Sadly, this morning I woke up and found my garden covered in frost. We've had a few days in a row that frosted overnight. So, at this point, my garden is pretty much dead. Last night finally killed off the tomato plants. My tomato plants were almost six feet tall! So, I have a lot of garden cleanup ahead.
But let's talk about chemistry. We've been investigating the phases of matter and phase changes. We've studied the common ones: melting, freezing, vaporization, and condensation. Each of these is straightforward. There is a clear path from one phase to the next. But, two of the phase changes are more complex. Our last lesson was on sublimation. That's the conversion of a solid directly to a gas. Today, we'll look at the opposite process, called deposition. I brought up my garden because it got covered with frost. Frost forms by deposition!
A Molecular Perspective on Deposition
At this point, I sound like a broken record. But, each phase of matter depends on the balance of kinetic energy and Van der Waals forces. But, for deposition and sublimation, there is a trick. We are changing from one extreme to the other. Deposition starts in the gas phase. So, the kinetic energy dominates. The molecules are freely moving and have no arrangement. There is a lot of empty space between the particles. So, they fill the volume of their container. We end up in the solid state. In solids, intermolecular forces dominate. They have a fixed particle arrangement. The molecules get locked into place and can't move past each other. So, solids have a fixed volume.
How do we transition from one extreme to the other? It still depends on the temperature change. But, the temperature has to drop rapidly. The temperature is our measure of the average kinetic energy. What happens when we drop the temperature below the freezing point? The electrostatic attraction begins to dominate. That's universally true. So, if we rapidly cool a gas below the freezing point, it becomes a solid. That's deposition!
Chemistry in the World: The Frost Point
When we studied condensation, we discussed the dew point. It turns out there is also a frost point. That's the temperature at which frost spontaneously forms. At freezing temperatures, it is easy for the air to become saturated with water vapor. But what happens when the saturated air comes into contact with a colder surface? The surface instantly cools the air to the freezing point of water. What's the result? The water vapor freezes. We've formed frost!
Why do some surfaces readily form frost? For example, we almost always think of cars covered in frost. They have low heat capacities! Cars are metal. Most elemental metals have a specific heat capacity below 1.0goCJ. But what about car bodies? We construct cars from an alloy called carbon steel. Remember, steel is a mixture of iron and carbon. In standard steel, the mass percentage of carbon is less than 0.5 percent. But for carbon steel, it's between 0.5 and 2.0 percent! Carbon steel has a specific heat capacity of 0.5goCJ. How does that compare to water? Remember, water's specific heat capacity is 4.18goCJ. So, you have to remove a lot more energy to cool water. That's why metal surfaces are often colder than the air. The air retains heat far longer than most other substances. When the air hits the metal, it rapidly drops below the freezing temperature. The water vapor phase changes from a gas to a solid. So, it's undergone deposition!
