Thermo for Normals is a short book (~100 pages) I wrote that conveys the basics of thermodynamics without all of the mathematical complexity. Over the next year I will be posting 1 or 2 sections of the text per week. I don't know whether I will ever try to publish it in any other form, though perhaps as an Amazon e-book it wouldn't be a bad idea. Questions, comments, criticisms, and corrections are welcome.
Freezing and melting
Next up, we'll try to get a handle on some phase changes, when something changes from one phase (gas, liquid, or solid) into another.
First, we should state what is meant by each phase.
A gas is matter where the molecules are flying around in all different directions. And they're flying really fast, about 380,000 miles per hour at room temperature. This is so fast that when you look at the trajectory they take, gravity hardly matters at all. Because of that, when you fill a container with a gas, there are just as many atoms near the top as near the bottom. A gas fills a container that it is put into completely. That implies that the gas can take on any volume that it is allowed to by the vessel it's in.
A liquid has molecules going more slowly that in the gas, though not that much slower. However, unlike the gas, the molecules bump into each other frequently, and they don't get too far away from each other. Molecules stick together much more than in the gas. A liquid will conform to a container that you put it in, but in a way so that it keeps the same volume of liquid. You can't really compress a liquid very much, nor can you expand it. If you have a liter of water you have a liter of water. That's not true of a gas.
A solid has atoms that are packed very closely, much more closely than either liquid or gas. Atoms are directly bonded to each other, and one atom cannot move without the adjacent one feeling it almost immediately. A solid will not fill a container in any way; it retains its overall shape unless the atoms are physically detached from each other by cutting or melting. And just like a liquid, volume never changes.
We are all familiar with the phases of water, H2O. In gaseous form we call it water vapor. In liquid form it's just water, and in solid form it's ice (this is how I'll refer to them from now on). Ice forms at 32°F (0°C) and vapor is formed at 212°F (100°C). Does this mean that if you have H2O at 32°F you necessarily have ice? No!
Here's why. Suppose you start out with some liquid at room temperature, and begin cooling it by removing heat in a refrigerator. Every time you remove some heat, the temperature falls. That is, until it just reaches 32°F. At that point you still have liquid. No ice forms. But, if you remove any more heat from the liquid, you don't decrease the temperature. Liquid water cannot exist below 32°F*. So any heat you remove instead makes ice form. If you remove a small amount of heat you create a small amount of ice, and the temperature remains 32°F for both the liquid and the ice crystals. Then you remove some more heat and more ice forms. Again, temperature does not fall. This continues until all the liquid has become ice. After that, removing heat resumes lowering the temperature, and ice can exist in principle all the way down to absolute zero.
This works the other way too. If you start out with warm ice tea and put an ice cube or 3 into it, what temperature is the liquid after a minute or so? 32°F! Why? Because heat will flow out of the liquid into the ice until the ice gets to 32°F (it's usually colder when it comes out of your ice box), and then any more heat you put into the ice cube doesn't change its temperature. It stays at 32°F unless it all melts. And the tea won't stop heating the ice until its temperature matches the ice. So, any fluid with ice in it, after mixing it and waiting a bit, is at 32°F. Putting in a whole lot of ice does two things only: 1. It makes the drink get to ice cold temperature faster and more uniformly (because the ice piles up and can cool the liquid near the bottom, as opposed to just a few ice cubes which will all float and only cool the top part of the drink). 2. It will keep the drink colder longer. Why? Because after it gets to 32°F, you let it sit, and heat begins to leak into the cup from the room. This heat would warm up the drink, but instead the liquid dumps any heat that leaks into it into the ice cubes instead of absorbing it itself. Only once all the ice melts will your drink start to warm up. What adding a lot of ice emphatically does not do is lower the temperature! It's stuck at 32°F, no matter how much ice you add.
At the same time that all of this is happening, though, water molecules from the ice cube are becoming liquid. So, of course, if you leave your drink with a lot of ice in it until half the ice melts, you get the pleasure of a drink that is still ice cold, but is also waterier than before.
The "desire" of substances to change phase is sometimes amazing. When water freezes into ice, it actually expands, taking up more volume in the solid phase than liquid. But what happens if the water is in a container that constricts its volume? The drive to expand is humongous. So, more than likely the container holding the water will explode! You can demonstrate this quite easily by putting a can of soda into the freezer and leaving it for awhile (but be prepared to clean up a mess). Water sitting inside a crack in a rock is capable of actually opening the crack even larger---it literally pulls solid rock apart. Freezing water can break pipes, tear apart trees, and actually lift the foundation of a house.