Physics · 2 Thermal physics

2.2.3 Melting, boiling and evaporation

Before you start

The particle model of solids, liquids and gases: how the particles are arranged, how far apart they are and how they move (2.1.2).
The temperature of a substance is linked to the average kinetic energy of its particles: hotter means faster-moving particles on average (2.1.2, 2.2.2).
Heating a substance increases its internal energy (2.2.2).
Power is energy transferred per unit time, $P = Δ E / t$ , with energy in joules and time in seconds (1.7.4).
Converting minutes to seconds.

Syllabus objectives covered (8)

Describe melting and boiling in terms of energy input without a change in temperature
Know the melting and boiling temperatures for water at standard atmospheric pressure
Describe condensation and solidification in terms of particles
Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid
Know that evaporation causes cooling of a liquid
Describe the differences between boiling and evaporation
Describe how temperature, surface area and air movement over a surface affect evaporation
Explain the cooling of an object in contact with an evaporating liquid

Part 1 of 7

Melting: energy goes in, the temperature stays still

Start from what you already know: the temperature of a substance tells you the average kinetic energy of its particles. Faster particles on average means a higher temperature.
Heat a solid and at first the energy does what you expect. The particles vibrate faster, their average kinetic energy rises, and the temperature rises.
When the solid reaches its melting point, something changes. The heater is still supplying energy, but the temperature stops rising, and it stays the same until all of the solid has melted.
The energy is not going into kinetic energy. It is being used to work against the attractive forces between the particles, freeing them from their fixed positions so that they can move past each other as a liquid.
Because the average kinetic energy of the particles does not change, the temperature does not change. The energy has still gone into the substance: its internal energy increases even though its temperature does not.
On a graph of temperature against time this shows up as a flat section. A flat section while energy is still being supplied is the sign of a change of state.
graph diagram — not drawn yet
Temperature in °C (vertical axis, −20 to 120) against time (horizontal axis, no values) for ice heated at a steady rate: a rising line from −10 °C to 0 °C, a horizontal section at 0 °C labelled 'melting', a rising line from 0 °C to 100 °C that is less steep than the first line, then a longer horizontal section at 100 °C labelled 'boiling'; the regions labelled 'solid', 'solid + liquid', 'liquid' and 'liquid + gas'.
The two flat sections are changes of state: energy is still going in, but the temperature does not rise.
The objective's own words are the answer to learn: melting is energy input without a change in temperature. Saying only 'the temperature stays the same' describes the graph; saying where the energy goes explains it.

Part 2 of 7

Boiling, and the two temperatures for water

Keep heating the water from the melted ice. Its temperature rises again, because the energy is now increasing the kinetic energy of the particles.
At the boiling point the temperature stops rising a second time. It stays constant until all the liquid has turned to gas. That is the second flat section on the graph.
The reason is the same as for melting. The energy is used to overcome the attractive forces between the particles so that they can move far apart as a gas. It is not increasing their average kinetic energy, so the temperature does not rise.
For pure water at standard atmospheric pressure, the melting point is 0 °C and the boiling point is 100 °C. The paper does not give you these values. You must know them.
Water also freezes at 0 °C. For a pure substance, the melting point and the freezing point are the same temperature. The name only tells you which way the change is going.
The words 'standard atmospheric pressure' are in the objective because the boiling point depends on the pressure above the liquid. Where the air pressure is lower, such as high on a mountain, water boils below 100 °C. You do not need to know by how much.
Always write the unit with the number: 100 °C, not 100.

Part 3 of 7

Running it backwards: condensation and solidification

Condensation is a gas turning into a liquid. Solidification (freezing) is a liquid turning into a solid. Both happen when the substance loses energy to its surroundings.
Condensation, in particles: in a gas the particles are far apart and move quickly. As they lose energy, they slow down. Once they are slow enough, the attractive forces between them pull them close together, and they form a liquid. They are now touching, but they can still move past each other.
Solidification, in particles: the liquid particles lose more energy and slow down further. The attractive forces then hold them in fixed positions, where they can only vibrate. The substance is now a solid.
Notice the direction of the energy. Melting and boiling take energy in. Condensation and solidification give energy out to the surroundings. That is one reason steam condensing on skin can burn it.
When a pure substance cools, these changes also happen at a constant temperature: a pure liquid solidifies at its melting point. As the particles are pulled together, energy is released. This energy replaces the energy that the substance is losing to its surroundings, so the temperature stays flat until the change of state is complete.
graph diagram — not drawn yet
Temperature (vertical axis) against time (horizontal axis) for a pure liquid cooling in air: a curve that falls steeply and then less steeply, a horizontal section labelled 'solidifying' at a temperature marked 'melting point' on the vertical axis, then a falling curve labelled 'solid cooling'.
Cooling has a flat section too: the temperature holds at the melting point while the liquid solidifies.
A particle answer needs three things: the particles lose energy and slow down; they move closer together (condensation) or into fixed positions (solidification); and it is the attractive forces between them that hold them there.

Part 4 of 7

Evaporation: the fastest particles escape, and the liquid cools

In a liquid the particles do not all move at the same speed. At any moment some are slow, most are near the average, and a few move much faster than the average.
A fast particle at the surface, moving outwards, can have enough energy to overcome the attractive forces of the particles around it. It escapes into the air as a gas. That is evaporation.
Two conditions go together: the particle must be one of the more-energetic ones, and it must be at the surface. A fast particle deep inside the liquid has other particles all around it and does not get out.
At any temperature, a few particles have enough energy to escape. So evaporation happens at any temperature, not only at the boiling point. A puddle dries on a cool day without getting near 100 °C.
Now the cooling. The particles that escape are the most energetic ones. The particles that stay behind are, on average, slower.
A lower average kinetic energy means a lower temperature. So evaporation causes cooling of the liquid that is left.
An analogy: if the tallest pupils leave a room, the average height of the pupils who stay goes down, but nobody got shorter. Evaporation lowers the average kinetic energy of the liquid in the same way, by removing the fastest particles.

Part 5 of 7

Boiling and evaporation are different processes

Both processes turn a liquid into a gas, and that is why people confuse them. The differences are about the temperature, the place in the liquid, and the energy.
Temperature. Evaporation happens at any temperature. Boiling happens only at the boiling point, which for water at standard atmospheric pressure is 100 °C.
Where. Evaporation happens only at the surface of the liquid. Boiling happens throughout the liquid: bubbles of vapour form inside it, rise, and burst at the surface. Evaporation makes no bubbles.
Energy and temperature change. Boiling needs a continuous supply of energy, and the temperature stays constant while the liquid boils. Evaporation does not need any heating, and it cools the liquid that is left.
When you write a difference, give both sides of it. 'Boiling happens throughout the liquid' is half of a difference. 'Boiling happens throughout the liquid, but evaporation happens only at the surface' is a complete one.

Part 6 of 7

What makes evaporation faster

Evaporation is particles escaping from the surface. Anything that lets more particles escape each second makes it faster. There are three factors, and each has its own particle reason.
Higher temperature. The particles have a higher average kinetic energy, so more of them have enough energy to escape. Evaporation is faster.
Larger surface area. More particles are at the surface, where escape is possible. Spread the same water over a wider dish and it evaporates faster.
Air movement over the surface. Some escaped particles stay in the air just above the liquid, and some of these go back into it. A draught or wind carries them away before they can return, so more particles leave the liquid than come back.
The reason for the air movement is not that the air gives the particles energy. The moving air removes the vapour from just above the surface. This is why washing dries faster on a windy day.
In a question with a table of results, a comparison shows the effect of one factor only if everything else is the same. Before you compare two dishes, check that they differ in one factor only.

Part 7 of 7

How an evaporating liquid cools the object it touches

The previous section showed that the liquid cools. This section shows that the object in contact with it cools as well. The explanation is a chain of four links. Give every link: an answer with a link missing does not explain the cooling.
Link 1. The more-energetic particles escape from the surface of the liquid.
Link 2. The average kinetic energy of the particles that stay falls, so the temperature of the liquid falls.
Link 3. The liquid is now colder than the object it touches. Thermal energy is transferred from the object to the liquid.
Link 4. The object has lost energy, so its temperature falls. The energy it gives to the liquid lets more particles evaporate, so the cooling continues while there is liquid left.
Sweating works in exactly this way. The sweat on your skin evaporates, the sweat cools, energy flows from your skin into the sweat, and your skin cools.
A clay water pot is a second example. Water seeps through the clay and evaporates from the outside of the pot, so the water inside stays cooler than the air around it.
The factors from the previous section apply here too. A breeze on wet skin cools you faster, because it makes the evaporation faster.

Worked examples

Example 1. A pure substance X is melted in a test tube and then left to cool in the air. Its temperature is recorded every minute from 0 min to 10 min. The readings, in order, are: 82, 75, 69, 64, 61, 61, 61, 61, 61, 56 and 52 °C. (a) Determine the melting point of X. (b) Describe what happens to the particles of X between 4 min and 8 min, and explain why the temperature stays constant although X is still losing energy to the air.

Show the working

Find the flat section first. The readings at 4, 5, 6, 7 and 8 min are all 61 °C, so the temperature is constant from 4 min to 8 min.
A flat section while a substance cools is solidification. A pure substance solidifies at the same temperature at which it melts, so the melting point of X is 61 °C.
Name the states. Before 4 min, X is a liquid that is cooling. After 8 min, X is a solid that is cooling. Between 4 min and 8 min, X is a mixture of liquid and solid while it solidifies.
Particles: X loses energy, so its particles slow down. The attractive forces between the particles then hold them in fixed positions, where they can only vibrate.
Why the line is flat: energy is released as the particles are held in fixed positions. This energy replaces the energy that X loses to the air. So the average kinetic energy of the particles, and so the temperature, does not change until all of X is solid.

Answer: (a) 61 °C. (b) X is solidifying. Its particles lose energy, slow down and become held in fixed positions by the attractive forces between them, where they vibrate. The energy released as the particles are held in place replaces the energy lost to the air, so the temperature stays at 61 °C until all of X is solid.

Example 2. An electric heater of power 120 W is placed in a funnel packed with crushed ice at 0 °C. The heater is switched on for 5.0 minutes. The thermometer in the ice reads 0 °C the whole time, and melted water drips out of the funnel. (a) Calculate the energy supplied by the heater in this time. (b) Explain why the thermometer reading does not rise.

Show the working

Recall the equation. Paper 4 gives you no formula sheet, so this must come from memory: power is energy transferred per unit time.
$P = \frac{Δ E}{t}$
The question asks for energy, so make $Δ E$ the subject: multiply both sides by $t$ .
$Δ E = P t$
Convert the time to seconds before you substitute, because one watt is one joule per second: $5.0$ min $= 5.0 \times 60 = 300$ s.
Substitute and give the unit.
$Δ E = 120 \times 300 = 36 000 J$
If you had used 5.0 instead of 300, you would get 600 J, which is 60 times too small. The time conversion is the step to check.
For (b), say where the energy goes. It is used to overcome the attractive forces between the particles of ice, so that they leave their fixed positions and form a liquid.
Then link this to temperature. The average kinetic energy of the particles does not increase, so the temperature stays at 0 °C while the ice melts.

Answer: (a) 36 000 J (36 kJ). (b) The energy is used to overcome the attractive forces between the ice particles so that the ice melts; it does not increase the average kinetic energy of the particles, so the temperature stays at 0 °C.

Example 3. Two identical wet towels are hung up to dry. Towel A is folded in half and hung indoors, in still air at 18 °C. Towel B is spread out flat on a washing line outdoors, on a windy day at 26 °C. Explain, in terms of particles, why towel B dries faster.

Show the working

Start from what drying is: water evaporates from the towel when more-energetic water particles escape from the surface of the water.
Notice that the towels differ in three ways. This is not a fair test of any one factor, but all three differences make towel B dry faster, so explain each one.
Temperature: towel B is warmer, so its water particles have a higher average kinetic energy and more of them have enough energy to escape.
Surface area: towel B is spread out, not folded, so more of its water is at the surface, where particles can escape.
Air movement: the wind carries escaped water particles away from the surface of towel B before they can return, so fewer go back into the towel.

Answer: Towel B is warmer, so more of its water particles have enough energy to escape; it has a larger surface area, so more particles are at the surface and able to escape; and the wind removes escaped particles from just above the surface, so fewer return. All three make evaporation faster.

Example 4. A runner stops after a race on a breezy day. Her skin is covered in sweat and she soon feels cold. Explain why the evaporation of the sweat cools her skin.

Show the working

Link 1: the more-energetic particles of the sweat escape from its surface.
Link 2: the particles that stay in the sweat have a lower average kinetic energy, so the temperature of the sweat falls.
Link 3: the sweat is now colder than her skin, so thermal energy is transferred from her skin to the sweat.
Link 4: her skin loses energy, so its temperature falls.
Add the breeze if you have time: moving air carries the escaped particles away, so evaporation is faster and the cooling is greater.

Answer: The more-energetic particles escape from the surface of the sweat, so the average kinetic energy of the remaining particles falls and the sweat cools. The sweat is then colder than the skin, so energy is transferred from the skin to the sweat and the skin cools. The breeze makes the evaporation faster, which increases the cooling.

Where people go wrong

Mistake: Expecting the temperature to keep rising while a substance melts or boils, because the heater is still on.

Instead: The temperature stays constant during a change of state. The energy supplied goes into overcoming the attractive forces between the particles.

Temperature follows the average kinetic energy of the particles. During melting and boiling the energy does not increase the kinetic energy, so the temperature does not change.

Mistake: Thinking that water has to be near 100 °C to evaporate, as if evaporation were slow boiling.

Instead: Evaporation happens at any temperature, and only at the surface. Boiling happens only at the boiling point, and throughout the liquid.

At every temperature a few particles at the surface have enough energy to escape, so evaporation never needs the liquid to reach its boiling point.

Mistake: Saying that an evaporating liquid cools an object because 'the cold' moves into the object.

Instead: The most energetic particles leave, so the liquid cools. Energy is then transferred from the warmer object to the colder liquid, so the object cools.

Cold is not something that moves. Energy moves, and it always moves from the hotter object to the colder one.

Mistake: Explaining that wind speeds up evaporation because it gives the particles more energy.

Instead: Wind removes escaped particles from just above the surface, so fewer of them return to the liquid.

The particle reason for each factor is different. Only temperature works by changing the energy of the particles.

Mistake: Thinking that condensation and solidification need energy to be supplied, like melting and boiling.

Instead: Condensation and solidification release energy to the surroundings.

For a gas to condense or a liquid to solidify, its particles must lose energy and slow down until the attractive forces can hold them together.

Check yourself

State the melting point and the boiling point of pure water at standard atmospheric pressure.
Answer
Melting point 0 °C; boiling point 100 °C.
A pan of water is boiling on a cooker that is still switched on. Explain why the temperature of the water does not rise.
Answer
The energy supplied is used to overcome the attractive forces between the particles so that they can separate and form a gas. The average kinetic energy of the particles does not increase, so the temperature stays at the boiling point.
A 60 W heater keeps a beaker of melting ice at 0 °C for 4.0 minutes. Calculate the energy that the heater supplies in this time.
Answer
$t = 4.0 \times 60 = 240$ s, and $Δ E = P t = 60 \times 240 = 14 400$ J.
Describe, in terms of particles, what happens when liquid water solidifies into ice.
Answer
The particles lose energy and slow down. The attractive forces between them then hold them in fixed positions, where they can only vibrate. Energy is released to the surroundings.
A glass of water at room temperature slowly evaporates. Explain how this can happen far below 100 °C, and why the evaporation cools the water that is left.
Answer
Some particles at the surface are more energetic than the average and have enough energy to escape; this happens at any temperature. The particles that escape are the most energetic ones, so the average kinetic energy of the remaining particles falls, and so does the temperature of the water.
Give three differences between boiling and evaporation.
Answer
Evaporation happens at any temperature, boiling only at the boiling point. Evaporation happens only at the surface, boiling throughout the liquid with bubbles of vapour. Evaporation cools the liquid, but during boiling the temperature stays constant while energy is supplied.
A fan blows air across the surface of a dish of water. State and explain the effect on the rate of evaporation.
Answer
The rate of evaporation increases. The moving air carries escaped water particles away from just above the surface, so fewer of them return to the liquid.
Water seeps through the walls of a clay pot and evaporates from its outer surface. Explain why the water inside the pot stays cooler than the air around it.
Answer
The more-energetic particles escape from the water on the outside of the pot, so that water cools. It is then colder than the pot and the water inside, so energy is transferred from the pot and the water inside to the evaporating water. The water inside loses energy and cools.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student puts crushed ice at −10 °C into a beaker with a thermometer and an electric heater of power 50 W. She stirs the contents and records the temperature at regular intervals until the water has been boiling for several minutes. The air pressure in the room is standard atmospheric pressure.

(a) State the two temperatures at which the reading on the thermometer stops rising for a time. 2 marks No calculator

Mark scheme

B1 0 °C (while the ice melts)
B1 100 °C (while the water boils)

(b) While the ice is melting, the heater is still switched on but the temperature does not rise. Explain why. 2 marks No calculator

Mark scheme

B1 the energy is used to overcome the attractive forces between the particles / to separate the particles / to break the bonds between particles / to increase the potential energy of the particles, owtte
B1 the (average) kinetic energy of the particles does not increase, so the temperature stays constant

(c) The reading stays at the lower of the two temperatures for 6.0 minutes. Calculate the energy supplied by the heater during this time. 2 marks

Mark scheme

C1 ΔE = Pt in any form (e.g. P = ΔE/t) or 50 × 360 seen
A1 18 000 J (accept 18 kJ); unit required

Question 2. A student puts the same mass of water at 20 °C into each of three dishes and leaves them in a room at 20 °C for 24 hours. Dish A has a surface area of 40 cm² and is in still air; it loses 5.8 g of water. Dish B has a surface area of 80 cm² and is in still air; it loses 11.4 g. Dish C has a surface area of 80 cm² and a fan blows air across it; it loses 19.6 g.

(a) Some of the water vapour in the room condenses on a cold window. Describe, in terms of particles, what happens as the water vapour condenses. 2 marks No calculator

Mark scheme

B1 the water particles lose energy / slow down (at the cold window)
B1 the particles move closer together and are held by attractive forces, forming a liquid

(b) Explain, in terms of particles, why dish B loses more water than dish A. 2 marks No calculator

Mark scheme

B1 dish B has a larger surface area (and the other conditions are the same)
B1 so more particles are at the surface, where they can escape

(c) Explain, in terms of particles, why dish C loses more water than dish B. 2 marks No calculator

Mark scheme

B1 the moving air removes escaped water particles / water vapour from above the surface
B1 so fewer particles return to the water and more leave than come back, owtte

Question 3. Two thermometers hang side by side in a room. The bulb of one is wrapped in cotton wool soaked in water at room temperature; the bulb of the other is dry. After a few minutes, the wet thermometer reads a lower temperature than the dry one.

(a) The water in the cotton wool evaporates. Describe three differences between evaporation and boiling. 3 marks No calculator

Mark scheme

B1 evaporation happens at any temperature, but boiling happens only at the boiling point
B1 evaporation happens only at the surface, but boiling happens throughout the liquid / boiling forms bubbles of vapour inside the liquid
B1 evaporation cools the liquid, but during boiling the temperature stays constant / boiling needs a continuous energy supply but evaporation does not; a statement about one process only is insufficient

(b) Explain why the wet thermometer reads a lower temperature than the dry thermometer. 3 marks No calculator

Mark scheme

B1 the more-energetic water particles escape from the surface (evaporate)
B1 the average kinetic energy of the remaining water particles falls, so the temperature of the water falls
B1 energy is transferred from the thermometer bulb to the colder water, so the bulb cools

(c) A fan is switched on and blows air across both thermometers. Predict what happens to the reading on the wet thermometer, and give a reason. 2 marks No calculator

Mark scheme

B1 the reading falls further / decreases
B1 the moving air removes escaped particles so evaporation is faster, giving more cooling

How this is examined

Why an object cools when a liquid evaporates from it: a three-mark Explain about a wet object, such as sweat on skin or a wet cloth on a thermometer bulb Three links, in order: the more-energetic particles escape from the surface; the average kinetic energy of the remaining particles falls, so the liquid's temperature falls; energy is transferred from the object to the colder liquid, so the object cools. 'The liquid takes heat away' with no particles in it does not give the chain.
The differences between boiling and evaporation: a Describe or State question worth up to three marks, one mark for each difference Each difference stated for both processes: any temperature against only the boiling point; surface only against throughout the liquid (with bubbles of vapour); cools the liquid against constant temperature while energy is supplied.
Where the energy goes when the temperature stays constant during melting or boiling: a two-mark Explain set against a heating curve or a table of temperatures, which can be paired with a $Δ E = P t$ calculation for the flat section Energy is used to overcome the attractive forces between the particles (to separate them); the average kinetic energy of the particles does not increase, so the temperature does not rise. In a calculation: time in seconds, $Δ E = P t$ recalled, and the answer in J.
How temperature, surface area and air movement change the rate of evaporation, each with its own particle reason: a two-mark Explain for each factor, set in a context such as dishes of water or washing drying in different conditions Temperature: more particles have enough energy to escape. Surface area: more particles are at the surface. Air movement: escaped particles are carried away from above the surface, so fewer return.
Condensation and solidification described with particles: a two-mark Describe 'in terms of particles' The particles lose energy and slow down; the attractive forces pull them close together (condensation) or hold them in fixed positions where they vibrate (solidification).
The melting and boiling temperatures of water: a one- or two-mark State, or a value you read off a heating curve 0 °C and 100 °C, each with the unit, for pure water at standard atmospheric pressure.

Words to know

Melting point: The temperature at which a pure solid melts. A pure liquid solidifies at the same temperature. For water at standard atmospheric pressure it is 0 °C.
Boiling point: The temperature at which a liquid boils. It depends on the pressure above the liquid. For water at standard atmospheric pressure it is 100 °C.
Standard atmospheric pressure: The normal pressure of the atmosphere used as a fixed reference, close to the average air pressure at sea level. The melting and boiling points of water are quoted at this pressure.
Evaporation: The escape of more-energetic particles from the surface of a liquid. It happens at any temperature and cools the liquid that is left.
Boiling: The change from liquid to gas throughout the liquid, at the boiling point, with bubbles of vapour forming inside it, while energy is supplied.
Condensation: The change from gas to liquid. The particles lose energy, slow down and are pulled close together by attractive forces.
Solidification: The change from liquid to solid (freezing). The particles lose energy, slow down and are held in fixed positions by attractive forces.
Internal energy: The total kinetic and potential energy of the particles in a substance. It increases when the substance is heated, including during melting and boiling, when the temperature does not change.

All Physics lessons Next: 2.3.1 Conduction