Evaporation

At the beginning of this unit, you learned about evaporation. Evaporation is when the molecules at the surface of a liquid state break away and enter a gas state. This is usually what happens when something wet is left out and dries. Over time, liquid water slowly becomes water vapor in the air.

Evaporation will occur at any temperature as long as you have liquid water. Water will evaporate on hot summer days and on cold wintry days. Even when the temperature is 0 °C (the freezing point of water), water will evaporate and wet things will dry when left outside. Remember, even at 0 °C, some of the water molecules in the liquid state will be moving faster than 1000 m/s (much faster than the average speed of water molecules in boiling water). If these fast moving water molecules happen to be at the surface of the liquid state and are moving in the right direction, they can break away and enter a gas state as water vapor in the air.

The only difference between evaporation on a hot summer day and on a cold wintry day is the rate of evaporation (how quickly evaporation occurs). A puddle of water will evaporate much more quickly on a very hot day than on a very cold day. There are four factors that affect how quickly a liquid will evaporate: the attraction between molecules, the surface area of the liquid, the temperature, and the air pressure.

How does molecular attraction affect the rate of evaporation?

When the force of attraction between molecules is strong, the molecules will hold on to each other tightly in the liquid state and it will be more difficult for molecules to break away and enter a gas state (evaporation will be slow). On the other hand, when the force of attraction between molecules is weak, the molecules will not hold on to each other as tightly in the liquid state and it will be easier for molecules to break away and enter a gas state (evaporation will be faster).

How does surface area affect the rate of evaporation?

Evaporation is a process that occurs at the surface of a liquid. If you look at the two containers of water in the simulation above, you will see that both containers hold the same amount of liquid water, but the liquid water in the container to the left has more surface area. While surface area does not affect the probability that a given water molecule will break away from the surface of the liquid state and enter a gas state, the rate of evaporation will be faster on the left simply because there are many more water molecules at the surface where evaporation can occur.

How does temperature affect the rate of evaporation?
Temperature
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Hot
Temperature:
Temperature
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Temperature:

The temperature of a substance is a measure of the average kinetic energy of the molecules in the substance. The more kinetic energy a molecule has, the faster it is moving. If the temperature of liquid water increases, that means the average speed of the water molecules has increased, and there will be more molecules with the speed needed to break away from the surface of the liquid state and enter a gas state (evaporation will be faster). On the other hand, if the temperature of liquid water decreases, then the average speed of the water molecules has decreased. While there will still be some water molecules with the speed needed to break away from the surface of the liquid state, there will be fewer of them (and evaporation will be slower). You can see how temperature affects the rate of evaporation by adjusting the temperature of the two containers of water in the simulation above.

How does air pressure affect the rate of evaporation?

Once again, air pressure is something that we usually do not think about very much since it is all around us all the time. However, if we think about it, we can see that air pressure can affect the rate of evaporation in two ways. If the air pressure increases, then the molecules in the air will be pushing harder on the surface of the liquid water. This means that the water molecules will be pushed closer together (increasing the force of attraction between them and slowing down evaporation) and the molecules in the air will be physically knocking water molecules breaking away from the liquid state back into the liquid state more often (also slowing down evaporation). Decreasing the air pressure will have the opposite effect, speeding up the rate of evaporation. So we would expect water to evaporate more quickly at higher altitudes where the air is thinner and the air pressure is lower, such as in Denver (1610 meters above sea level), Nairobi (1660 m), Quito (2800 m), or Lhasa (3650 m). It is also why, when you watch the weather report on the news, that the air is “drier” (contains less water vapor) when a high-pressure system is passing through the area.

Investigating Evaporation Rates*
The Cooling Effect of Evaporation

In our study of evaporation, we have introduced two other extremely important concepts in natural processes: rates and probabilities. When the air pressure above a pool of liquid water increases, the probability that a given water molecule at the surface of the liquid state is going to be able to break away and enter a gas state as water vapor in the air goes down because there are more molecules in the air above it knocking it back into the liquid state. It is still possible; the probability is just lower. And if the probability of a water molecule breaking away is lower, then it will happen less frequently, which means that the rate of evaporation will be slower. It is the same with temperature. There is no cut off point where, below this temperature, evaporation does not happen. As the liquid water gets colder, the average speed of the water molecules goes down (the key word here is “average,” not the speed for all molecules), which means that the probability that a given water molecule at the surface of the liquid state will have the speed needed to break away is lower. The probability of evaporation and the rate of evaporation goes down as the temperature gets cooler, but it is never an on or off situation.