Difference Between Boiling and Evaporating

Boiling vs Evaporating

Boiling and evaporating are physical properties of an object and are frequently used concepts in daily life and in the study of physics. Many people take boiling and evaporation to be same whereas there are fundamental differences between the two terms and this article intends to make a clear distinction between the two. Every liquid has a boiling point which is different for different liquids.

Boiling point

The boiling point of a liquid substance is the temperature at which the vapor pressure of the liquid equals the external pressure on the liquid. This is the temperature at which the vapor pressure of the liquid is able to overcome the atmospheric pressure and bubbles are formed in the liquid.

To understand boiling point, we need to talk a little about vapor pressure. It is the indication of a liquid’s evaporation rate. All liquids have a tendency to evaporate into gaseous form. The particles or molecules of a liquid have a tendency to escape from the surface of the liquid. Liquids which have a higher vapor pressure tend to evaporate quickly and are known as volatile. A good example of such a liquid is petrol.

At boiling point, which is the temperature at which a liquid begins to boil is the temperature at which this vapor pressure equals the atmospheric pressure which allows the molecules of the liquid to quickly evaporate (or escape) into atmosphere.

As we apply heat to water, its vapor pressure begins to increase. It begins to boil as soon as this vapor pressure becomes equal to the atmospheric pressure.

Evaporation

It is a process by which the molecules of a liquid spontaneously become gaseous, without applying heat to the liquid. Generally it can be seen as gradual disappearance of the liquid when exposed to atmosphere. Why does evaporation take place at all? The answer to this puzzle lies in the fact that molecules in a liquid are in a constant state of random motion and keep on colliding with each other. Normally molecules do not have enough energy to escape out of the surface of the liquid, but this collision transfers energy to some of the molecules more than others and if these molecules happen to be near the surface of the liquid, they may actually fly off and become gaseous. This is known as evaporation.

Thus evaporation is a type of boiling without the application of heat. But if the liquid is kept in a closed container, the evaporated molecules remain inside the container finally making the air in the container saturated. Then comes a stage of equilibrium and the rate of evaporation becomes equal to the condensation of the vapor back to the liquid form. Thus there is no loss of liquid.

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