Question

Question #387ac

Answer 1

See the discussion that follows.

Explanation:

Evaporation occurs when molecules in a liquid achieve two criteria:

• They are located at the surface of the liquid
• The acquire sufficient kinetic energy (through random collisions) to break from of the intermolecular forces at the surface, and enter the gas phase.

This results in an increase in the number of molecules in the space above the liquid, and builds a vapour pressure in that space.

This model explains a great many features regarding evaporation and the behaviour of liquids and solutions, such as

• why evaporation produces a significant cooling effect (the process removes the highest KE molecule from the liquid, meaning the average temperature is reduced)
• why vapour pressure increases with temperature (higher average KE means a greater % of molecules at the surface have "enough" KE to escape the attractions of the other molecules)
• why solutions have lower vapour pressures than pure liquids (solute particles occupy space at the surface of the solution, reducing the number of molecules that achieve the first criterion above)