During a change of state (like melting or boiling), the temperature stays constant because all the heat energy you supply is used to change the arrangement and bonding of particles, not to make them move faster.

Quick Scoop

When a substance changes state at its melting point or boiling point, it absorbs (or releases) a special kind of energy called latent heat. This energy goes into breaking or forming the forces of attraction between particles instead of increasing their kinetic energy.

As long as the phase change is happening and the pressure is constant, the average kinetic energy of the particles does not change, so the temperature remains fixed at that value. Only after the whole substance has finished changing state can extra heat start raising (or lowering) the temperature again.

A simple picture

  • Imagine particles in a solid held together by “spring-like” attractive forces.
  • When you heat the solid to its melting point, further heat is spent stretching and breaking these “springs” so particles can move into the liquid arrangement.
  • Because energy is going into changing structure, not speeding particles up, there is no temperature rise during this stage.

The same idea applies when a liquid boils into a gas or when the reverse changes (condensation, freezing) occur; energy is used or released to make or break intermolecular bonds, while the temperature stays constant at that particular pressure.

In short: during a change of state, heat changes state (particle bonding), not temperature (particle speed), so the thermometer reading stays flat until the phase change is complete.

Information gathered from public forums or data available on the internet and portrayed here.