🌡️ The temperature rises
The energy increases the kinetic energy of the particles. They vibrate or move faster, so the temperature of the substance goes up.
Change of State » Heating and Changes of State
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 5.8P, 5.9P
- Why heating a system changes the energy stored within it
- When heating raises the temperature and when it changes the state
- What the particles do when a solid melts
- How evaporation and boiling are different
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Unlock This CourseHeating changes the energy stored in a system
This lesson is examined in Paper 2 only.
When you heat something, energy is transferred to it. The energy does not vanish. It is stored in the particles of the object, so the energy stored within the system goes up. Remember from the lesson Particles in Solids, Liquids and Gases that the particles in a solid vibrate, in a liquid slide past each other and in a gas fly around freely.
The extra energy can do one of two things. Which one happens depends on the situation.
Key terms:
- Kinetic energy: the energy a particle has because it is moving.
- Melting: a solid changing into a liquid.
- Melting point: the temperature at which a substance melts.
- Evaporation: a liquid changing into a gas at the surface of the liquid.
- Boiling: a liquid changing into a gas throughout the liquid.
- Boiling point: the temperature at which a liquid boils.
Two things heating can do
❄️ The state changes
The energy is used to overcome the forces between the particles, pulling them away from each other. The particles are not speeding up, so the temperature stays constant while the state changes.
A good way to remember it: the energy either makes the particles move faster or it pulls them apart. While a substance is melting or boiling, the energy goes into overcoming the forces, so the temperature stays still until the change is finished.
Melting: solid to liquid
Here is what happens when a solid is heated until it melts.
- Energy is transferred to the solid. The particles vibrate more and more, and the temperature rises.
- The temperature reaches the melting point. Now the energy overcomes the forces holding the particles in their fixed positions.
- The regular pattern breaks down. The particles are free to move around each other, and the solid becomes a liquid.
- The temperature stays at the melting point until all of the solid has melted. After that, more heating raises the temperature of the liquid.
A pure substance melts at one particular temperature. For example, pure ice melts at 0°C. Each pure substance has its own melting point.
Evaporating and boiling: liquid to gas
A liquid can turn into a gas in two different ways. Both happen when particles escape from the liquid, but the details are different.
| Evaporation | Boiling | |
|---|---|---|
| Where does it happen? | Only at the surface of the liquid | Throughout the liquid, with bubbles of gas forming inside it |
| What temperature? | Any temperature below the boiling point | Only at the boiling point |
| Which particles escape? | The fastest particles near the surface | Particles throughout the liquid gain enough energy to break away from each other |
| What happens to the liquid? | The liquid left behind gets cooler | The temperature stays constant at the boiling point |
Why does evaporation cool the liquid? The particles in a liquid move at different speeds. The fastest ones have enough kinetic energy to overcome the forces holding them in the liquid and escape from the surface. The particles left behind are slower on average, so the liquid becomes cooler. This is why wet clothes feel cold, and why sweating helps to cool you down.
When a liquid boils, energy is transferred to it continuously. This energy is used to overcome the forces between the particles, so the temperature stays at the boiling point until all of the liquid has become a gas. Water boils at 100°C in normal conditions.
Worked example
Describe what happens to the particles when a block of ice at −5°C is heated until it has become water at 20°C.
Stage 1: the ice warms to 0°C. The energy increases the kinetic energy of the particles, so they vibrate faster.
Stage 2: the ice melts at 0°C. The energy overcomes the forces between the particles, so they break out of the regular pattern. The temperature stays at 0°C.
Stage 3: the water warms from 0°C to 20°C. The energy increases the kinetic energy of the particles again, so they move faster.
Common mistakes
- Saying the temperature keeps rising while a substance melts or boils. It stays constant, because the energy is overcoming the forces between particles.
- Saying the particles themselves melt or get bigger. The particles stay the same. Only their arrangement and motion change.
- Saying evaporation only happens at 100°C. It happens at any temperature, but only at the surface.
- Mixing up the two changes. Boiling happens throughout the liquid, evaporation only at the surface.
Exam-style question
A pan of water is heated on a cooker until it boils.
(a) Explain why the temperature of the water rises before it boils. [2 marks]
(b) Explain why the temperature stays constant while the water is boiling. [2 marks]
(c) State one difference between boiling and evaporation. [1 mark]
Model answer
(a) Energy is transferred to the water (1). This increases the kinetic energy of the particles, so the temperature rises (1).
(b) The energy is used to overcome the forces between the particles (1), not to make them move faster, so the temperature stays constant (1).
(c) Boiling happens throughout the liquid but evaporation happens only at the surface (1). (Also accept: boiling happens only at the boiling point but evaporation happens at any temperature.)
Exam tip
In a "constant temperature" question, always say where the energy goes: it overcomes the forces between the particles. Saying only "it is changing state" is not enough.