Physics

# The Physics Philes, lesson 122: Heat of the Sublime

WE are elbow deep in heat. We’ve been talking about what heat is and quantities of heat or how heat is measured. But we haven’t talked much about what the addition or subtraction of heat from a system actually means. How does heat effect different materials? The addition or subtraction of heat can obvious lead to a change in temperature. It can also lead to something called a phase change.

A phase is a state of matter, like solid, liquid, and gas. (These are the only states of matter we’ll deal with.) You’re probably familiar with states of matter. Water is a common example. Ice is water’s solid state, liquid water is its liquid state, and steam is its gaseous state. But water doesn’t just magically jump from one state to another. It has to got through a phase transition or a phase change.

Let’s think about melting ice. Let’s say we have some ice at 0°C. If we add heat to the ice at normal atmospheric pressure, the temperature of the ice doesn’t go up. Instead some of the ice melts to form liquid water. If we continue to slowly add heat and keep the ice in thermal equilibrium, we can melt the whole block of ice without increasing the temperature. We’ve changed the phase of the material. The heat required to change one kilogram of ice into one kilogram of water at 0°C at normal atmospheric pressure is called the latent heat of fusion. Latent heat of fusion is measured in joules per kilogram or calorie per gram. So if we know the latent heat of fusion, we can determine the amount of heat that will be needed to melt a substance my multiplying the mass of the substance by the latent heat of fusion.

There is a nice symmetry to this, as well, because the entire process is reversible. We need to remove heat in order to freeze water at 0°C. The magnitude of the heat needed to freeze the water will be the same as the amount of heat needed to melt the water, but in the latter situation the heat will be negative (because we’re removing heat from the system).

This, though, just deals with one phase change, the change from solid to liquid. However, the story is pretty much the same when we look the phase change from liquid to gas, known as evaporation or boiling. The heat per unit mass in this case is called the latent heat of vaporization. This process is reversible, as well. When we remove heat from a substance like water at boiling temperature, the gas condenses, or returns to its liquid phase. It gives up the same quantity of heat that was needed to initially push the substance to its gaseous state.

But wait! There’s more! Sometimes a substance can just leapfrog right over a state of matter. That is, it can go straight from solid to gas and vice versa. When a substance goes from its solid state to its gaseous state, that process is called sublimation and a solid substance is said to sublime. The heat needed for sublimation is called the latent heat of sublimation. Dry ice is a good example of this. Since liquid carbon dioxide can’t exist at pressures any lower than five times the atmospheric pressure, it sublimes at normal atmospheric pressure. The reverse of this process also occurs, like when frost forms on cold bodies.

If you have a very pure water, that water can be cooled to a few degrees below the normal freezing temperature without freezing. This is an unstable state described as supercooled. When supercooled water is agitated somehow, or if an ice crystal is introduced, the water will freeze in less than a second. Water vapor can also be supercooled. When that happens, the water vapor will quickly condense into fog droplets when disturbed. This is the principle behind seeing clouds.

You might guess that, since supercooling is a thing, superheating is, too. You’d be right. A liquid can be superheated to above it’s normal boiling temperature and any small agitation can cause the substance to boil.

This is all a little weird when you think about it. I tend to think of heat as causing a temperature change. It’s weird to think of heat being added to a system that doesn’t result in a change in temperature. And yet, physics doesn’t care what I think is weird, and that’s pretty cool.

Featured image credit: Chris Potako via Flickr