IGCSE · Practice questions · Solids, liquids and gases

Think in particles.

Six original Cambridge-style questions on the kinetic particle theory: the three states, changes of state, melting and boiling point data, heating curves and gases.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Arrangement, motion, forces.

Solid: close, regular, vibrate on the spot.
Liquid: close, irregular, move around each other.
Gas: far apart, random, fast in all directions.
Flat on a heating curve: energy overcomes forces, temperature constant.

Recall

[3 marks]

Describe the arrangement and the motion of the particles in a solid, a liquid and a gas.

Solid: particles close together in a regular arrangement, vibrating about fixed positions. ✓
Liquid: particles close together but irregularly arranged, moving around each other. ✓
Gas: particles far apart and randomly arranged, moving quickly in all directions. ✓

Recall

[4 marks]

Name each change of state: (a) solid to liquid, (b) liquid to gas throughout the liquid at its boiling point, (c) gas to liquid, (d) liquid to solid.

(a) melting ✓
(b) boiling ✓
(c) condensation ✓
(d) freezing ✓

Analysis

[4 marks]

The table gives the melting and boiling points of four substances.

Substance	Melting point / °C	Boiling point / °C
P	−210	−196
Q	801	1413
R	−95	56
S	17	118

State the physical state of each substance at 25 °C.

P: gas (25 °C is above its boiling point). ✓
Q: solid (25 °C is below its melting point). ✓
R: liquid (between its melting and boiling points). ✓
S: liquid (17 °C < 25 °C < 118 °C). ✓

Explain

[3 marks]

(Extended) A sample of solid wax is heated steadily from room temperature until it is a liquid well above its melting point. Sketch and explain the shape of its heating curve.

The temperature rises while the wax is solid, as the particles gain kinetic energy and vibrate faster. ✓
At the melting point there is a flat section: the energy supplied is used to overcome the forces of attraction between particles, so the temperature stays constant while solid and liquid are both present. ✓
Once all the wax has melted the temperature rises again as the liquid particles move faster. ✓

Explain

[2 marks]

Give two differences between boiling and evaporation.

Boiling happens only at the boiling point; evaporation happens at any temperature below the boiling point. ✓
Boiling happens throughout the liquid, with bubbles forming inside it; evaporation happens only at the surface. ✓

Explain

[2 marks]

(Extended) Use the kinetic particle theory to explain what happens to the volume of a gas when the pressure on it is increased at constant temperature.

The volume decreases. ✓
There are large spaces between gas particles, so the extra pressure pushes the particles closer together. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.