O-Level Excretion in Humans: worked solution

Question

Daniel, whose body mass is normally about 65 kg, spent 3 hours in a hot desert environment and lost some body mass. Most of this lost mass was water. Explain how losing this mass helped Daniel cope with the hot conditions.

In the hot desert Daniel sweats. The water in sweat lies on the skin surface and evaporates. Evaporation of water requires heat energy (latent heat of vaporisation), which is taken from the body/skin. Removing this heat lowers his body temperature, so the mass lost (mainly water) helped him stay cool and avoid overheating.