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?

VEscape Velocity
VFundamental Interactions

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.

Factors affecting the rate of 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.

Rate of Condensation: 
Surface Area: 
Surface Area: 
Water Concentration: 
Water Concentration: 
Condensation in the real world

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 and the dew point

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.

VCondensation and Temperature Change
VThe Loss of Energy and Speed
VCondensation and Other Molecules in the Air

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.