Correct Answer - Option 3 : cetyl alcohol
(i) By applying a thin chemical film on the water surface, the evaporation can be reduced. Currently the application of the chemical over the water surface is the only feasible method available for reduction of evaporation of reservoirs to moderate extent.
(ii) Certain chemicals such as cetyl alcohol (hexadecanol) and stearyl alcohol (octadecanol) form monomolecular layers on a water surface and these layers act as evaporation inhibitors by preventing the water molecules to escape past them.
(iii) The thin film formed has the following desirable features:
- The film is strong and flexible and does not break easily due to wave action.
- If punctured due to the impact of raindrops or by birds, insects, etc., the film closes back soon after.
- It is pervious to oxygen and carbon dioxide: the water quality is therefore not affected by its presence.
- It is colourless, odourless and nontoxic.
Cetyl alcohol is found to be the most suitable chemical for use as an evaporation inhibitor. It is a whit waxy crystalline solid and is available as lumps, flakes or powder. It can be applied to the water surface in the form of powder, emulsion or solution in mineral turpentine.