Fig. 27.1 shows an experiment to demonstrate evaporation. A thin layer of volatile liquid (ether) is placed on the surface of a glass slide resting on a sma...

(a) Explain why the ether evaporates when air is blown across it. [2]

The moving air sweeps away ether vapour from above the liquid surface, preventing saturated vapour from building up and stopping molecules from returning to the liquid. [1] At the surface, the fastest-moving (most energetic) molecules have enough kinetic energy to overcome the attractive forces of neighbouring molecules and escape into the air. This process is evaporation. [1]

(b) Explain why the water underneath the glass slide freezes. [3]

This is a three-step process:

1. As the ether evaporates, it is the most energetic molecules that escape from the liquid surface. [1]
2. The molecules left behind have a lower average kinetic energy, so the temperature of the remaining ether drops significantly. [1]
3. The cold ether absorbs thermal energy from the water below it (conducted through the glass slide). The water loses enough energy to cool below 0 °C and freeze. [1]

Ether is volatile (has a low boiling point), so it evaporates rapidly and produces a dramatic cooling effect sufficient to freeze water even though the room is at 22 °C.

(c) State two factors that affect the rate of evaporation of a liquid. [2]

1. Temperature of the liquid: a higher temperature gives more molecules enough energy to escape, increasing the rate of evaporation. [1]
2. Air movement across the surface (or surface area, or humidity above the surface): moving air removes vapour and prevents saturation; a larger surface area exposes more molecules; lower humidity means fewer vapour molecules returning to the liquid. [1]

(d) Explain why a person feels cold when they step out of a swimming pool on a windy day. [2]

Water on the skin evaporates. The most energetic water molecules escape from the skin surface, reducing the average kinetic energy of the remaining water and the skin beneath it. This lowers the skin temperature, producing the sensation of cold. [1] Wind increases the rate of evaporation by blowing away the water vapour from above the skin, preventing the air near the skin from becoming saturated. Faster evaporation means faster cooling, making the person feel even colder. [1]