There is a common misconception that the rate of evaporation of water is slower when the relative humidity is high. But that is actually not true. How would the amount of water vapor in the air affect how often a water molecule at the surface of a liquid state breaks away and enters a gas state? One of the keys to developing a deeper understanding of science is to be able to visualize and analyze atoms and molecules as physical objects, and to not accept explanations unless you can understand them on a physical level. The only way water molecules in the air would possibly slow down the rate of evaporation of liquid water is because they contribute to the overall air pressure above the liquid surface (knocking some of the water molecules breaking away from the liquid state back into the liquid state). But it does not make any difference if the molecules in the air above the liquid surface are water molecules, carbon dioxide molecules, oxygen molecules, or some other kind of molecule.
But doesn’t water evaporate more slowly on extremely humid days? Isn’t that what we observe in the real world? This is where we have to be really precise with our observations and our language. Liquid water will “dry” more slowly on a humid day, but that is not the same as saying that water is “evaporating” more slowly. Evaporation and condensation are always happening simultaneously. At the same time that water molecules are breaking away from the liquid state and entering a gas state, water molecules in the gas state are getting “captured” and entering the liquid state. Students often have difficulty understanding that both processes are happening at the same time; they struggle to understand when evaporation takes over and when condensation takes over, without realizing that evaporation and condensation are never just turned on or off. When a towel is drying, water molecules are still condensing on the towel. When water is beading up on the outside of an ice-cold glass on a hot summer day, water molecules are still evaporating from those beads of water.
Probability on a Molecular ScaleThere are about 1 × 1021 water molecules in a single drop of water. About 1 × 1014 of those water molecules will be at the surface of the drop. Even at 0 °C (the freezing point of water), about 0.00005% of those water molecules will be moving at speeds faster than 2000 m/s (an average speed of 590 m/s is fast enough for water to boil). While 0.00005% seems like an extremely small percentage, on a molecular scale, it means that there are about 50,000,000 water molecules at the surface of the drop that are moving faster than 2000 m/s. Some of those water molecules will break away from the liquid state and enter a gas state. This means that if you have liquid water, you will have evaporation. (Actually, even if you have ice, water molecules will be breaking away from the solid state and entering a gas state. This is why ice cubes will shrink if you leave them in a freezer for a long time.)
Conversely, if you have water vapor in the air, you will have condensation. Imagine you are in the Mojave Desert. The temperature is 40 °C (104 °F) and the air is extremely dry. As long as a single drop of water evaporates, you will put about 1 × 1021 water molecules into the air. Once those water molecules reach thermal equilibrium with the other molecules in the air (you will learn more about thermal equilibrium soon), at 40 °C, about 0.0005% of those water molecules will be moving at speeds slower than 10 m/s (an average speed of 500 m/s is slow enough for water to freeze). That means that there are about 5,000,000,000,000,000 water molecules in the air moving slower than 10 m/s. If those water molecules meet each other or any other water molecules already in a liquid state, some of them will condense. The only way you have no condensation is if you have no water vapor in the air. But if you have any evaporation at all, you will have some water vapor in the air. So if you have evaporation, you will have condensation.
When we introduced the concepts of evaporation and condensation, we were very careful to keep them separated for clarity. But now it is time to update our simulations to better reflect the real world.
Is this simulation modeling evaporation or condensation? Actually, it is modeling both. Overall, the liquid water at the bottom of the container ends up “drying” because the rate of evaporation is faster than the rate of condensation.
If we double the humidity (the amount of water vapor in the air), then the liquid water at the bottom of the container does take much more time to dry. But is the rate of evaporation any slower? No. Water molecules are still evaporating at the same rate; it is just that the rate of condensation has increased. When water condenses on the outside of an ice-cold glass on a hot summer day, evaporation and condensation are both occurring. Condensation is just happening faster, so the net result is a build up of liquid water on the outside of the glass.
When the rate of evaporation equals the rate of condensation, then we have reached a state of equilibrium where the amount of liquid water at the bottom of the container will stay the same. However, even though the overall amount of liquid water is not changing, water molecules are still evaporating and condensing. This is called a dynamic system because things are changing over time. When a dynamic system reaches equilibrium, it looks like nothing is changing from the outside, but if you look closer, things are still changing but the changes are balanced so the net effect is zero.
Thermodynamics and KineticsThere are two branches of science known as thermodynamics and kinetics. Loosely speaking, thermodynamics is the study of what will eventually happen in a dynamic system (when the system reaches an equilibrium state), while kinetics is the study of how and how long it will take for the system to reach that equilibrium state. So thermodynamics would tell us if a puddle of water will eventually dry up or not, and kinetics would tell us how long it would take the puddle to dry up and what will happen along the way.
Here is an interesting thought experiment for you to consider. Imagine a container with a layer of liquid water at the bottom. However, if instead of an open container (which is what we have been simulating so far), the container is sealed so that water molecules cannot get in or out of the container. The air above the liquid water is completely dry (there are no water molecules in the air) and at standard atmospheric pressure. The water, the container, and the air are all at room temperature (20 °C). What will happen over time?
Water molecules will begin evaporating from the liquid state. The rate of evaporation will depend on the attraction between water molecules, the surface area, the temperature, and the air pressure. There will be no condensation at first because there are no water molecules in the air to condense. But once evaporation begins (and water molecules enter a gas state), condensation will begin as well. The rate of condensation will depend on the attraction between water molecules, the surface area, the temperature, and the concentration of water molecules in the air above the liquid water.
The rate of evaporation stays the same in our simulation (actually, in real life, it would slow down a little bit because the addition of water vapor into the air will increase the air pressure slightly), but the rate of condensation speeds up as the amount of water molecules in the air increases. At some point, this dynamic system will reach a state of equilibrium where the rate of evaporation will equal the rate of condensation. When that happens, we say that the air is “saturated” and cannot “hold” any more water… relative humidity is now 100%.
The Importance of Precise LanguageWhen I introduced the concepts of relative humidity and saturation to you earlier, I used some extremely imprecise language. I said that once air was saturated with water, adding any more water would start condensation. By now, you should realize that condensation is happening the whole time, not just when the air is saturated with water. And saying that air is “holding” water is also very imprecise. What I should have said is that, when the relative humidity is less than 100%, in general, the rate of evaporation will be faster than the rate of condensation. But when the relative humidity is 100%, the rate of evaporation and the rate of condensation will be equal and the system will be in equilibrium. Trying to add more water to the air at this point will cause the rate of condensation to be faster than the rate of evaporation, and there will be a net transfer of water molecules from the gas state to the liquid state. (A “net transfer,” means that more molecules are entering the liquid state than the gas state, so the overall amount of liquid water is increasing and the overall amount of water vapor is decreasing.)
Scientists often use imprecise language because it is shorter and feels more natural. Unfortunately, many scientific terms also have less precise meanings in general usage. However, imprecise language can lead to imprecise thinking, especially when students are learning concepts for the first time. You should try to be precise in your language so that you are precise in your thinking.
Once the system modeled in the simulation above has reached equilibrium, what would happen if we lowered the temperature of the water, the container, and the air to 5 °C (41 °F)? Changing the temperature of the system would disrupt the equilibrium of the system. The rate of evaporation would change and the rate of condensation would change (both depend on the temperature), and the two rates would no longer be in balance. As a second interesting thought experiment, predict what will happen after the temperature is lowered and explain why it happens. The system would eventually find a new equilibrium: what would that second equilibrium state look like compared to the first equilibrium state?
Investigating Dynamic SystemsNow that you have a much deeper understanding of evaporation, condensation, and dynamic systems, there are a number of experiments that you can conduct. Start by repeating the experiments that you did for evaporation. However, instead of leaving the containers of water open to the air in the room, seal the containers of water inside of separate large, clear containers. Make sure that these large, clear containers are the same size. This will trap the water molecules in a closed system.
You can then measure how long the system takes to reach equilibrium (kinetics) and the equilibrium state itself (thermodynamics) as you vary the surface area or the temperature. To measure the equilibrium state itself, wait until the water level inside the inner container stops changing. This means that the rate of evaporation and the rate of condensation are balanced and equilibrium has been reached. Measure the amount of water left in the inner container. This will tell you how much water has entered the air as water vapor. (Remember, matter is conserved and this is a closed system.) If you have measured the volume of the large, clear containers, then you can use this information to estimate the concentration of the water in the air when equilibrium is reached. Because there will have been some water in the air to start with, you will be underestimating the actual amount of water in the air. But if the humidity of the air was low when you sealed the container, then most of the water in the air should have come from the liquid water in the inner container. It would be good methodology, however, to record the humidity in the room at the start of the experiment. Unfortunately, humidity is very difficult to measure accurately, even with a hygrometer.
If you are measuring the water levels at regular intervals, then you should see that the rate of drying is slowing down over time (and becoming zero when equilibrium is reached). You can then analyze how the rate of drying varies based on the amount of water in the air in the outer, sealed container. And finally, control for the surface area and temperature in both systems, and vary the size of the outer, sealed containers. See how that variable affects the time it takes to reach equilibrium (kinetics) and the equilibrium state itself (thermodynamics).
After we lower the temperature in our thought experiment, the rate of evaporation will slow down and the rate of condensation will speed up. The net effect will be an increase in the amount of water in the liquid state and a decrease in the amount of water in the gas state. Will this continue until there is no more water in the gas state and all of the water is in the liquid state? No. The system will find a new equilibrium state before that happens. As the amount of water in the gas state decreases, the rate of condensation will also slow down. At some point (before the amount of water in the gas state is zero), the rate of condensation will slow down to the same speed as the rate of evaporation. Lowering the temperature has driven the system to a new equilibrium where the percentage of water in the gas state is lower.
This is a graph of what our data might look like if we could collect it from our thought experiment. The blue and red bars represent how much water (in grams) is in the liquid and gas states. The total amount of water in the entire system is 200 g. The blue and red lines show the rates of evaporation and condensation (in grams per second). You can see that the system starts out in equilibrium (the rate of evaporation and the rate of condensation are balanced, both at 0.24 g/s), the equilibrium gets disrupted (the rate of condensation jumps to 0.3 g/s and the rate of evaporation drops to 0.2 g/s), and then the system gradually moves to a new equilibrium. In real life, measuring the rate of evaporation separately from the rate of condensation would be very difficult.
What would have happened if we had increased the temperature instead?
