Skip to content

Course home

Sign up

14.2.1 Heating, temperature and changes of state

Heating transfers energy

Definition

Heating

Heating is the transfer of energy from a region at a higher temperature to a region at a lower temperature.

Definition

Internal energy

Internal energy is the total kinetic energy and potential energy of all the particles in a system.

  1. A system is the object or group of objects being studied, such as the water inside a kettle.
  2. Heating is an energy transfer pathway and not an energy store, so an object never contains heat.
  3. Energy transferred into a system by heating increases the internal energy of that system.
  4. Internal energy has two parts: the kinetic energy of the moving particles and the potential energy associated with the forces between them.
  5. Increasing the internal energy of a system produces one of two outcomes: a rise in temperature or a change of state.
  6. Which outcome happens depends on whether the substance is below a change of state temperature or already at one.
  7. Energy is conserved, so the energy supplied by a heater equals the energy gained by the system plus the energy transferred wastefully to the surroundings.
  8. Cooling is the same process running backwards, so a cooling system loses internal energy to its surroundings.

Raising the temperature

Definition

Temperature

Temperature is a measure of the average kinetic energy of the particles in a substance.

  1. When a substance is heated and stays in the same state, the transferred energy increases the average kinetic energy of its particles.
  2. Particles in a solid vibrate more rapidly about their fixed positions.
  3. Particles in a liquid or a gas move more rapidly from place to place.
  4. Because temperature is a measure of the average kinetic energy of the particles, the temperature of the substance rises.
  5. The arrangement of the particles does not change during this stage, so the substance stays in the same state throughout.
  6. Temperature is not the same as internal energy, because a small mass at a high temperature can hold less internal energy than a large mass at a low temperature.
  7. Cooling a substance in a single state slows its particles down, so the average kinetic energy and the temperature both fall.

A diagram showing how particle arrangement, motion, and interactions change with temperature across the three states of matter. As temperature increases, molecular motion increases and molecular interactions decrease.

Producing a change of state

Definition

Change of state

A change of state is a physical change in which a substance changes between the solid, liquid and gas states without becoming a different substance.

Definition

Melting point

The melting point is the temperature at which a solid changes into a liquid, and at which the liquid freezes back into a solid.

  1. Once a substance has reached its melting point or its boiling point, continued heating produces a change of state instead of a temperature rise.
  2. The transferred energy is used to break up the existing arrangement of the particles and to overcome the forces of attraction holding them to their neighbours.
  3. This increases the potential energy part of the internal energy, so the internal energy of the system still rises.
  4. The average kinetic energy of the particles does not increase during the change, so the temperature stays constant.
  5. The temperature only begins to rise again after the last particle has changed state.
  6. A mixture of ice and water therefore stays at 0 ∘C0\,^\circ\text{C}0∘C while any ice remains, however hard it is heated.
  7. Melting and boiling need energy to be transferred into the substance, so they cannot happen without a supply of energy.

Cooling and releasing energy

  1. Freezing and condensing transfer energy out of the substance and into the surroundings.
  2. The temperature stays constant while a substance freezes or condenses, for the same reason that it stays constant while it melts or boils.
  3. The energy released when a given mass condenses is equal to the energy needed to boil the same mass.
  4. Steam at 100 ∘C100\,^\circ\text{C}100∘C therefore scalds far more severely than liquid water at 100 ∘C100\,^\circ\text{C}100∘C, because it releases a large amount of energy as it condenses on the skin.
  5. Evaporation works the other way and takes energy from its surroundings, which is why sweat cools the skin as it dries.
  6. A substance in a fridge freezer keeps losing internal energy while it freezes, even though a thermometer in it shows no change.
Example

Heating ice through to steam

  • Ice at −10 ∘C-10\,^\circ\text{C}−10∘C is heated steadily in an insulated container.
  • The molecules vibrate faster about their fixed positions, their average kinetic energy rises, and the temperature climbs to 0 ∘C0\,^\circ\text{C}0∘C.
  • At 0 ∘C0\,^\circ\text{C}0∘C the energy now separates the molecules from their fixed positions and overcomes the forces of attraction between them.
  • The average kinetic energy does not increase during this stage, so the thermometer holds at 0 ∘C0\,^\circ\text{C}0∘C until the last of the ice has melted.
  • Further heating then raises the average kinetic energy of the water molecules, so the temperature rises from 0 ∘C0\,^\circ\text{C}0∘C towards 100 ∘C100\,^\circ\text{C}100∘C.
  • At 100 ∘C100\,^\circ\text{C}100∘C the temperature stops rising again, because the energy is being used to pull the molecules apart into a gas.
  • The internal energy of the water has increased at every stage, including the two stages where the temperature did not change.
Exam technique

Writing a heating explanation

  • Say that energy is transferred by heating and that the internal energy of the system increases, because that opening statement is usually worth a mark on its own.
  • Use the words average kinetic energy when the temperature changes, since the mark scheme looks for that phrase rather than the word speed alone.
  • Use the words arrangement and forces of attraction when the state changes.
  • Add the consequence in the same sentence, so a rise in average kinetic energy is followed by so the temperature rises.
  • Deal with each stage separately when a question covers both a temperature rise and a change of state, because each stage carries its own marks.
  • State that internal energy still increases during a change of state, which is the point that separates a top answer from a middling one.
Common Mistake
  • Do not write that an object contains heat, because heating is a way of transferring energy and not a store.
  • Do not assume that heating always raises temperature, because the temperature holds steady throughout a change of state.
  • Do not write that the internal energy is constant while ice melts, because it rises even though the temperature does not.
  • Do not use temperature and internal energy as though they mean the same quantity, because internal energy also depends on the mass and the state of the substance.
  • Do not write that particles gain kinetic energy while a substance boils, because the extra energy goes into separating them instead.
Self review
  • State the two possible effects of heating a system.
  • Name the two parts that make up the internal energy of a system.
  • Explain why the temperature stays constant while ice is melting.
  • Explain what happens to the particles of a solid when its temperature rises.
  • Explain why steam causes a worse scald than boiling water at the same temperature.
  • State why heating is not an energy store.
PreviousNext

How was this guide?

Teach Genie

Review 14.2.1 Heating, temperature and changes of state by teaching Genie

Teach it back in your own words, spot gaps, and remember it better.

Start teaching
Genie and Baby Genie

14.2.1 Heating, temperature and changes of state Revision Guide

  1. GCSE
  2. /Physics
  3. /14.2.1 Heating, temperature and changes of state

Revision notes for Edexcel GCSE Physics 14.2.1 Heating, temperature and changes of state: explanations and worked examples.

Revision guides