Atoms, elements and compounds - AQA GCSE Chemistry Exam Questions

Practise 31 exam-style questions on Atoms, elements and compounds for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

31 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

1 mark

Chemical reactions always involve the formation of one or more new substances.

Give one other feature that chemical reactions often involve, which can be detected.

Model answer

A detectable energy change, for example the mixture getting hotter or colder.

Explanation

Chemical reactions transfer energy to or from the surroundings, which is often detected as a temperature change or as light. Colour changes, gas bubbles and precipitates are evidence that new substances have formed.

Mark scheme

1 mark
an energy change
allow temperature change / heat or light given out or taken in; ignore colour change, fizzing or precipitate (these show new substances)

Question 2

2 marks

Hydrochloric acid reacts with sodium hydroxide solution to form sodium chloride solution and water.

Write the ionic equation for this reaction. Include state symbols.

Model answer

H⁺(aq) + OH⁻(aq) → H₂O(l)

Explanation

In neutralisation, the hydrogen ions from the acid react with the hydroxide ions from the alkali to form water. The sodium and chloride ions stay in solution unchanged (they are spectator ions), so they are left out of the ionic equation.

Mark scheme

1 mark
H⁺ + OH⁻ → H₂O
do not accept the full equation HCl + NaOH → NaCl + H₂O; do not accept if Na⁺ or Cl⁻ are included
1 mark
state symbols: H⁺(aq) + OH⁻(aq) → H₂O(l)

Question 3

3 marks

Sodium is a soft, highly reactive metal. Chlorine is a toxic, green gas. Sodium and chlorine react to form sodium chloride, a white solid that is safe to eat in small amounts.

Explain why sodium chloride has different properties from the elements it is made from.

Model answer

When sodium and chlorine react, a chemical reaction takes place and a new substance is formed. In sodium chloride the sodium and chlorine are chemically bonded together in fixed proportions (one sodium for every chlorine), as Na⁺ and Cl⁻ ions. Because the particles and the way they are joined are different, the compound has its own properties, unlike a mixture in which each substance keeps its properties.

Explanation

A compound is a new substance, not just the elements side by side. Its atoms are chemically bonded (here as ions), so its properties depend on these new particles and bonds rather than on the properties of the original elements. This is why a toxic gas and a reactive metal can make a harmless solid.

Mark scheme

1 mark
a chemical reaction takes place / a new substance is formed
max 3
1 mark
sodium and chlorine are chemically combined / bonded together (in a compound)
allow sodium ions and chloride ions are held together by ionic bonds
1 mark
in fixed proportions
allow one sodium for every chlorine / the formula NaCl
1 mark
sodium atoms and chlorine atoms change into ions / the particles are different from those in the elements
allow sodium atoms lose electrons and chlorine atoms gain electrons
1 mark
in a mixture the substances would keep their own properties, but a compound has its own new properties

Question 4

4 marks

Calcium reacts with water to form calcium hydroxide, Ca(OH)₂, and hydrogen gas.

Write a word equation for this reaction. Write a balanced symbol equation for the reaction, including state symbols.

Model answer

Word equation: calcium + water → calcium hydroxide + hydrogen
Symbol equation: Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

Explanation

Ca(OH)₂ contains 2 oxygen atoms and 2 hydrogen atoms, and H₂ contains 2 more hydrogen atoms, so 4 hydrogen atoms and 2 oxygen atoms are needed on the left: 2H₂O. Water is a liquid (l) and hydrogen is a gas (g).

Mark scheme

1 mark
calcium + water → calcium hydroxide + hydrogen
1 mark
correct formulae: Ca + H₂O → Ca(OH)₂ + H₂
1 mark
balanced: Ca + 2H₂O → Ca(OH)₂ + H₂
allow multiples
1 mark
state symbols: Ca(s), H₂O(l), Ca(OH)₂(aq), H₂(g)
allow Ca(OH)₂(s) because calcium hydroxide is only slightly soluble; allow on correct formulae even if unbalanced

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