Revision — Key Ideas
1. Define the terms atom, molecule and ion. [3]
2. Write the correct description for each substance. Choose from the list below. [5]
the acid found in the stomach | the acid found in car batteries and used widely in industry | the acid used in fertiliser manufacture | a strong alkali used in drain cleaners | a weak alkali produced by dissolving a gas in water
| Substance | Description |
|---|
| Hydrochloric acid (HCl) | |
| Sulfuric acid (H₂SO₄) | |
| Nitric acid (HNO₃) | |
| Sodium hydroxide (NaOH) | |
| Ammonia solution (NH₃) | |
3. State whether each of the following is an acid, a base, or neither. [5]
| Substance | Acid / Base / Neither |
|---|
| Copper(II) oxide (CuO) | |
| Carbon dioxide (CO₂) | |
| Potassium hydroxide (KOH) | |
| Sodium chloride (NaCl) | |
| Nitric acid (HNO₃) | |
Section 1 (2.35–2.36) — Acids and Bases as Proton Transfer
4. There are different ways to define acids and bases, each more complete than the last. [6]
a) Give a simple definition of an acid and a base based on pH values.
b) Give a more precise definition based on the ions that acids and bases produce in solution. State one limitation of this definition.
c) Give a definition of acids and bases based only on H⁺ ions. Suggest one type of substance that the definition in part (b) struggles to explain, and use it to show why this definition is more useful.
5. For each reaction below, identify the acid and the base, and explain your reasoning. [6]
a) H₂SO₄ + H₂O → H₃O⁺ + HSO₄⁻
b) HNO₃ + NaOH → NaNO₃ + H₂O
c) HCl + H₂O → H₃O⁺ + Cl⁻
d) H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
e) HCl + NH₃ → NH₄⁺ + Cl⁻
6. Water can act as both an acid and a base. [3]
a) State what water must do in order to act as a base.
b) Complete the equation to show water acting as a base:
HCl + H₂O → _____________ + _____________
c) Using your equation, identify the proton donor and the proton acceptor.
Section 2 (2.37) — Reactions of Acids to Form Salts
Key reactions:
- Acid + metal → salt + hydrogen
- Acid + metal oxide (base) → salt + water
- Acid + metal hydroxide (base) → salt + water
- Acid + carbonate → salt + water + carbon dioxide
The name of the salt depends on the
acid used: hydrochloric acid → chloride salt; sulfuric acid → sulfate salt; nitric acid → nitrate salt.
7. Complete the word equations. [7]
a) Zinc + sulfuric acid → __________ + __________
b) Copper(II) oxide + hydrochloric acid → __________ + __________
c) Sodium hydroxide + nitric acid → __________ + __________
d) Calcium carbonate + hydrochloric acid → __________ + __________ + __________
e) Iron + hydrochloric acid → __________ + __________
f) Magnesium oxide + sulfuric acid → __________ + __________
g) Sodium carbonate + nitric acid → __________ + __________ + __________
8. Write balanced symbol equations, including state symbols, for the following reactions. [6]
a) Magnesium (Mg) reacting with dilute sulfuric acid
b) Zinc oxide (ZnO) reacting with dilute hydrochloric acid
c) Calcium carbonate (CaCO₃) reacting with dilute hydrochloric acid
9. A student adds excess zinc powder to dilute sulfuric acid. Describe what the student would observe, and explain what happens to the rate of reaction over time. [3]
10. Name the gas produced when an acid reacts with a carbonate and describe a test for it. [2]
Section 3 (2.38) — Bases: Metal Oxides, Hydroxides and Ammonia
Key concept: A base is any substance that neutralises an acid to form a salt and water only. Metal oxides, metal hydroxides and ammonia (NH₃) are all bases. A soluble base is called an alkali — it produces OH⁻ ions in aqueous solution.
11. From the following list, identify which substances are bases: HCl, MgO, NaOH, CO₂, NH₃, CuO, HNO₃, Ca(OH)₂. [2]
12. Explain the difference between a base and an alkali. Give one example of each. [3]
13. Explain why metal oxides act as bases using the idea of proton transfer. [2]
14. Fill in the missing balancing numbers in the equations below. One equation is already balanced — if you think so, write "already balanced" in the answer space. [7]
a) KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)
b) ___ NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + ___ H₂O(l)
c) ___ NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
d) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
e) Ca(OH)₂(aq) + ___ HCl(aq) → CaCl₂(aq) + ___ H₂O(l)
f) ___ KOH(aq) + H₂SO₄(aq) → K₂SO₄(aq) + ___ H₂O(l)
g) Mg(OH)₂(aq) + ___ HNO₃(aq) → Mg(NO₃)₂(aq) + ___ H₂O(l)
Section 4 (2.34) — Solubility Rules
Key rules:
- All nitrates are soluble.
- All sodium, potassium and ammonium salts are soluble.
- Most chlorides are soluble — except silver chloride (AgCl) and lead chloride (PbCl₂).
- Most sulfates are soluble — except barium sulfate (BaSO₄), lead sulfate (PbSO₄) and calcium sulfate (CaSO₄).
- Most carbonates are insoluble — except sodium, potassium and ammonium carbonates.
- Most hydroxides are insoluble — except sodium, potassium and ammonium hydroxides (and calcium hydroxide is slightly soluble).
15. Predict whether each compound is soluble or insoluble in water. [8]
| Compound | Soluble / Insoluble |
|---|
| Potassium nitrate | |
| Silver chloride | |
| Barium sulfate | |
| Ammonium carbonate | |
| Copper(II) carbonate | |
| Lead(II) nitrate | |
| Calcium sulfate | |
| Iron(III) hydroxide | |
16. A student mixes aqueous solutions of barium chloride and sodium sulfate. A white precipitate forms immediately. [3]
a) Complete the word equation for the reaction:
barium chloride + sodium sulfate → __________________ + __________________
b) The formulae of the reactants are BaCl₂ and Na₂SO₄. Write a balanced symbol equation for the reaction, including state symbols.
Section 5 (2.39) — Preparing a Pure, Dry Soluble Salt ★ Practical
Key method: To make a soluble salt from an
insoluble base and an acid:
- Warm the acid gently and add the insoluble base in excess until no more dissolves.
- Filter to remove the excess, unreacted solid.
- Evaporate the filtrate to concentrate the salt solution.
- Allow to cool and crystallise.
- Filter the crystals, then dry between filter paper or in a warm oven.
Using the base
in excess ensures all the acid is used up, giving a pure (acid-free) product.
17. A student prepares copper(II) sulfate crystals by reacting copper(II) oxide with dilute sulfuric acid. Place the following steps in the correct order by numbering them 1–5. [3]
| Order | Step |
|---|
| ___ | Filter the mixture to remove excess copper(II) oxide. |
| ___ | Filter and dry the crystals between filter paper. |
| ___ | Warm the sulfuric acid and add copper(II) oxide in excess. |
| ___ | Allow the solution to cool so crystals form. |
| ___ | Evaporate the filtrate to concentrate the solution. |
18. Explain the purpose of each step below. [4]
a) Adding the base in excess:
b) Filtering the mixture after heating:
c) Evaporating then cooling rather than evaporating to dryness:
19. The formulae of copper(II) oxide and sulfuric acid are CuO and H₂SO₄. Write the balanced symbol equation for the reaction used to produce copper(II) sulfate, including state symbols. [2]
20. Suggest what colour change the student would observe in the filtrate as copper(II) sulfate is formed. [1]
Total marks: 80
Mark Scheme
Revision — Key Ideas
Q1. [3 marks — 1 per definition]
- Atom: the smallest particle of an element that can exist and retain the chemical properties of that element; consists of a nucleus (protons and neutrons) surrounded by electrons. [1]
- Molecule: a group of two or more atoms chemically bonded together — can be the same element (e.g. O₂) or different elements (e.g. H₂O). [1]
- Ion: an atom or group of atoms that has gained or lost one or more electrons, giving it an overall positive or negative electrical charge. [1]
Q2. [5 marks — 1 per row]
Correct letter matches:
- Hydrochloric acid (HCl) — D (the acid found in the stomach)
- Sulfuric acid (H₂SO₄) — C (found in car batteries; used in industry)
- Nitric acid (HNO₃) — E (used in fertiliser manufacture)
- Sodium hydroxide (NaOH) — A (strong alkali; drain cleaners)
- Ammonia solution (NH₃) — B (weak alkali; dissolving a gas in water)
Q3. [5 marks — 1 per row]
- Copper(II) oxide (CuO) — Base (metal oxide; neutralises acids)
- Carbon dioxide (CO₂) — Neither (it is an acidic oxide but does not neutralise acids in the way a base does; it does not accept protons — do not accept "acid")
- Potassium hydroxide (KOH) — Base (metal hydroxide; also an alkali as it is soluble)
- Sodium chloride (NaCl) — Neither (it is a neutral salt)
- Nitric acid (HNO₃) — Acid
Note: accept "acid" for CO₂ if the student gives a valid justification based on its oxide chemistry, but "neither" is the expected answer at this level.
Section 1 — Proton Transfer
Q5. [6 marks — 2 per part]
- a) pH-based definition (observational, no mechanism): [2]
- An acid has a pH below 7 (the lower the pH, the stronger the acid). [1]
- A base has a pH above 7 (the higher the pH, the stronger the base/alkali). [1]
Note: this definition is purely observational — it describes what we measure, but gives no explanation of why.
- b) Ion-based definition (explains the chemistry in solution): [2]
- An acid produces H⁺ ions in aqueous solution. A base produces OH⁻ ions in aqueous solution. [1]
- Limitation: this definition only works in aqueous (water-based) solutions, and cannot explain why ammonia (NH₃) acts as a base — because ammonia contains no OH⁻ and does not directly produce it when dissolved. [1]
- c) Proton transfer definition (most complete): [2]
- An acid is a proton (H⁺) donor. A base is a proton (H⁺) acceptor. [1]
- This is more powerful because it explains how ammonia acts as a base without needing OH⁻: NH₃ accepts a proton from the acid directly (e.g. HCl + NH₃ → NH₄⁺ + Cl⁻), forming the ammonium ion. The definition also works beyond aqueous solutions. [1]
Q5. [10 marks — 2 per part]
For each part, award 1 mark for correctly identifying the acid with a reason, and 1 mark for correctly identifying the base with a reason.
- a) H₂SO₄ + H₂O → H₃O⁺ + HSO₄⁻
Acid: H₂SO₄ — it donates H⁺ to water. [1]
Base: H₂O — it accepts the H⁺ to form the hydronium ion H₃O⁺. [1]
- b) HNO₃ + NaOH → NaNO₃ + H₂O
Acid: HNO₃ — it donates H⁺ to the hydroxide ion. [1]
Base: NaOH (specifically the OH⁻ ion) — it accepts the H⁺ to form water. [1]
- c) HCl + H₂O → H₃O⁺ + Cl⁻
Acid: HCl — it donates H⁺ to water. [1]
Base: H₂O — it accepts the H⁺ to form H₃O⁺. [1]
Note: this is similar to (a) but uses a different acid; water again acts as the base.
- d) H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
Acid: H₂SO₄ — it donates H⁺ to the hydroxide ions (note: sulfuric acid can donate two protons, one at a time). [1]
Base: KOH (specifically the OH⁻ ion) — it accepts the H⁺ to form water. [1]
- e) HCl + NH₃ → NH₄⁺ + Cl⁻
Acid: HCl — it donates H⁺ to ammonia. [1]
Base: NH₃ — it accepts the H⁺ to form the ammonium ion NH₄⁺. NH₃ has no OH⁻ ion — it acts as a base by accepting the proton directly. [1]
Q6. [3 marks]
- a) Water must accept a proton (H⁺) from an acid. [1]
- b) HCl + H₂O → H₃O⁺ + Cl⁻ [1]
Accept: products written as H₃O⁺ and Cl⁻ in either order.
- c) Proton donor: HCl (the acid). Proton acceptor: H₂O (acting as the base). [1]
Section 2 — Reactions of Acids
Q7. [7 marks — 1 per part]
- a) zinc sulfate + hydrogen
- b) copper(II) chloride + water
- c) sodium nitrate + water
- d) calcium chloride + water + carbon dioxide
- e) iron(II) chloride + hydrogen — accept iron(III) chloride if student justifies, but iron(II) is the expected product with dilute HCl
- f) magnesium sulfate + water
- g) sodium nitrate + water + carbon dioxide
Q8. [6 marks — 2 per part: 1 for correct balanced equation, 1 for all state symbols correct]
- a) Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
- b) ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)
- c) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Note: award the equation mark even if state symbols are missing or wrong, as long as the equation is balanced and correct.
Q9. [3 marks]
- Bubbles / effervescence are observed as hydrogen gas is produced. [1]
- The rate of reaction (amount of bubbling) slows down over time. [1]
- This is because the acid is being used up, so its concentration decreases — fewer successful collisions per second. The reaction eventually stops, with unreacted zinc remaining. [1]
Q10. [2 marks]
- Gas produced: carbon dioxide (CO₂). [1]
- Test: bubble the gas through limewater (calcium hydroxide solution) — it turns milky / cloudy white if carbon dioxide is present. [1]
Section 3 — Bases
Q11. [2 marks]
Bases from the list:
MgO, NaOH, NH₃, CuO, Ca(OH)₂
HCl and HNO₃ are acids, not bases. CO₂ is an acidic oxide — it does not neutralise acids.
- Award 2 marks for all five correct with no incorrect inclusions.
- Award 1 mark if four are correct, or if all five are listed but one incorrect substance is also included.
- Award 0 if fewer than four correct.
Q12. [3 marks]
- A base is a substance that neutralises an acid to form a salt and water. [1]
- An alkali is a base that dissolves in water to produce OH⁻ ions in solution — all alkalis are bases, but not all bases are alkalis. [1]
- Example of a base (not an alkali): copper(II) oxide, CuO / magnesium oxide, MgO — any insoluble base. [½]
Example of an alkali: sodium hydroxide, NaOH / potassium hydroxide, KOH — any soluble base. [½]
Award the full mark for one valid example of each.
Q13. [2 marks]
- When a metal oxide reacts with an acid, the oxide ion (O²⁻) accepts H⁺ ions from the acid. [1]
- Accepting a proton is the definition of a base in the proton transfer model, so the metal oxide is acting as a proton acceptor / base. [1]
Q14. [7 marks — 1 per part]
- a) KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)
Already balanced — all coefficients are 1. Award the mark for "already balanced" or for leaving the equation unchanged.
- b) 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
Na₂SO₄ requires two Na⁺ ions, so two NaOH are needed. This produces two water molecules.
- c) 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
The product contains two NH₄⁺ groups, so two NH₃ molecules are needed as reactants.
- d) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
This equation is already balanced — all coefficients are 1. Award the mark for "already balanced" or for leaving it unchanged.
- e) Ca(OH)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)
Ca(OH)₂ has two OH⁻ groups, so two HCl molecules are needed, producing two water molecules.
- f) 2KOH(aq) + H₂SO₄(aq) → K₂SO₄(aq) + 2H₂O(l)
The sulfate product K₂SO₄ requires two K⁺ ions, so two KOH molecules are needed, producing two water molecules.
- g) Mg(OH)₂(aq) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + 2H₂O(l)
Mg(OH)₂ has two OH⁻ groups; each reacts with one HNO₃, giving two water molecules. The product Mg(NO₃)₂ also confirms two nitrate groups are present.
Section 4 — Solubility Rules
Q15. [8 marks — 1 per row]
- Potassium nitrate — Soluble (all nitrates are soluble; all potassium salts are soluble)
- Silver chloride — Insoluble (AgCl is one of the exceptions to "most chlorides are soluble")
- Barium sulfate — Insoluble (BaSO₄ is one of the exceptions to "most sulfates are soluble")
- Ammonium carbonate — Soluble (all ammonium salts are soluble)
- Copper(II) carbonate — Insoluble (most carbonates are insoluble; only Na, K, NH₄ carbonates are soluble)
- Lead(II) nitrate — Soluble (all nitrates are soluble)
- Calcium sulfate — Insoluble (CaSO₄ is one of the exceptions to "most sulfates are soluble")
- Iron(III) hydroxide — Insoluble (most hydroxides are insoluble; only Na, K, NH₄ hydroxides are soluble)
Q16. [3 marks]
- a) barium chloride + sodium sulfate → barium sulfate + sodium chloride [1]
- b) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) [2]
Award 1 mark for a correct, balanced equation. Award the second mark for all four state symbols correct — note BaSO₄ must be (s) as it is the insoluble precipitate, and both reactants and NaCl must be (aq).
Section 5 — Salt Preparation Practical
Q17. [3 marks]
The correct order (reading down the table as presented) is:
- Filter the mixture to remove excess copper(II) oxide → 2
- Filter and dry the crystals between filter paper → 5
- Warm the sulfuric acid and add copper(II) oxide in excess → 1
- Allow the solution to cool so crystals form → 4
- Evaporate the filtrate to concentrate the solution → 3
Award 1 mark for steps 1–2 in correct relative order, 1 mark for steps 2–3–4 in correct relative order, and 1 mark for steps 4–5 in correct relative order. Award full 3 marks automatically if all five are in the correct sequence.
Q18. [4 marks]
- a) The base is added in excess to ensure that all of the acid is used up / neutralised. If any acid remained in the filtrate, it would contaminate the final crystals. [1]
- b) The mixture is filtered to remove the excess, unreacted copper(II) oxide. If it were not removed, it would contaminate the salt crystals. [1]
- c) Two marks available — award 1 mark for each of the following points:
- Evaporating to complete dryness would drive off the water of crystallisation (the water molecules that are part of the crystal structure), destroying the hydrated crystals. [1]
- By evaporating to concentrate the solution and then allowing it to cool slowly, proper hydrated crystals of copper(II) sulfate (CuSO₄·5H₂O) can form. [1]
Q19. [2 marks]
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
- Award 1 mark for a correctly balanced equation with correct formulae.
- Award 1 mark for all state symbols correct: CuO is a solid reactant (s), H₂SO₄ is in aqueous solution (aq), CuSO₄ dissolves to give an aqueous solution (aq), and H₂O is liquid (l).
Q20. [1 mark]
The filtrate turns blue as copper(II) sulfate solution forms. Accept "blue/blue-green". Do not accept "green" alone.