Illustrated · 1 min
What is condensation?
Condensation is the process of turning a gas into a liquid.
It is the reverse of evaporation.
Image source: By Caroline Monahan
How does condensation occur?
Condensation occurs when a vapor is cooled enough to turn into a liquid (this occurs when the substance reaches its "saturation limit").
Example: Water vapor would turn into a liquid at any temperature between 0 and 100 Celsius.
Image source: By Caroline Monahan
Video
by Tyler DeWitt
In this video, Tyler Dewitt describes phase changes. This section of the video illustrates liquid condensation. He shows you what happens to the molecules as you transition from gas to liquid.
Read · 4 min
This morning I woke up and found frost in my yard. It's the first sign of autumn. For the past few weeks, it has just been dew. But, last night, the air temperature got below freezing. Unfortunately, it's also a sign that I should finish picking my garden. Soon, all the plants will die! The zucchini and cantaloupe already look bad. But what does any of this have to do with chemistry?
We've been studying phase changes. We've already examined the individual phases of matter. Now, we're looking at how we transition between them. We started with the transition from liquid to solid. That's freezing! Then we went in the other direction, from liquid to gas. That's vaporization or evaporation! Today, we'll look at condensation. That's the transition from gas to liquid.
As we've said before, the phases of matter are a balance between kinetic energy and Van der Waal's forces. In gases, kinetic energy dominates. What are the results? They don't have a fixed arrangement. The molecules are freely moving at very high speeds. That's why they fill the volume of their container and are expandable and compressible.
At STP, most gases behave as ideal gases. Do you remember the kinetic molecular theory? We assume that collisions between ideal gases are elastic. They merely bounce off one another! We know that's not exactly true. But it's a good enough approximation. How do we get them to behave like real gases? We adjust the temperature or pressure.
What happens when we lower the temperature? Remember, the temperature is our measure of the average kinetic energy. So, a lower temperature equals less kinetic energy. As the particles slow down, the collisions become inelastic. They don't simply bounce off. Instead, they begin to experience electrostatic interactions. Van der Waal's forces start to influence their movements.
What happens when we increase the pressure? At high pressures, there is less empty space between the molecules. Remember, electrostatic interactions depend on distance. So, the molecules' proximity determines their interactions. When molecules are close together, Van der Waal's forces begin to impact them.
Let's consider an example. Liquid carbon dioxide cannot exist at STP. If you want to learn more, check out our lesson phase diagrams. To create liquid carbon dioxide, we use pressures above five atmospheres! Remember, carbon dioxide is nonpolar. The carbon-oxygen double bonds would be polar based on the electronegativity differences. But, the molecule has a linear geometry. The dipoles cancel out. So, it only experiences dispersion forces. Dispersion forces are weak. So, we expect carbon dioxide to be gaseous. As we increase the pressure, the molecules get forced close together. Soon, the dispersion forces cause them to be attracted to one another. As the dispersion forces balance the kinetic energy, we transition from gas to liquid. The molecules still have enough kinetic energy to move past each other. But they are attracted to the other molecules. So they can't break away.
One of the most common occurrences of condensation is morning dew. Water is constantly evaporating. The water vapor comes from local ponds, grass yards, and even trees. In the troposphere, the air becomes saturated with water vapor. It can no longer hold water vapor. So cloud droplets begin to form, and we get rain. A similar effect happens on the ground. Grass readily loses heat. Why? It has a low heat capacity! The ground cools the air directly above it. What's the result? The water vapor in the air condenses on the grass. We've created dew! It's the same as when you take a cold soda outside. The warmer air collides with the can, cooling it. What's the result? The water vapor condenses. Now, you have water droplets on your soda can!
If you've ever watched the meteorologist on the local news, you may have heard the term dew point. The dew point is the temperature required for dew to form. It's similar to what happens in clouds. At that temperature, the air becomes saturated with water vapor. So, any excess moisture condenses. It's the same principle as solution precipitation.
Make sure you don't confuse the dew point with humidity. Humidity measure the concentration of water vapor in the air. We express it as a percentage of how much water the air can hold. So, fifty percent humidity means the air is halfway to its saturation point. How do we calculate it? It's the ratio of the water vapor's partial pressure to the equilibrium vapor pressure. What does that mean? The equilibrium vapor pressure occurs when the condensation and evaporation rates are equal. That's the point where we get condensation. So, if the partial pressure equals the equilibrium vapor pressure, our ratio is one. We would be at one hundred percent humidity. That would mean we were also at the dew point. Remember, the equilibrium constant is affected by temperature. So, the concentration of water the air can hold varies with temperature. Barometric pressure also influences the equilibrium concentration. As a side note, sometimes the air can become supersaturated. So, our humidity is above one hundred percent. But, it's unbelievably rare.
Are you also wondering about the frost point? You might assume that water vapor condenses and then freezes. It turns out that at the frost point, water vapor transitions directly from gas to solid. So, we'll explore it in our lesson on deposition.