When the temperature of the gas is lowered, both the volume of the gas and the kinetic energy of the molecules decrease. The molecular motion becomes slow and molecules become sluggish. The progressive decrease of temperature brings the molecules closer and closer because they are unable to resist the attractive force that starts operating between them. Ultimately, at sufficiently low temperature, the voids between the molecules become less than 10-5cm and the gas changes into liquid state.
This process of liquefaction by bringing gas molecules closer can also be achieved by increasing the pressure of the gas: this also decreases the volume of the gas. For example, sulphur dioxide can be liquefied at 265 K if pressure is 760 mm of Hg. It can also be liquefied at 293K if the pressure is increased to 2470 mm of Hg.
Thus, liquefaction of gases can be achieved by either decrease of temperature or by increase of pressure.
The phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.
- Evaporation requires energy. A liquid draws heat energy from the surrounding thereby cooling the surrounding.
Water placed in a porous pot becomes very cool after some time. This is because water molecules draw energy from the water itself for evaporation and hence, the temperature of water in the post falls.The process, by which a soluble solid can be obtained from a solution by allowing the solvent to vaporize, is called evaporation.