If a change of state happens: The energy needed for a substance to change…

The idea

Internal energy is the total kinetic energy plus the total potential energy of the particles. During melting or boiling the energy supplied does not speed the particles up; it pulls them apart against the forces holding them together, so the potential energy store rises while the kinetic energy store, and therefore the temperature, stays the same. That is why a heating graph shows a flat section at the melting point and another at the boiling point even though the heater is still running. Internal energy is increasing the whole time. Reverse the process and the same energy is given out: condensing steam releases a great deal of energy at a constant 100 °C, which is why steam scalds far more severely than boiling water.

What earns the marks

• Stating that the potential energy of the particles increases as the forces between them are overcome.
• Stating that the kinetic energy of the particles does not change, because the temperature does not change.
• Concluding that the internal energy of the substance increases.
• Identifying a flat section of a heating or cooling graph as a change of state.

Common mistakes and how to put them right

• Saying the temperature keeps rising while a liquid boils.

• Saying the energy supplied during a change of state is lost, wasted or destroyed.

• Claiming the kinetic energy of the particles increases during melting.

• Reading the flat part of a heating graph as a period when no energy is being supplied.

Exam tips

• Split any internal energy answer into two sentences, one about potential energy and one about kinetic energy, then state the effect on internal energy.
• If a graph question mentions a plateau, name the change of state and the temperature at which it happens.
• Do not say heat when you mean internal energy; examiners expect the phrase internal energy in these answers.