Now that you have had a chance to study water molecules in a liquid and a gas state, and the process of evaporation, it is time for you to start building up your own powers of visualization and analysis. Here are our sixteen water molecules again, but this time, in a slightly larger box than before.
Do you see any of the water molecules entering a liquid state where the molecules are pulled together and no longer free to move independently? Do you see any of the water molecules breaking away so completely from the clump of water molecules in the liquid state that they enter a gas state where they are able to move freely and would be able to drift away if they were not trapped inside of a box? Keep running the simulation until you are sure that you have seen at least one water molecule “evaporate”… that is transition from a liquid state to a gas state.
What ended up happening to that water molecule? If you were able to watch long enough, you probably would have seen that water molecule at some point re-enter the liquid state. Why does that happen?
Escape velocity is a physics term that is most often applied in space science, but can also be useful when thinking about the process of evaporation. If you throw an object straight up into the air, what happens? Well, usually it will eventually fall back down because of the force of gravity (the attraction between the object’s mass and the mass of planet Earth). However, there is a certain speed known as the escape velocity, where if you were to throw an object straight up at the escape velocity or faster, the object would actually be able to escape Earth's gravity and keep going, moving farther and farther away without ever falling back down (this is not the same as launching an object into orbit; an object in orbit has not escaped Earth's gravity). The escape velocity from the Earth's surface is about 11.2 km/s (25,000 mph).
There are only four known forces (or fundamental interactions) in the universe. Two of them are the gravitational force and the electromagnetic force. Molecular attraction is an electromagnetic force. The other two forces are the strong and weak nuclear forces that are responsible for the interactions between subatomic particles (the particles that make up atoms). All other “forces” that we see in the real world, such as friction, are actually based on one or more of the four known forces.
To make sure there is no question whether a water molecule is truly in a gas state or not, we are going to start out with one water molecule that is definitely in a gas state before releasing it into the box with the other water molecules. And to make it easier to keep track of this water molecule, we are going to color it blue.
Does the blue water molecule ever enter a liquid state by getting pulled into a clump of other water molecules? Keep running the simulation until you are sure that you have seen the blue water molecule transition from its original gas state to a liquid state at least once. Whenever a molecule in a gas state gets close to a group of molecules in a liquid state, there is a chance that the molecule will be “captured” by the other molecules because of molecular attraction and enter the liquid state itself. This process is known as condensation.
Did you notice if the blue water molecule got “captured” and entered the liquid state the first time it came into contact with the other molecules? Or did it bounce away the first time hard enough to achieve an “escape velocity” that would have kept it drifting away except that it was trapped in a box? Just like there is a probability that an individual water molecule will break away from the surface of a liquid state and enter a gas state, there is a probability that a water molecule in a gas state, colliding with molecules at the surface of a liquid state, will enter the liquid state. Sometimes it will, but sometimes it will not. By now, you should be able to figure out what factors will affect the rate of condensation and how the rate will be affected.
By studying and thinking about the simulation above, you should be able to identify the four factors that will affect the rate of condensation: the attraction between molecules, the surface area of the liquid, the temperature, and the concentration (or amount) of water molecules in the air. Adjust the settings for the simulation below to see how the last three factors affect the rate of condensation.
We are all familiar with the process of evaporation. Every day, we see water evaporating and things drying all around us. Condensation feels a little less familiar. When was the last time you put out something wet, only for it to get wetter and wetter over time? The classic example of condensation is a glass of ice water on a hot summer day. What happens to the glass? Over time, droplets of water build up on the outside of the glass. You’ve learned about the conservation of matter, so where does that water come from? It is water vapor in the air condensing on the cold surface of the glass.
Another common example of condensation is the “steam” you see above a pot of boiling water. You actually cannot see steam or water vapor. What you are actually seeing are tiny droplets of liquid water forming in the steam as the water molecules in the gas state begin to cool down and condense back into a liquid state. The temperature of the water molecules in the boiling water is 100 °C, but the air in the room is only at 20 °C. So as hot water molecules leave the pot in a gas state as steam, they immediately begin to cool off (losing energy and speed) as soon as they start mixing with the colder air molecules in the room. The same thing is happening when you see your breathe on a freezing cold day and when your mirror fogs up after a hot shower. Fogs and mists are really tiny water droplets (or ice crystals) suspended in the air, usually created by the condensation of water vapor.
Relative humidity is a measure of the amount of water vapor in the air. At room temperature, the water molecules in a puddle of water and the water molecules in the air are both moving at similar speeds. So why are some of the water molecules in a liquid state while others are in a gas state? The water molecules in the air are in a gas state because they are moving freely instead of being attracted and held by other water molecules. However, if you were to start increasing the concentration (or amount) of water in the air, the water molecules in the air would start getting closer together, and if they got close enough, some of those water molecules would start attracting and holding on to each other, condensing into a liquid state as water droplets.
There is a maximum amount of water that air can hold before the water molecules in the air get so concentrated and close together that they start to condense. When that happens, we say that the air is saturated and the relative humidity is 100%. The maximum amount of water that air can hold depends on the temperature of the air and the air pressure.
Warm air can hold more water vapor than cold air. You should be able to explain why that is the case. So, on a nice spring day, the temperature might be 25 °C (77 °F) and the relative humidity might be 70%. While the air would feel fairly dry to us, there would still be a significant amount of water vapor in the air. However, over night, as the temperature drops to 10 °C (50 °F), suddenly the water molecules in the air are moving at slower speeds, and the space between them is no longer far enough to keep them from condensing and entering a liquid state. The dew point tells you the temperature at which the air would be saturated with water and the relative humidity would be 100%. It’s called the dew point because the dew (drops of water) you find on blades of grass in the morning are caused by the temperature falling below the dew point over night.
So far, all of the examples of condensation we have discussed have involved changes in the temperature… water molecules in a gas state in the air cooling down (losing energy and speed) and then condensing into a liquid state as water droplets. With the glass of ice water, it was warm water molecules in the air coming into contact with the ice-cold surface of the glass. With the steam above the boiling pot of water, it was hot water molecules leaving the pot and coming into contact with the room temperature air above the pot. However, condensation does not have to involve a temperature change. Water vapor in the air is condensing all the time, whether the temperature is changing or not. The only reason why we associate condensation with temperature changes is because, when the temperature drops, condensation is much faster and much more noticeable.
Earlier in the unit, I pointed out that when molecules are attracted to each other and clump together, that this does not mean that they slow down. Molecules will continue moving at the same speed unless they lose kinetic energy. However, several times in our discussion of condensation, I have casually said that the water molecules cool off, losing energy and speed. What does cooling off mean? How does a molecule lose energy and speed? You will be studying this process and the concept of thermal equilibrium later in this unit. But for now, if you are curious, you can find an example of a water molecule losing energy and speed in the simulation above with the blue water molecule. Watch the blue water molecule carefully. What causes the blue water molecule to lose energy and speed? Where does the energy go? Just like matter can neither be created nor destroyed; neither can energy… it has to go somewhere.
So far, we have been focused on water molecules. But that means that our simulations have been incomplete. There are other molecules in the air that are coming into contact with the water molecules at the surface of the liquid state. What happens to those molecules? Like the water molecules in the air that condense and enter the liquid state, other molecules in the air can be “captured” by the water molecules in the liquid state as well. This is not called condensation, even though it is basically the same process. If oxygen (O2) molecules in the air get captured by other oxygen molecules and are held together by their mutual attraction, this is called condensation and you would have liquid oxygen. But when oxygen molecules get captured by water molecules in a liquid state, we say that oxygen gas has dissolved in water. The solubility of oxygen in water depends on the attraction between oxygen molecules and water molecules. You will learn much more about solubility later in this unit. It is a very important concept and it is responsible for enabling fish to breathe underwater and for making our sodas nice and fizzy.
If you have noticed that evaporation and condensation are opposite sides of the same coin, then you are on the right track. In evaporation, water molecules in a liquid state enter a gas state to become water vapor in the air. In condensation, water molecules in a gas state in the air get captured by other water molecules and enter a liquid state, often as small drops of water. Where does water vapor come from? Evaporation. If all liquid water eventually evaporates and becomes water vapor, how do we ever get our liquid water back? Condensation. It is one of those circle of life things.