AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2019: Question 6

✅ Correct answer

2 Na + Cl₂ → 2 NaCl

1 mark for selecting the fully balanced equation.

💡 Key knowledge

• Chlorine exists as a diatomic molecule, so it is written as Cl₂ , not Cl .
• Sodium forms Na⁺ ions and chlorine forms Cl⁻ ions, giving sodium chloride in a 1:1 ratio.

🧠 Exam technique

• Check the equation is balanced by counting atoms on both sides.
• If chlorine is shown as Cl₂ , you must use 2 sodium atoms and make 2 units of NaCl.

❌ Common errors

• Choosing Na + Cl → NaCl because it “looks balanced” but ignores chlorine being diatomic.
• Writing Na + Cl₂ → NaCl₂ or 2Na + Cl → Na₂Cl .

✅ Correct answers

• Before reaction: the sodium is a glowing splint.
• During reaction: it relights.
• After reaction: there is a white solid/powder formed.

Any equivalent correct observation about the glowing splint and relighting scores the marks.

💡 Key knowledge

• The reaction is very energetic because sodium is a reactive Group 1 metal.
• The product is sodium chloride, which is a white ionic solid.

🧠 Exam technique

• Give one observation for each stage: before, during and after.
• Use simple observable facts: colour, flame, light, solid formation.
• The mark scheme accepts the exact wording glowing splint and relights.

❌ Common errors

• Describing the chemistry instead of the observation, e.g. “sodium loses an electron”.
• Writing “burns” without saying what is seen.
• Not giving a separate observation for each time point.

✅ Correct answer

Sodium is less reactive than potassium because its outer electron is closer to the nucleus and is held more strongly, so it is harder to lose.

Potassium has more electron shells, so there is more shielding and the outer electron is further from the nucleus.

Therefore potassium loses its outer electron more easily and reacts faster.

This question is worth 4 marks, so students needed multiple linked points, not just “potassium is lower down the group”.

💡 Key knowledge

• Reactivity of Group 1 metals increases down the group.
• Down the group, atoms have more shells.
• This increases distance and shielding.
• The attraction between the nucleus and outer electron gets weaker.

🧠 Exam technique

• Use the chain: more shells → more shielding → weaker attraction → easier electron loss → more reactive.
• To gain full marks, link the cause and effect clearly.
• Use the comparison word than to show you are comparing sodium and potassium.

❌ Common errors

• Saying “potassium has more electrons so it is more reactive” without explanation.
• Claiming sodium is less reactive because it is “more stable” with no link to electron shells.
• Forgetting the words shielding and attraction to the nucleus.

✅ Correct answer

Sodium chloride:

• Has ionic bonding.
• It is made of Na⁺ and Cl⁻ ions.
• The ions are arranged in a giant ionic lattice.
• There are strong electrostatic attractions between oppositely charged ions.

Hydrogen chloride:

• Has covalent bonding.
• It is made of small molecules with shared pairs of electrons.
• Each molecule has a strong covalent bond between H and Cl.

Comparison: sodium chloride is a giant ionic structure, whereas hydrogen chloride is simple molecular.

💡 Key knowledge

• Ionic bonding = attraction between oppositely charged ions.
• Covalent bonding = sharing a pair of electrons.
• Giant ionic lattice = many ions in a regular structure.
• Simple molecular = small separate molecules.

🧠 Exam technique

• This is a 6-mark comparison, so you need both similarities and differences if relevant, but the main focus is on contrast.
• Use precise science words: ionic , covalent , lattice , molecules , electrostatic attraction .
• Top responses usually organise the answer into two clear parts: one paragraph for sodium chloride and one for hydrogen chloride.

❌ Common errors

• Saying hydrogen chloride is ionic because it contains a metal and a non-metal — it does not contain a metal.
• Mixing up ionic bonds inside the lattice with covalent bonds in molecules.
• Calling sodium chloride “a molecule” instead of a giant lattice.
• Not mentioning that hydrogen chloride is made of small molecules.

📐 Calculation-style thinking for this paper

There are no full calculations here, but the same exam rule applies: show the correct reasoning in small steps and use the right scientific terms.

• Equation mark: you either select the fully balanced equation or you do not.
• Observation marks: each correct observation is a separate mark.
• Explanation marks: each linked idea in the chain can gain credit.
• Comparison marks: contrast the two substances clearly, not just describe one of them.

🧠 What distinguished top-level answers

• They used exact practical observations for 06.2.
• They explained reactivity using shells, shielding and attraction for 06.3.
• They compared giant ionic lattice with simple molecular covalent structure for 06.4.

❌ Biggest traps across the question

• Forgetting chlorine is Cl₂ .
• Giving explanations when the question asks for observations.
• Using vague language like “it changes” or “there is a reaction” instead of a visible observation.
• Confusing ionic and covalent bonding.