, e.g. water. |
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