Similarities Between Evaporation and Boiling

What is Evaporation?

If you left a glass full of water in your room for long enough, the entire glass would be empty, and the air humidity in your room would be increased. This is because all molecules, no matter if they’re in the gaseous, liquid or solid phase, are constantly moving and thus have some kinetic energy. While moving, they often collide under different conditions and transfer a portion of that energy. If you consider the top layer of water in the mentioned glass, chances that some of the water molecules that are moving underneath that layer could come up and collide with them are pretty high. You can imagine that as if a marble hit another marble – the water molecule on the surface would get “kicked out” of the glass, thus going from liquid to gaseous phase. This occurs from time to time to time, and at any temperatures, as long as the air humidity is lower than 100%, and is called evaporation. In short words, evaporation would be the process of slow vaporization of water molecules, or the slow transfer from the liquid to the gaseous phase.

What is Boiling?

Boiling is a fast vaporization process, as opposed to evaporation. It can occur only at specific temperatures, determined by the liquid constituents and structure. For example, the well-known boiling temperature of water is 100 degrees Celsius, or 212 degrees Fahrenheit. At that point, the entire volume of the liquid starts rapidly switching from liquid to gaseous phase, and that can occur in one of the three distinct ways:

• Nucleate boiling

In this type of boiling, bubbles form at different discrete points in the liquid and emerge on the surface, releasing heated water molecules into the air.

• Critical heat flux boiling

If the boiling surface is heated enough so that a thin film of vapor can form, it’s called critical heat flux boiling. This happens at a certain critical temperature, which is usually higher than the one needed for nucleate boiling.

• Transition boiling

When the temperature is slightly above the maximum needed for nucleate boiling and slightly lower than the minimum needed for critical heat flux boiling, a transition-type of boiling occurs, which has features of both types.

Similarities between Evaporation and Boiling

Both evaporation and boiling are fundamentally the same process – molecules with high kinetic energy leave the liquid, thus transitioning to gaseous phase. The only differences are the conditions under which these processes occur, as well as the mechanism and speed of each process. Briefly covered, the main similarities between evaporation and boiling would be:

• Similarities between Evaporation and Boiling in terms of “ Transformation”

Both represent the transformation from the liquid to the gaseous phase

Structures that are formed when a substance is in liquid phase are broken down during both processes, transforming it into the gaseous phase.

• Similarities between Evaporation and Boiling in terms of “ Energy”

Both require energy

Usually thermal, but other forms of energy can also be used in order to increase the kinetic energy of the molecules, in order to make them evaporate or boil.

• Similarities between Evaporation and Boiling in terms of “ External Conditions”

Both depend on external conditions

Both the rate of evaporation and the critical boiling temperature highly depend on the surrounding air pressure, humidity and temperature of both the air and liquid.

Summary points on Similarities between Evaporation and Boiling

Both evaporation and boiling are processes that we encounter every day. Evaporation occurs at all times while the air humidity is lower than 100%, while on the other hand boiling occurs at a certain critical temperature, which depends on air pressure. Fundamentally, however, they are the same mechanism – water molecules gain enough kinetic energy to escape the liquid and transition into the gaseous phase. This can either occurs slowly, only from the surface, in which case we’re talking about evaporation, or it can occur simultaneously at many places in the liquid volume, in which case we’re talking about boiling.