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.

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).

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.

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.

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*

There are many simple experiments you can conduct to test the theories (or hypotheses) about evaporation presented here. Of the four factors that affect how quickly a liquid will evaporate, the surface area of the liquid and the temperature are going to be easiest for you to vary as independent variables. There is not much you can do about the attraction between water molecules (although, once you know more about the attraction between molecules, you could work with liquids other than water) and the air pressure will be difficult for you to control.

To test the affect of surface area on the rate of evaporation, find two containers with different surface areas. Measure the surface area of both containers. Then, measure the same amount of water into both containers. Place the containers side-by-side, and begin recording your observations. Since the surface area is your independent variable, you will need to control for the other three variables: the attraction between molecules, the temperature, and the air pressure. By using water in both containers, the attraction between molecules should be the same. By placing the containers side-by-side, the temperature and the air pressure should be the same. This does not mean that the temperature and the air pressure will be constant. If the containers are left out for a number of hours, the temperature and/or air pressure may certainly change during the period of the experiment. But both containers should experience the same conditions. To increase the precision of your observations, you could measure the temperature in both containers and use a barometer to measure the air pressure above them over time.

There are three methods you can use to estimate the rate of evaporation for both containers. First, you could simply measure the time it takes for the water to completely evaporate from both containers, and then divide the amount of water you started with by the time to find the average rate of evaporation. If you are measuring the amount of water by its mass, then your rate might be in grams per second (g/s). If you are measuring the amount of water by its volume, then your rate might be in milliliters per second (mL/s). (Using Avogadro’s number, you could then convert these rates to molecules per second.) However, since the water in one container may take longer to evaporate than the water in the other container, then the conditions for both containers would not be the same even though they are side-by-side. For example, if you did this experiment overnight and the temperature dropped during this time period, then the average temperature would be lower for the container that took longer to evaporate. This is a significant flaw in methodology.

Second, you could allow the water in both containers to evaporate for a set amount of time, and then measure how much water is left after the time has elapsed. This ensures that the conditions for both containers should be the same. And third, you could measure the water level in both containers (using this to indirectly measure the amount of water left) at regular intervals over a period of time. You could automate this work by using a camera to take time-lapse photography of the containers, possible including temperature and barometer readings. (There is free, open source software available for Mac OS X that will enable you to take time-lapse photography with the built-in camera in Apple Macintosh computers.) You could also use temperature probes to measure the temperature over time.

To test the affect of temperature on the rate of evaporation, you would use two containers with the same surface area and vary the temperature. One way to do this would be to use electric warming blankets beneath the containers. I have not tried this myself, but it should work.

*When you learn about dynamic systems, you will see that these experiments do not actually measure the rate of evaporation. Evaporation and condensation are both happening at the same time (you will learn about condensation next). What is actually being measured in these experiments is the rate of “drying.” The rate of drying is the rate of evaporation minus the rate of condensation. This is not a poor approximation if the humidity is low and the rate of condensation is slow. However, the rate of drying is not the same as the rate of evaporation.

The Cooling Effect of Evaporation

You may have heard that one way that we keep ourselves cool in hot weather is through evaporation: panting for dogs, licking the bottom of paws for cats, and sweating for us. When water molecules break away from the surface of the liquid state and enter a gas state, the molecules breaking away tend to be the faster moving molecules. This means that the average speed of the water molecules left behind in the liquid state is lowered, which also means that the temperature of the liquid water left on our skin (or on your dog’s tongue or your cat’s paw) goes down. Heat from our body is lost to the air through water vapor. Many warm-blooded animals regulate their body temperatures in some way through 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.