Question

Why is ethylene glycol used in car radiators for both winter as well as summer seasons?

Answer 1

When we add a non-volatile solute to a solvent, it displaces some solvent molecules at the surface of the solution and hence reduces the vapour pressure of the solvent. Ethylene glycol is added to water in winter since it acts as antifreeze.

Complete step-by-step answer: In order to solve this question we first need to have an understanding about colligative properties. Let us understand colligative properties and the factors on which they depend.
Colligative properties of solutions come into picture due to the presence of non-volatile solute in a solution. They only depend on the number of the particles of the solute and not on their nature. When we add a non-volatile solute to a solvent, it displaces some solvent molecules at the surface of the solution and hence reduces the vapour pressure of the solvent which is termed as the relative lowering of vapour pressure.
When the vapour pressure of a liquid becomes equal to the atmospheric pressure at a particular temperature, then that temperature is called the boiling point of the liquid. Since the vapour pressure of a liquid gets reduced on the addition of a non-volatile solute, so its temperature needs to be raised to increase the vapour pressure and when the vapour pressure becomes equal to the atmospheric pressure, the liquid will start to boil. Therefore, the elevation in boiling point of a liquid is observed when a non-volatile solute is added to a solvent.
Similarly the freezing point of a substance is the temperature at which the solid and the liquid phase of the substance are at equilibrium i.e. their vapour pressure is the same. Now, in addition to the non-volatile solute, the vapour pressure of the solution will decrease than that of the pure solvent and hence it will freeze at a lower temperature.
Now, although ethylene glycol is volatile, it still has a higher boiling point than that of water and hence has lower vapour pressure than that of water due to which a binary solution of water and ethylene glycol will show elevation in boiling point and depression in freezing point. Also the rate of evaporation for such a solution will be less than that of the pure solvents.

Note: If there is dissociation of the non-volatile solute in the solvent, then the number of particles of the solute will increase. Due to the dependence of the colligative properties on the number of the particles, the van’t Hoff factor is used to account for the new number of particles of the solute.