Heating Curve Model

Heater and sample

Heater power P Mass m

c = ΔE / (mΔθ)

Particle colour shows speed: slower, faster.

Not to scale. The particles are drawn roughly ten million times bigger than real ones, and their motion is slowed down enormously. On the graph, the two flat sections are drawn the same length to show the shape: their lengths are illustrative and do not show how long melting or boiling really takes. The time axis is rescaled to fit each setting of the sliders, so compare the rate of temperature rise Δθ / t shown in °C/s. Substance X uses made-up values.

Boiling and evaporation compared

	Evaporation	Boiling
Temperature	any temperature	only at the boiling point
Where	only at the surface	all through the liquid
Bubbles	no	yes, bubbles of vapour
Energy	the liquid cools unless energy is supplied	energy must be supplied; the temperature stays constant

How to write it in the exam

When a solid or a liquid is heated and does not change state, its temperature rises and its internal energy increases. The average kinetic energy of its particles increases.
During melting and boiling, energy is supplied but the temperature stays constant until the change of state is complete.
The energy is used to overcome the forces of attraction between the particles. It does not increase their kinetic energy.
At standard atmospheric pressure, pure water melts at 0 °C and boils at 100 °C.
Specific heat capacity is the energy needed per unit mass per unit rise in temperature: c = ΔE / (mΔθ), in J/(kg °C).
Evaporation is the escape of the more-energetic particles from the surface of a liquid. It happens at any temperature.
The particles left behind have a lower average kinetic energy, so the temperature of the liquid falls: evaporation causes cooling.