Every physical matter has three distinct states—solid, liquid and vapor. At a particular temperature and pressure, a specific matter may exist in only one state. Change of state is associated with the change in heat content (either the matter itself or the surroundings). In general to convert a solid matter to liquid or a liquid matter to vapor, heat must be supplied to the matter from external source. However, for the reverse case, heat should be extracted from the matter.

Liquid to vapor conversion may take place in two distinct ways, namely evaporation and boiling. Evaporation is a slow and silent but natural process that occurs when a liquid is exposed to a gaseous medium that is not saturated with that particular liquid vapor. It is independent of temperature of liquid or surrounding gas but depends on saturation limit of surrounding gas. At a temperature and pressure, every gas has a specific saturation limit for a particular liquid vapor. So evaporation will continue until the gas is saturated with that particular liquid. Even after the saturation, evaporation will persist but condensation and precipitation will occur simultaneously and thus no palpable change in liquid volume will be observed. Evaporation also needs heat supply; however, here the source of heat is the liquid itself (so liquid temperature drops automatically with evaporation).

In boiling, heat is supplied externally and liquid temperature is elevated to its boiling point. Once again, every liquid has a particular boiling point at a constant pressure. When liquid temperature is raised to its boiling point, it starts boiling. During the boiling process (i.e., until entire liquid is converted into vapor), temperature of liquid remains constant at its boiling point. Boiling is independent of saturation limit of surrounding gas. Similarities and differences between evaporation and boiling are discussed in the following sections.

Similarities between evaporation and boiling

• Both evaporation and boiling are physical change (not chemical change).
• Both cause change of state from liquid to vapor.

Differences between evaporation and boiling

Evaporation	Boiling
Evaporation can occur at any temperature of the liquid.	Boiling of a specific liquid occurs only at a particular temperature (boiling temperature) at particular pressure.
Evaporation occurs only from the top most layer of the liquid.	Boiling occurs from the entire liquid once its temperature reaches boiling temperature.
Evaporation is a very slow vaporization process.	Boiling occurs rapidly.
Latent heat required for evaporation is collected from liquid itself.	For boiling, necessary latent heat is supplied externally.
Temperature of liquid decreases gradually due to evaporation.	During boiling, temperature of the liquid remains constant.
No bubble formation occurs in evaporation.	Boiling is associated with bubble formation.
Evaporation is a quite process.	Because of continuous bubble formation and collapse, boiling is noisy process.
Rate of evaporation increases with increase in open surface area of the liquid.	Rate of boiling is independent of open surface area of the liquid.