Question

Which one of the processes is slow?
A ) Evaporation
B ) Boiling
C ) Both have same rate
D ) None of the above

Answer 1

Hint: Slow processes require a long period of time to occur. Slow processes occur on their own without addition of external heat energy. Fast processes are rapid, and require only a short span of time. Fast processes do not occur on their own. They require an external heat source. Consider which process, evaporation or boiling occurs at the room temperature and which process occurs at high temperature.

Complete answer:
> During boiling a liquid changes its phase from liquid to vapor, at the temperature that is equal to boiling point. During evaporation liquid slowly changes its phase from liquid to gas, over a wide range of temperatures between freezing point and boiling point.
> To evaporate one liter of water, several hours (or several days) are needed. However, to boil the same volume of water, only a few minutes (or an hour) are required. This suggests that evaporation is a slow process and boiling is a rapid process.
> Evaporation of liquid occurs at any temperature as long as the temperature is above the freezing point of the liquid. Water has a freezing point of 273 K and boiling point of 373 K. Water can evaporate at any temperature between 273 and 373 K. But water will boil only at 373 K. With increase in temperature, the rate of evaporation of water increases.
> For boiling to occur, an external source of energy (heat) in the form of a burner is required. However, for evaporation, the energy comes from within the liquid. For evaporation, no additional source of heat (such as a burner) is required. Evaporation is the natural process whereas boiling is an artificial process.
Thus, evaporation occurs without external heat supply and is a natural process. It is a slow process.

Hence, the option A ) evaporation is the correct answer.

Note: Boiling is an unnatural process, in which a liquid is heated (by supplying heat with a heat source such as burner), and due to continuous supply of heat, liquid changes its phase from liquid to vapor. The process of boiling occurs only at the temperature that is equal to boiling point. On the other hand, evaporation is the natural process that occurs without an external source of heat (such as a burner), and occurs at any temperature between freezing point and boiling point of the liquid, and through this process, liquid slowly changes its phase from liquid to gas.