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When we apply the heat energy to the substance, the molecule's kinetic energy increases due to the supplied energy, increasing its temperature.
But in some cases, it won't happen, 
Water boiling and evaporating into water vapour (steam) when heated on the stove.
In this example, - Energy is needed to vaporise a liquid because in so doing, the molecules are separated, and attractive molecular forces are to be overcome.
The applied energy is used only to separate the molecules; no part increases their kinetic energy. There is no temperature change until a phase change is complete.
Changes of Phase:
The term change of phase means the same thing as the term change of state. The change of phase always occurs with a change of heat. If heat energy is supplied to or taken out from a substance, it will undergo a change from one state of matter to another. However, the temperature does not change.
During a change in state, the heat energy is used to change the bonding between the molecules.
There are four states of matter in the universe - plasma, gas, liquid, and solid. But, matter on Earth exists mainly in three distinct phases - gas, liquid, and solid.
A phase is a distinctive form of a substance, and matters can change among the phases. It may take extreme temperature, pressure, or energy, but all matter can be changed.
One of the following transformations may take place,
  • Solid to Liquid
  • Liquid to Gas
  • Solid to Gas
  • Gas to Liquid
  • Liquid to Solid
  • Gas to Solid
Description of phase change 
Term for phase change 
Heat movement during phase change
 Temperature change during phase change
Solid to Liquid
Heat goes into the solid as it melts. 
Liquid to Gas
Vaporisation, which includes boiling and evaporation
Heat goes into the liquid as it vaporises. 
Solid to Gas
Heat goes into the solid as it sublimates.
Gas to Liquid
Heat leaves the gas as it condenses.
Liquid to Solid
Heat leaves the liquid as it freezes.
Gas to Solid
Heat leaves the gas as it freezes


Water is the only matter on Earth that is found naturally in all three states - solid, liquid, and gas.