Evaporation is Changing Liquid to Gas

by Ron Kurtus (7 February 2007)

Evaporation is the process where an amount of a liquid becomes a gas at temperatures below the boiling point. In other words, its molecules escape the body of the liquid and move freely away, as a gas.

An example is when water in a pan evaporates until there is no liquid left in the pan. The reason this happens is because some molecules have high enough energy to escape the surface tension of the liquid. Evaporation results in the liquid becoming cooler.

Blowing on the liquid increases the evaporation.

Questions you may have include:

• How does evaporation occur?
• Why does the liquid become cooler?
• How does blowing increase evaporation?

How evaporation occurs

A liquid is made of many molecules in rapid motion. The temperature of the liquid is proportional to the average kinetic energy of the molecules:

KE = ½mv²

where

• KE is the kinetic or moving energy of a molecule
• m is the mass of the molecule
• v is its velocity
• v² is v-squared or v times v
• ½mv² is ½ times m times v²

If the temperature is below the boiling point of the liquid, the average kinetic energy of the molecules is not enough to escape the surface tension of the liquid.

Although there is an average kinetic energy of the molecules, there is a range of velocities to make up that average. This means that there are some molecules that are moving almost fast enough to escape from the liquid. They get just beyond the surface of the liquid, but are then pulled back into the liquid.

Some molecules are able to escape the liquid

But there are other molecules that have such a high velocity and kinetic energy that they are able to escape and become free of the constraints of the liquid. Those molecules have evaporated into gas.

Liquid cools from evaporation

The temperature of the liquid is proportional to the average kinetic energy of its molecules. If the molecules that had the highest kinetic energy were able to escape the liquid through evaporation, that would mean that the average velocity and kinetic energy of the remaining molecules would be less than before. That means the temperature of the liquid would now be less than before.

Verification

You can verify that by wrapping the bulb of a thermometer with a wet piece of cloth. As the water evaporates, the temperature will go down slightly.

A more obvious verification would be to rub some alcohol on your skin, blow on it and feel how cool it feels as the liquid evaporates.

The reason you perspire in hot weather is so that liquid can evaporate and thus cool your body.

Why evaporation continues

A good question is: If the temperature of the liquid goes down with evaporation, won't there be a point where the energy of the remaining molecules is too low to escape? If that is the case, how come a pan of water will completely evaporate?

The answer is that although the temperature of the liquid goes down with evaporation, the surrounding air and the container are still at room temperature. They heat up the liquid back to room temperature, which then creates more higher energy molecules. So, the process continues.

Blowing to enhance evaporation

If you put some liquid on are skin and blow on it, the liquid will evaporate more quickly. The same thing happens when you blow on a pan of liquid, except that you don't notice the evaporation as readily.

What happens when you blow on a liquid, or when you get cooled by a breeze on a hot summer day, is that the air molecules not only carry away the liquid molecules that have escaped, but they also pull away some of the liquid molecules that are barely leaving the surface.

The flow of air on the surface of a liquid thus enhances the evaporation process.

Summary

Evaporation when some liquid becomes a gas at temperatures below the boiling point. Molecules escape the body of the liquid and move freely away, as a gas. This happens because some molecules have high enough energy to escape the surface tension of the liquid. Evaporation results in the liquid becoming cooler. Blowing on the liquid increases







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