IGCSE Physics Melting, Boiling and Evaporation Questions and Worked Solutions (2.2.3) | 0625

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IGCSE Physics 0625 · Topic 2.2.3

IGCSE Physics Melting, Boiling and Evaporation Questions and Worked Solutions

50 topic-wise practice questions on Melting, Boiling and Evaporation (Unit 2: Thermal Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions22 Core18 Extended10 Challenge117 marksTextbook §2.2 · pp. 100–114
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Melting, Boiling and Evaporation

During melting and boiling, energy is supplied but the temperature stays constant because the energy is used to separate particles. Evaporation happens at any temperature and causes cooling.

Key formulas

No equations to learn in this subtopic — flat sections on heating/cooling curves = changes of state.

Prerequisite knowledge

  • States and particle model (2.1)
  • Internal energy (2.2.2)

Exam tips & common mistakes

  • Explain the flat section of a heating curve: energy breaks bonds/separates particles, not raising kinetic energy
  • For evaporation questions, say 'most energetic particles escape'
  • Three factors for evaporation rate: temperature, surface area, draught

Key facts from the syllabus (2.2.3)

  • Melting and boiling happen at constant temperature even though energy is supplied
  • Melting point of ice: 0 °C; boiling point of water: 100 °C at standard pressure
  • Condensation and solidification release energy
  • Evaporation: the more energetic particles escape from the surface, so the average kinetic energy (temperature) of the remaining liquid falls
  • Extended boiling occurs at a fixed temperature, throughout the liquid, with bubbles; evaporation occurs at any temperature, only at the surface
  • Extended evaporation rate increases with temperature, surface area and air movement over the surface
  • An object in contact with an evaporating liquid is cooled

Textbook: §2.2 Thermal properties and temperature, pp. 100–114

1
Core[2 marks]

Describe what happens to the temperature of ice while it melts, even though energy is supplied.

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Method: Temperature stays constant.

It stays constant at 0 °C; the energy is used to separate the particles
2
Core[1 mark]

State the melting point of ice at standard atmospheric pressure.

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Method: 0 °C.

0 °C
3
Core[1 mark]

State the boiling point of water at standard atmospheric pressure.

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Method: 100 °C.

100 °C
4
Core[2 marks]

Describe condensation in terms of particles.

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Method: Particles slow, come closer, attractions pull them together.

Gas particles lose energy, slow down and come close together to form a liquid
5
Core[2 marks]

Describe solidification in terms of particles.

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Method: Particles slow and settle into fixed positions.

Liquid particles lose energy and settle into fixed positions in a regular arrangement
6
Core[2 marks]

Describe evaporation in terms of particles.

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Method: More energetic particles escape the surface.

The more energetic particles near the surface escape from the liquid
7
Core[2 marks]

Explain why evaporation cools a liquid.

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Method: Most energetic particles leave.

The most energetic particles escape, so the average kinetic energy (temperature) of those left falls
8
Core[2 marks]

Explain why sweating helps to cool the body.

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Method: Evaporation takes energy from skin.

Sweat evaporates, taking energy from the skin, cooling it
9
Core[2 marks]

A temperature–time graph for heating ice has two flat sections. State what is happening at each.

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Method: Changes of state.

Lower flat section: melting (0 °C); upper flat section: boiling (100 °C)
10
Core[1 mark]

State whether energy is absorbed or released during boiling.

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Method: Absorbed.

Absorbed
11
Core[1 mark]

State whether energy is absorbed or released during condensation.

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Method: Released.

Released
12
Core[2 marks]

State the temperature of pure water while it boils at standard pressure.

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Method: Constant.

100 °C (it stays constant)
13
Core[2 marks]

Describe melting in terms of particles.

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Method: Particles gain energy and break free.

Particles gain enough energy to break out of their fixed positions and move past each other
14
Core[2 marks]

Describe boiling in terms of particles.

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Method: Particles escape throughout.

Particles throughout the liquid gain enough energy to escape and form bubbles of gas
15
Core[2 marks]

Explain why steam at 100 °C causes worse burns than water at 100 °C.

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Method: Condensation releases energy.

Steam releases extra energy as it condenses on the skin
16
Core[2 marks]

State whether energy is absorbed or released when water freezes.

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Method: Released.

Released
17
Core[2 marks]

State the shape of a cooling curve for a substance as it solidifies.

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Method: Flat section.

A flat (horizontal) section at the melting point
18
Core[3 marks]

A solid is heated at a steady rate. Its temperature rises, stays constant at 54 °C for 4 minutes, then rises again. Explain the shape of the graph.

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Method: Melting at 54 °C.

The temperature rises as the solid heats; at 54 °C it melts and the energy separates particles, so temperature is constant; then the liquid heats up
19
Core[2 marks]

Explain why a drink is cooled more by ice at 0 °C than by the same mass of water at 0 °C.

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Method: Melting absorbs energy.

The ice absorbs extra energy as it melts, taking more energy from the drink
20
Core[2 marks]

State why a dog pants on a hot day.

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Method: Evaporation cools.

Water evaporates from its tongue, removing energy and cooling it
21
Core[1 mark]

State the boiling point of water in kelvin.

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Method: Add 273.

373 K
22
Core[2 marks]

Explain why clay water pots keep water cool in hot countries.

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Method: Evaporation through porous clay.

Water seeps through the porous clay and evaporates from the surface, cooling the pot and its contents
23
Extended[3 marks]

State three differences between boiling and evaporation.

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Method: Temperature, location, bubbles.

Boiling occurs at a fixed temperature, throughout the liquid, with bubbles; evaporation occurs at any temperature, only at the surface, with no bubbles
24
Extended[2 marks]

State and explain the effect of temperature on the rate of evaporation.

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Method: More particles have enough energy.

Higher temperature increases the rate: more particles have enough energy to escape
25
Extended[2 marks]

State and explain the effect of surface area on the rate of evaporation.

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Method: More surface particles.

Larger surface area increases the rate: more particles are at the surface and can escape
26
Extended[2 marks]

State and explain the effect of air movement (a draught) on the rate of evaporation.

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Method: Removes vapour.

Increases the rate: escaped particles are carried away, so fewer return to the liquid
27
Extended[2 marks]

Wet clothes dry faster on a windy day. Explain why.

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Method: Air movement.

Wind removes water vapour near the clothes, so fewer molecules return and evaporation is faster
28
Extended[3 marks]

A thermometer bulb is wrapped in cotton wool soaked in alcohol. The reading falls below room temperature. Explain why.

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Method: Evaporation cools the bulb.

The most energetic alcohol molecules escape, lowering the temperature of the remaining alcohol; energy then flows from the bulb to the alcohol
29
Extended[2 marks]

Explain why a puddle dries faster on a hot day than on a cold day.

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Method: Temperature effect.

More water molecules have enough energy to escape from the surface
30
Extended[2 marks]

Explain why water in a shallow dish evaporates faster than the same volume in a narrow glass.

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Method: Surface area.

The dish has a larger surface area, so more molecules can escape per second
31
Extended[2 marks]

Explain why an object in contact with an evaporating liquid becomes cooler.

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Method: Energy flows from object to liquid.

The liquid cools as it evaporates, so energy is transferred from the warmer object to the liquid by conduction
32
Extended[2 marks]

State one difference between evaporation and boiling in terms of where they occur.

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Method: Surface vs throughout.

Evaporation occurs only at the surface; boiling occurs throughout the liquid
33
Extended[2 marks]

State one difference between evaporation and boiling in terms of temperature.

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Method: Any vs fixed.

Evaporation happens at any temperature; boiling only at the boiling point
34
Extended[2 marks]

Explain why a wet towel dries faster when spread out than when folded.

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Method: Surface area.

Spreading increases the surface area, so more water molecules can escape
35
Extended[2 marks]

Explain why perfume on the skin feels cold.

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Method: Evaporation.

It evaporates quickly, taking energy from the skin
36
Extended[3 marks]

Two beakers of water at 40 °C are left in a room: one has a fan blowing across it. Predict and explain which cools faster.

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Method: Draught.

The one with the fan: air movement removes vapour so evaporation is faster, removing more energy
37
Extended[2 marks]

Explain why evaporation rate is reduced by covering a liquid with a lid.

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Method: Vapour trapped.

Escaped molecules are trapped and many return to the liquid
38
Extended[3 marks]

Explain why the temperature of a liquid that is evaporating falls, in terms of the average kinetic energy.

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Method: Fast ones leave.

The most energetic molecules escape, so the average kinetic energy of those remaining is lower, and temperature depends on average kinetic energy
39
Extended[2 marks]

Explain why sweating is less effective at cooling in humid weather.

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Method: Air already saturated.

The air already contains a lot of water vapour, so evaporation is slower
40
Extended[2 marks]

Explain why a wet finger held up feels cooler on the side facing the wind.

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Method: Faster evaporation.

Wind increases the rate of evaporation on that side, removing more energy

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

A cooling curve for wax shows a flat section at 54 °C lasting 5 minutes. Explain what happens to the particles during this time and why the temperature stays constant even though energy is lost.

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Method: Solidification releases energy.

The wax is solidifying; as particles form bonds and settle into fixed positions, energy is released, balancing the energy lost to the surroundings, so temperature is constant
42
ChallengeCore[3 marks]

Explain why burns from steam at 100 °C are worse than from water at 100 °C.

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Method: Condensation releases energy.

Steam releases a large amount of extra energy when it condenses on the skin, before cooling as water
43
ChallengeCore[4 marks]

Explain why sweating cools you, and why you feel colder when you step out of a swimming pool on a windy day.

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Method: Evaporation.

Evaporating water takes energy from the skin; wind removes vapour so evaporation is faster, removing more energy
44
ChallengeCore[3 marks]

Explain why the temperature of boiling water does not rise even if the heater power is increased.

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Method: Energy goes into changing state.

The extra energy makes water boil faster (more steam per second), not hotter
45
ChallengeExtended[4 marks]

Give three differences between boiling and evaporation and explain why evaporation causes cooling but boiling does not cool the remaining liquid.

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Method: Selective escape.

Boiling: at boiling point, throughout, bubbles; evaporation: any temperature, surface only, no bubbles. Evaporation removes the most energetic particles, lowering the average; in boiling energy is supplied continuously
46
ChallengeExtended[4 marks]

Two identical dishes of water at 30 °C are placed in a room. One is in a draught. Predict and explain which loses mass faster and which cools faster.

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Method: Draught.

The one in the draught loses mass faster (vapour removed) and cools faster (more energetic particles leave)
47
ChallengeExtended[4 marks]

Explain how a refrigerator uses evaporation to cool its contents.

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Method: Evaporating coolant.

A liquid coolant evaporates in pipes inside the fridge, taking energy from the inside; it is compressed and condenses outside, releasing energy
48
ChallengeExtended[3 marks]

Explain why water evaporates faster from a wide shallow dish than from a tall narrow glass at the same temperature.

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Method: Surface area.

More molecules are at the surface in the dish, so more can escape per second
49
ChallengeExtended[4 marks]

A thermometer bulb is covered with wet cloth (wet bulb) and another is dry. On a dry day the wet bulb reads much lower; on a humid day the readings are close. Explain.

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Method: Evaporation rate.

On a dry day water evaporates quickly, cooling the wet bulb a lot; in humid air evaporation is slow, so little cooling occurs
50
ChallengeExtended[4 marks]

Explain why ether spilled on the hand feels much colder than water at the same temperature.

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Method: Evaporates faster.

Ether evaporates much faster, so it removes energy from the skin at a greater rate
All IGCSE Physics topics← 2.2.2 Specific Heat Capacity2.3.1 Conduction →

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