Q. Why is eau-de-cologne applied to the forehead of a sick person?
A. Eau-de-cologne, which is volatile in nature, will evaporate as soon as it is applied on the forehead of a sick person. During the process of evaporation, some heat from the body will be taken away which results in lowering the temperature of the sick person. Thus eau-de-cologne helps to bring down the body temperature and provides relief to the sick person.
Q. Why do we perspire before rains ?
A. Just before the rain falls, the atmosphere gets saturated with water vapours. The perspiration exuded by us, therefore, does not evaporate quickly, but appears on the surface of the skin. Hence, we feel the perspiration at that time.
Q. Why does a perspiring man feel relief when air floats by his side ?
A. The flow of air increases the rate of evaporation of perspiration from the body. During the process of evaporation, some body heat is taken away thus giving a sense of coolness to the body and providing relief to the perspiring person.
Q. How does the thermos keep the liquid hot for a long time ?
A. In a thermos flask, loss or gain of heat through conduction and convection is reduced to a minimum. It is used for keeping a hot liquid hot and a cold liquid cold for a good length of time.