Energy transfer, temperature and phase changes

What you will learn

Energy input causing temperature change or state change; particle-level explanation and heating curves.

Specification mapping: C4.3

Prerequisites: C4.2

Physical principles

Internal energy includes the random kinetic energy of particles and the potential energy associated with their interactions. Heating transfers energy because of a temperature difference. Electrical work can also increase internal energy. Temperature and total internal energy are different: a larger mass can contain more internal energy at the same temperature.

Within a single phase, energy input usually increases temperature. The amount depends on mass and specific heat capacity. During an ideal equilibrium change of state of a pure substance at constant pressure, temperature remains constant while energy changes the arrangement and separation of particles. For melting or boiling, energy increases intermolecular potential energy rather than raising temperature.

A constant-power heating curve can show rising segments and plateaux. The gradient of a rising temperature–time segment depends on power, mass, heat capacity and losses. Plateau duration depends on mass changing phase and latent heat, again with losses and pressure conditions considered. Real mixtures can change state over a range rather than a perfectly flat plateau.

Cooling reverses the energy flow. Freezing and condensation release latent energy to surroundings. Evaporation can occur below boiling temperature; boiling takes place throughout the liquid at the boiling condition for the pressure. Do not confuse breaking intermolecular attractions during boiling with breaking atoms or chemical bonds within molecules.

Scientific diagram and interpretation

Schematic constant-power heating shows rising temperature within a phase and plateaux during melting and boiling at constant pressure. Horizontal widths and temperatures are not numerical measurements.

Worked examples and applications

Interpret a plateau

A heater continues transferring energy while a pure sample melts at steady pressure. A flat temperature trace does not mean zero energy transfer: the energy supports the phase change.

Compare curve slopes

For equal mass, equal heater power and negligible losses, a sample with twice the specific heat capacity warms at half the rate. If losses differ, this simple comparison needs correction.

Common misconceptions

Constant temperature during a phase change can coincide with substantial energy transfer. Boiling does not normally split molecules into atoms.

Examination technique

Identify the phase and process before choosing mcΔT or mL.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.