PhysicsICSE Class 8 · Heat Transfer

Evaporation and Boiling: Differences, Factors and Cooling Effect (ICSE Class 8 Physics)

Updated 17 October 2026 · Fact-checked

Evaporation is the slow change of a liquid into vapour from its surface at any temperature, and it causes cooling. Boiling happens at a fixed temperature throughout the liquid, with bubbles, and needs a steady supply of heat. Evaporation is faster in warm, dry, windy air and with a larger surface.

Evaporation happens only at the surface at any temperature and cools; boiling makes bubbles throughout at a fixed temperature.

Illustration drawn for QuestionBuddy.

Understand Evaporation and Boiling

Heat is energy that flows from a hotter to a colder body. When a liquid gains heat it can change into vapour in two ways: evaporation and boiling.

Evaporation	Boiling
takes place at all temperatures	takes place at a fixed temperature (the boiling point)
from the surface only	throughout the liquid (bubbles form inside)
slow and silent	rapid, with bubbles
causes cooling	needs a continuous supply of heat

Why evaporation cools. Particles in a liquid move with different speeds. The fastest ones near the surface escape as vapour and take their energy with them, so the average energy of the remaining liquid falls: the liquid cools. The cooling uses heat from the liquid itself and from what it touches (your skin, a pot).

Factors increasing evaporation.
1. Higher temperature: more particles move fast enough to escape.
2. Larger exposed surface area: more particles are at the surface.
3. Moving air (wind): it carries the vapour away so more can leave.
4. Dry air (low humidity): air that already holds a lot of water vapour takes in less.

Everyday cooling by evaporation.

We sweat; evaporating sweat takes heat from the body. A fan helps by moving the air.
Water in an earthen pot stays cool: water seeps through tiny pores to the surface and evaporates, drawing heat from the water inside.
A volatile liquid like spirit or perfume feels cold on the skin because it evaporates fast.
Wet clothes dry faster in sun, wind and when spread out.

Boiling. At the boiling point (100 °C for pure water at normal pressure), bubbles of vapour form throughout the liquid. The temperature stays constant while the liquid boils, even though heat is still supplied; the heat is used to turn liquid into vapour.

Remember. Evaporation happens only at the surface and at any temperature; boiling is a bulk change at one fixed temperature.

Evaporation and Boiling: formulas and key facts

Evaporation versus boiling: Evaporation: any temperature, surface only, slow, cooling | Boiling: fixed temperature, whole liquid, bubbles, heat needed; Both change liquid to vapour.
Factors for faster evaporation: Higher temperature
Larger surface area
Moving air
Dry air; Four factors.
Boiling point of pure water: 100 °C at normal atmospheric pressure | stays constant during boiling; Heating curve is flat.

How to solve Evaporation and Boiling questions

For "explain" questions, name the process, give the cause at particle level, then state the effect.

1Decide: surface at any temperature (evaporation) or whole liquid at a fixed temperature (boiling).
2For cooling, say the fastest particles escape and take energy with them.
3For drying and speed, list the factors that apply (temperature, area, wind, dryness).
4In a comparison experiment, change one factor only and keep the others the same.

Evaporation and Boiling in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Clothes on a line

Suppose four identical wet towels are put out: A spread in sun and wind, B folded in sun and wind, C spread in a shaded still room, D folded in the shaded still room.

Which dries first and which last? Give reasons.

Try it first, then show the solutionSolution

A has a large surface area, higher temperature and moving air, so it dries first.
D has a small surface, lower temperature and still air, so it dries last.

Answer: A first, D last.

Memory hook: Hot, wide, windy, dry: fastest drying.

Sweat and the fan

Imagine a hot, humid evening when you sweat a lot but feel cooler as soon as the fan is switched on.

Explain what the fan does.

Try it first, then show the solutionSolution

The fan moves the air, carrying the water vapour away from the skin.
Sweat evaporates faster, taking more heat from the body, so you feel cooler.

Answer: The fan speeds up evaporation of sweat, so more heat is removed.

Memory hook: Moving air helps sweat leave.

Tricks to remember Evaporation and Boiling

Short memory aids. Say each one aloud once, then use it on a question the same day.

S-S-S
Evaporation: Surface, Slow, Silent. Boiling: bubbles, bulk, a fixed point.
Use it when: Comparisons.
THWD
Temperature, Humidity (low), Wind, Dimension (surface area): four ways to speed evaporation.
Use it when: Factors.
Escape of the fast
Fastest particles leave, so the average energy of the rest falls: the cool-down.
Use it when: Why cooling.

Evaporation and Boiling: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Evaporation only · 1 mark · MCQBoard level

Which statement is true for evaporation but not for boiling?
(a) It needs heat
(b) It produces vapour
(c) It takes place at all temperatures
(d) It happens at 100 °C for water

Try it first, then show the solutionSolution

Evaporation occurs at every temperature, whereas boiling occurs at a fixed boiling point.

Answer: (c) It takes place at all temperatures

Example 2: Two differences · 2 marks · Short answerBoard level

State two differences between evaporation and boiling.

Try it first, then show the solutionSolution

Evaporation occurs at all temperatures; boiling occurs at a fixed temperature (boiling point). (1)
Evaporation occurs only at the surface and is slow; boiling occurs throughout the liquid with bubbles and is rapid. (1)

Answer: Temperature (all vs fixed) and place (surface vs throughout).

Example 3: Why does a wet cloth dry quicker? · 3 marks · Short answerHarder board level

Three identical wet cloths are dried (i) spread out in the sun, (ii) spread out in a shaded room and (iii) folded in the shaded room. Arrange them in order of drying time, fastest first, and give the reason for each step in the order.

Try it first, then show the solutionSolution

Fastest: (i), because the sun gives a higher temperature than the shade. (1)
Then (ii): same large area as (i) but lower temperature. (1)
Slowest: (iii), because folding reduces the exposed surface area. Order: (i), (ii), (iii). (1)

Answer: Order (fastest first): (i), (ii), (iii)

Example 4: The earthen pot · 3 marks · Short answerHarder board level

Explain why water stored in an earthen pot (matka) in summer is cooler than the air around it, and why it stays cooler in a breezy place.

Try it first, then show the solutionSolution

Water seeps through the tiny pores to the outer surface and evaporates. (1)
Evaporation takes heat from the pot and the water inside, cooling it. (1)
A breeze carries the vapour away, increasing the rate of evaporation, so cooling is greater. (1)

Answer: Evaporation through the pores removes heat; wind speeds it up.

Example 5: Testing area · 4 marks · Case-basedHOTS / competency

Two beakers X and Y of the same water volume stand in the same room. X has a surface area of 50 cm² and Y of 100 cm². After a day X has lost 8 mL and Y has lost 16 mL. (a) Which loses water faster? (b) Find the ratio of the water lost. (c) What hypothesis do the results support? (d) Name one more factor you could test and how you would keep the test fair.

Try it first, then show the solutionSolution

(a) Y, which has the larger surface. (½)
(b) $\frac{16}{8} = 2$ , so Y loses twice as much. (1)
(c) The rate of evaporation is proportional to the exposed area (here the area is also doubled: $\frac{100}{50} = 2$ ). (1½)
(d) Wind or temperature: use identical beakers with equal volumes, one near a fan and one without, in the same room; change only one factor. (1)

Answer: (a) Y (b) 2 : 1 (c) rate ∝ area (d) fair test with one factor changed

Where students lose marks in Evaporation and Boiling

The slips examiners see most often, and what to write instead.

Mistake: Saying evaporation needs a boiling point
Evaporation happens at all temperatures.
Wrong working
"Water evaporates at 100 °C."
Right working
"Water evaporates at all temperatures and boils at 100 °C."
Do this: Link boiling with a fixed temperature and evaporation with any.
Mistake: Forgetting why evaporation cools
Fast particles leave and take energy.
Wrong working
"Evaporation cools because the liquid is wet."
Right working
"The fastest particles escape, so the average energy and temperature of the rest fall."
Do this: Give the particle reason.
Mistake: Unfair test
More than one factor changed gives no clear conclusion.
Wrong working
"A hot dish and a cold dish in different winds."
Right working
"Same dish, same temperature; only wind differs."
Do this: Change one factor, keep the rest.

Exam tips

Use a two-column table for differences and give at least three points for 3 marks.
Name the factor and the effect: "larger area, so more particles escape".
In experiment answers state what is kept the same.
Never say that evaporation "needs 100 °C".

Practice questions on Evaporation and Boiling

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
Name the process that causes cooling when a liquid changes to vapour at its surface.
Hint
Not boiling.
Answer
Evaporation.
Q2 · 2 marks · Short answerHarder board level
Water boils at 100 °C while the stove keeps supplying heat, but its temperature does not rise. Where does the supplied heat go?
Hint
Think of change of state.
Answer
The heat is used to change water into steam (to separate the particles), not to raise the temperature, so the temperature stays constant during boiling.
Q3 · 3 marks · Short answerHarder board level
Why does spirit poured on your palm feel cold? Why does it feel colder than water poured in the same way?
Hint
Which one evaporates faster?
Answer
Spirit evaporates quickly, taking heat from your palm, so the palm feels cold. It evaporates faster than water, so it removes more heat in the same time and feels colder.
Q4 · 3 marks · Short answerHarder board level
On which day will wet clothes dry fastest: (i) hot, dry and windy, (ii) hot, humid and still, (iii) cool, dry and still? Give reasons.
Hint
Count the favourable factors.
Answer
Day (i): high temperature, dry air and moving air all favour evaporation. (ii) has high temperature but humid, still air; (iii) has dry air but low temperature and no wind.
Q5 · 4 marks · Long answerHOTS / competency
A student says: "Evaporation can happen only when the water is boiling." Using the idea of a wet floor that dries by itself, show why this is wrong. Plan a fair experiment to show that wind increases evaporation.
Hint
Change only wind.
Answer
A wet floor dries at room temperature without boiling, so evaporation occurs at any temperature. Experiment: pour equal volumes of water in two identical dishes, place one in front of a fan and one away from it, in the same room and temperature; measure the remaining volume after an hour. The dish in the wind has less water left.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Evaporation takes place at?Show answer
All temperatures, from the surface.
2.Boiling takes place at?Show answer
A fixed temperature, throughout the liquid.
3.Why does evaporation cool?Show answer
Fast particles escape, taking heat.
4.Four factors for faster evaporation?Show answer
Temperature, area, wind, dryness.
5.Does the temperature rise during boiling?Show answer
No, it stays constant.

Evaporation and Boiling: frequently asked questions

What is the difference between evaporation and boiling?

Evaporation occurs at all temperatures, only from the surface, slowly and with cooling. Boiling occurs at a fixed temperature, throughout the liquid, rapidly with bubbles, and needs continuous heating.

Why does evaporation cause cooling?

The fastest particles escape from the surface and take energy with them, so the remaining liquid has a lower average energy and becomes cooler.

What factors affect the rate of evaporation?

Temperature, exposed surface area, wind (moving air) and the dryness of the air: higher temperature, larger area, more wind and drier air all increase the rate.