Melting point and heat of fusion
When heat is applied to ice (solid), the temperature rises gradually. By the way, while the ice is melting, the temperature does not rise. Temperature is kept constant, and the temperature at this time is called the melting point of ice.
At the melting point, even if heat energy is supplied, the temperature is kept constant. This is because all of the supplied thermal energy is used to change the state of a solid into a liquid. The heat energy absorbed by a substance when it changes state from solid to liquid is called 'heat of fusion.'
Boiling point and heat of vaporization
Also, when the heat is applied to water (liquid), the temperature gradually rises. By the way, while the liquid is boiling, the temperature does not rise. The temperature at this time is called the boiling point.
At the boiling point, even if heat energy is supplied, the temperature is kept constant. This is because all of the supplied thermal energy is used to change the state of the liquid into a gas. The heat energy absorbed by a substance when it changes state from liquid to gas is called 'vaporization heat.'