Physical and chemical change, rates of reaction — GCSE Chemistry practice questions

Question 1

Limescale in a kettle is mainly calcium carbonate. It can be removed by filling the kettle with a solution of citric acid.

Which change makes the limescale react more slowly?

Accepted Answer

B: using a more dilute citric acid solution. A more dilute solution has fewer acid particles in each unit volume, so collisions with the limescale are less frequent and the reaction is slower. Warming the acid and breaking the limescale into flakes both increase the rate. A larger volume of the same solution has the same concentration, so the rate does not change.

Question 2

Which equation shows a chemical change?

Accepted Answer

D: Ag+(aq) + Br-(aq) → AgBr(s). In D, silver ions and bromide ions join to make a new substance, the precipitate silver bromide: a chemical change. A is melting, B is a change of state of iodine and C is potassium bromide dissolving; in each the substance is still the same, so they are physical changes.

Question 3

A student adds an excess of zinc (5.0 g) to dilute sulfuric acid in four experiments.

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

| experiment | volume of acid / cm³ | concentration of acid / mol/dm³ | zinc | temperature / °C |
|---|---|---|---|---|
| 1 | 60 | 1.0 | granules | 30 |
| 2 | 25 | 2.0 | powder | 40 |
| 3 | 50 | 1.0 | powder | 40 |
| 4 | 20 | 2.0 | powder | 30 |

Which row is correct?

Accepted Answer

B: 2 · 1. Experiment 2 has the highest concentration, the higher temperature and powdered zinc, so it has the fastest initial rate. The acid is the limiting reactant, so the volume of hydrogen depends on the moles of acid: experiment 1 has 60 × 1.0 / 1000 = 0.060 mol, the most of the four, so it gives the largest volume of hydrogen even though it is the slowest.