Changes of State Exam Questions
Practice IGCSE Chemistry exam questions on changes of state. Covers melting, boiling, evaporation, freezing, condensation, sublimation, heating curves, and energy changes with mark schemes and examiner notes.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
These questions cover every aspect of changes of state tested on the 0620 papers. Write full answers before checking mark schemes.
Question 1 (3 marks, Core)
Name the change of state that occurs in each of the following:
(a) Ice turns to water. (1)
(b) Steam forms from boiling water. (1)
(c) Solid carbon dioxide changes directly to carbon dioxide gas. (1)
Mark scheme
(a) Melting [1]
(b) Boiling / evaporation [1]
(c) Sublimation [1]
Examiner note: “Vaporisation” is accepted for (b). For (c), “evaporation” is not accepted — sublimation specifically refers to solid directly to gas without passing through the liquid state.
Question 2 (4 marks, Core)
The graph below shows the heating curve when a solid is heated steadily from -20 degC to 120 degC.
The curve shows: a rising section (A), a flat section at 0 degC (B), a rising section (C), a flat section at 100 degC (D), then a rising section (E).
(a) State what is happening during section B. (1)
(b) State what is happening during section D. (1)
(c) Explain why the temperature remains constant during section B, even though heating continues. (2)
Mark scheme
(a) The solid is melting / changing from solid to liquid [1]
(b) The liquid is boiling / changing from liquid to gas [1]
(c) The energy being supplied is used to break/overcome the forces between particles [1] rather than increasing the kinetic energy of the particles / rather than raising the temperature [1]
Examiner note: “Energy is used to change state” alone is worth 1 mark. For full marks you must state that the energy overcomes intermolecular forces AND that kinetic energy (temperature) does not increase.
Question 3 (3 marks, Core)
State two differences between boiling and evaporation.
Mark scheme
Any two from:
- Boiling occurs at a fixed temperature (boiling point) / evaporation occurs at any temperature below the boiling point [1]
- Boiling occurs throughout the liquid / evaporation occurs only at the surface [1]
- Boiling requires a continuous supply of heat / evaporation can occur without external heating [1]
- Boiling produces bubbles within the liquid / evaporation does not [1]
Examiner note: Each difference must be a comparison. Writing “boiling happens at 100 degC” without contrasting it with evaporation scores 0. Pair each statement with its counterpart.
Question 4 (3 marks, Supplement)
Explain, in terms of particles and energy, what happens when a liquid evaporates from an open container.
Mark scheme
- Particles at the surface have enough kinetic energy to overcome the intermolecular forces [1]
- These faster-moving particles escape from the surface into the gas phase [1]
- The remaining liquid is cooled because the average kinetic energy of the remaining particles decreases [1]
Examiner note: The cooling effect is a key Supplement concept. It is the most energetic particles that escape, leaving behind particles with lower average kinetic energy, which is why the liquid cools.
Question 5 (2 marks, Core)
A student places a cold glass plate above a beaker of boiling water. Water droplets form on the underside of the plate.
(a) Name the change of state occurring on the plate. (1)
(b) Explain why this happens. (1)
Mark scheme
(a) Condensation [1]
(b) The steam / water vapour loses energy / is cooled when it contacts the cold plate, and the particles slow down enough for the intermolecular forces to pull them together into a liquid [1]
Examiner note: “The steam turns back into water” restates the observation — it does not explain it. Reference energy loss and/or particle speed.
Question 6 (4 marks, Supplement)
The cooling curve of a pure substance is shown below. The substance starts as a gas at 350 degC and is cooled steadily.
The curve shows: falling section, flat section at 240 degC, falling section, flat section at 85 degC, falling section to room temperature.
(a) State the boiling point and the melting point of this substance. (2)
(b) On which sections of the curve is the substance present as two states simultaneously? Explain your answer. (2)
Mark scheme
(a) Boiling point = 240 degC [1], melting point = 85 degC [1]
(b) During the two flat/horizontal sections [1]. At these points the substance is changing state, so both states are present — liquid and gas at 240 degC, solid and liquid at 85 degC [1].
Examiner note: On a cooling curve, the flat sections represent the reverse changes of state (condensation at the boiling point, freezing at the melting point). The temperatures are the same whether heating or cooling for a pure substance.
Question 7 (3 marks, Supplement)
Naphthalene has a melting point of 80 degC.
(a) Describe the energy changes occurring at the particle level when solid naphthalene is heated from 70 degC to 90 degC. (3)
Mark scheme
- From 70 to 80 degC: energy increases the kinetic energy of particles / particles vibrate faster [1]
- At 80 degC: energy is used to overcome/break the intermolecular forces between particles / to separate the particles [1]
- From 80 to 90 degC: once all the solid has melted, energy again increases the kinetic energy of the liquid particles / particles move faster [1]
Examiner note: The bonds broken during melting are intermolecular forces, not covalent bonds. Saying “bonds between atoms are broken” is incorrect and will lose the mark. Be precise: forces between molecules are overcome.
Question 8 (2 marks, Core)
Explain why the melting point of a pure substance is sharp (occurs at one specific temperature) but an impure substance melts over a range of temperatures.
Mark scheme
- In a pure substance all the particles are the same / identical, so the forces between them are uniform, and they are all overcome at the same temperature [1]
- In an impure substance the different types of particles have different forces between them, so they are overcome at different temperatures / the structure is disrupted and less energy is needed to start melting [1]
Examiner note: This is a higher-demand question. The key concept is uniformity of forces in a pure substance versus a mixture of different intermolecular forces in an impure one.
What to revise if you scored below 6
If heating and cooling curves were difficult (Questions 2, 6), practise drawing and labelling both types of curve. If you confused boiling and evaporation (Question 3), revisit the changes of state notes and memorise the three key differences. For the particle-level explanations, always follow: energy input → overcomes forces → state change (not temperature change).
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