Any state of a substance (solid/ liquid/ gas) can be changed into another by heating or cooling. The transition of a substance from one state to another is called a change of state.
Some common changes of states :
(i) Melting :
When heat is supplied, solid substance changes into liquid, this change of state of substance is called melting.
The temperature at which the solid and the liquid states of a substance coexist in thermal equilibrium with each other is called its melting point.
(ii) Freezing :
When heat is released, liquid changes into solid, this change of state of substance is called freezing.
(iii) Condensation :
When vapour is cooled, it changes into liquid, this change of state is called condensation
(iv) Evaporation :
Conversion of liquid into gaseous state at all the temperatures is called evaporation or boiling.
The temperature at which the liquid and vapour states of a substance coexsist in thermal equilibrium with each other is called its
boiling point.
It is a phenomenon that occurs at the surface of liquids. The rate of evaporation increases with the rise in temperature. The heat required to change a unit mass of liquid into vapor at a given temperature is called heat of evaporation at that temperature.
(v) Sublimation :
It is the conversion of a solid directly into vapors.
Sublimation takes place when boiling point is less than the melting point.