Change Of State

Decreasing Temperature By Cooling

(i) Change of state from liquid to solid (solidification): When water is cooled (by lowering its temperature) it gets changed into solid ‘ice’. The process of changing a liquid into a solid by cooling is called “freezing”. When the temperature is lowered particles of the matter lose energy due to which they move slowly. If we continue to lower the temperature then the particles of substance stops moving and vibrates around their fixed position. At this point the liquid freezes and gets converted into solid. 
                                     \( Liquid\xrightarrow{{Cool}}Solid \)     
Freezing is the reverse of melting. So the freezing point of a liquid is the same as the melting point of its solid form.

(ii) Change of state from gas to liquid (condensation): When the temperature of gaseous state of matter is lowered, it is converted into liquid state. So, the process of changing a gas (or vapour) to a liquid by cooling, is called condensation. At which temperature a gas turns to liquid state is called as condensation point.
For example, when temperature of water vapour is lowered it gets converted into liquid water.

Explanation: when the temperature is lowered then the particle of gaseous state lose energy and their movement slow down, because of this they move closer together until they start being attracted to each other and form a liquid.
                                      \( Gas\xrightarrow{{Cool}}Liquid \)
Condensation is the reverse of vaporization.
Conclusion: From the above discussion, we led to conclude that one state of matter can be changed into another or vice-versa by changing the temperature.
                                           \( SolidState\underset{{Cool}}{\overset{{Heat}}{\longleftrightarrow}}LiquidState\underset{{Cool}}{\overset{{Heat}}{\longleftrightarrow}}GaseousState \)
Imp Note: Melting point of ice is same as the freezing point of water. It is O⁰C or 273K under one atmospheric pressure. In other words, at O⁰C both ice and water exist together. But particles in water have more energy as compared to particles in ice at same temperature i.e. at 0⁰C. This is because during the change of state from ice to water, heat energy equal to latent heat of fusion has been absorbed.
The particles of steam have higher energy than the particles of liquid water at same temperature i.e. at 100⁰C. This is again because, during change of state from liquid water to steam or vapours, heat energy equal to latent heat of vaporization has been absorbed.