Question

What factors affect the vapour pressure of a liquid?

Answer 1

Hint: As we know that vapor pressure (also known as equilibrium vapor pressure) is referred to the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a particular temperature in a closed system. It is an indication of the evaporation rate of liquids. So here we have to explain the factors that affect the vapour
pressure.

Complete answer:
We know that vapour pressure is a measure of the tendency of a material to change into the gaseous or vapour state from liquid state. Here are the main factors that affect the vapour pressure of a liquid:-
(A) Nature of liquid: If the molecules of liquid show strong interaction with each other, then there will be less tendency for the molecules to escape as gas due to which vapor pressure will get lowered. Let say for an example: polar molecules that can form hydrogen bonds between themselves such as water and alcohols have relatively low vapour pressures.
 In other cases where there is only weak interaction between the liquid molecules then there will be a greater tendency for the molecules to evaporate and thus have a higher vapor pressure.
(B) Temperature: It also affects the vapor pressure. If the system in equilibrium is disturbed by raising the temperature, then according to Le Chatelier’s principle the system will tend to react in favour of relieving this stress. As the temperature is increased, the evaporation process (that absorbs heat) speeds up to a greater degree than the condensation process (that releases heat), as a result of which the vapor pressure is higher when equilibrium is restored at the new temperature.
If the temperature is increased enough to raise the vapor pressure till it becomes equal to the atmospheric pressure, the liquid will boil. Hence we can say that with the increase in temperature, vapour pressure increases as well.

Note:
-Another minor factor that affects vapor pressure is the presence of dissolved substances in the liquid or solid as according to Raoult's law, “the vapor pressure of a pure liquid or solid is usually lowered by the addition of a solute”.