Revision summary

Internal energy

Total kinetic energy of random particle motion plus total potential energy between particles. Temperature concerns an average; it does not give a sample's total internal energy.

Temperature change

E = CΔT = mcΔT, with C = mc. Use a temperature interval, no change of state, and c treated as constant over the interval. Match kg with J/(kg °C), or g with J/(g °C).

Change of state

E = ml. Use the mass that changes state and the correct fusion or vaporisation value. Lowercase l is energy per mass; the sample's total latent heat L is in J.

Heater input

With constant power, E_input = Pt. It equals energy reaching the sample only when apparatus heating and other transfers are negligible or accounted for.

Follow the particles and the energy

• Melting and boiling take in energy; solidification and condensation transfer it out. For a pure substance at its transition temperature and fixed pressure, average random kinetic energy stays unchanged while particle arrangement and potential energy change.
• Evaporation occurs at the surface and can happen below boiling temperature. Preferential escape of higher-energy particles can cool the liquid if incoming energy does not replace the loss.
• Cooling-curve slopes show falling temperature within a state. A transition plateau contains two states and can involve continuing energy loss.
• For a change involving several states, separate the warming, cooling and state-change stages before choosing equations.

Keep the distinctions clear

Measurement reminders

Identify the sample mass, measure a temperature difference and distinguish heater input from the sample's gain. For melting, measure the mass that changes state and account for background melting. A control needs comparable conditions. For a cooling curve, keep the probe in good contact, choose a useful sampling interval and retain the actual readings. Repetition does not correct poor placement, response lag or a wrong energy account.