Question

Sodium chloride and calcium chloride are used to clear snow from the roads. Why?

Answer 1

To answer the question we need to understand the concept of depression of freezing point. Depression in freezing point is a Colligative property i.e. depends on the amount of solute. Freezing point depression is the lowering of the freezing point when the solute is added to the solvent. When a solute is added to solvent the vapor pressure of solvent will decrease and the equilibrium between solid and liquid which is attained at the freezing point will now be achieved at a lower temperature.

Complete answer: The property of freezing point depression is highly beneficial in preventing snow deposits on the roads in cold countries. This helps prevent drivers from slipping. Since this lower freezing point will cause melting of ice at lower temperature and hence it will melt and water flows off easily and hence can be removed from the roads.
Sodium chloride depress the freezing point of water to such an extent that it cannot freeze to form ice. Therefore, it melts off easily at the prevailing temperature.
So we can say that adding these salts does not melt the ice rather lowers the freezing point.
The freezing point depression is expressed as-
$\Delta {T_f} = i{K_f}m$
Where,
$\Delta {T_f}$ is the depression in freezing point
$i$ is the van’t Hoff factor (It is the measure of the effect of solute on Colligative properties)
$m$ is the molality of the solution
${K_f}$ is the freezing point depression constant
Since $CaC{l_2}(i = 3)$ dissociates into 3 ions as compared to $NaCl(i = 2)$ which dissociates into 2 ions only the depression caused by Calcium chloride will be greater and hence mostly preferred over Sodium chloride. The Reaction with Calcium chloride is highly exothermic.

Note:
Normally we use Sodium and Calcium salts due to their high availability and low cost. These have been used for ages. Other than Sodium and Calcium chloride we can also use some other chemicals for melting like Magnesium chloride, Potassium Acetate, Calcium Magnesium Acetate, Urea, Potassium Acetate, etc.