Well, finally we're going to get right down to it. After circling around the issue like wolves around the sheep we are now going to begin the process of figuring out how that refrigerator gets heat to go from a cold place to a hot place. This is really the mystery of refrigerators and heat pumps. It will not be a quick or simple task but I think you will all get there. We have to begin with some study of the behavior of fluids when they change to gasses and vica versa. These are called changes of state and in our experience there are generally three states of matter. These are solid, fluid, and gaseous.
To begin with lets consider a material you have lots of experience with - water! You all know how to change water from one state to another. The two tools of choice to accomplish any desired change of state are a stovetop and a freezer. You will notice that you have no doubt at all about how to use these two appliances to accomplish any task I should set you. To get water from ice you just pop a pan of ice on the range and wait patiently. To get water from steam you just catch a little steam in a handy plastic bag and stuff it in the freezer and very soon you will have water. If you stopped to think about it you might notice that to change ice to water or water to steam you add heat to the material and to reverse the process you take heat away. You may also have noticed (or maybe just wondered) that you can't heat water or ice above a certain temperature. They just won't do it. You can't have ice at 5 degrees Centigrade, you can only have water (at least when limited to normal everday environments)!
All materials show the same properties as water although the particular melting and boiling points change dramatically depending on the material. What is less obvious but equally true is that for each material there is a characteristic quantity of heat that is needed to melt 1 gram. There is a characteristic quantity of heat that is needed to boil away 1 gram of the liquid material. These quantities of heat are respectively called the heat of fusion and the heat of vaporization. Collectively they are sometinmes known as the latent heats. You might spend some time making sure you know why they are different than the specific heats (or heat capacity) we talked about a couple of weeks ago.
As if this isn't confusing enough these latent heats, as well as the boiling points and melting points, depend on the physical environment in which the material is found. If you can fruits and vegetables, or are an ardent tea drinker you will have had some experience with this. A tea kettle has a closed lid which raises the pressure inside the kettle as the water begins to boil. This causes the boiling point of the water to rise noticably which allows you to pour a very hot cup of tea at an altitude where this is not normally possible. This is also why many of us burn our lips on coffee and tea when we go to Portland or the coast. Atmospheric pressure is noticably higher and it leads to hotter hot drinks. Pressure cookers work on the same principle. The point I am trying to make is that neither the melting point or the boiling point are constants and depend quite strongly on the specific circumstances of the experiment. In our particular case we are going to be most concerned with the behavior of fluids and gasses and the characteristics of the transition between these two states.
You are likely to have heard of the Ideal Gas Law at some point in your life and it contains a host of important conceptual information. You may even remember it as some variation on PV=nRT. From our point of view what is critically important is that you have an solid understanding of what is means conceptually and the heck with the numbers. In that little equation are condensed years of your life experience. Bike tires sometimes explode in the hot summer sun - we have learned to attribute this to the increased pressure resulting from the high temperature produced by the sun baking the tire. The ideal gas law says the same thing in different words - if the volume is constant and I increase the temperature then the pressure must also increase. If I take a balloon outside on a cold day it shrinks up a whole lot - a behavior we may attribute to things getting smaller when they get colder (yes, it is related to the same process that produces thermal expansion in solids). Refering to the ideal gas law we find that it indicates that if we allow the pressure to remain constant then the volume occupied by a gas will diminish as the temperature drops. Returning to our experiences with bikes, we know that if we take a gas and squeeze it as we do in a pump that its pressure will increase (so we can get some more air in the tire). The list goes on and on. It would be especially helpful if the situations we are going to consider would hold at least one of the variables (P, V, or T) constant but alas it is rarely the case except in carefully controlled laboratory experiments. Spend some time thinking about what the mathematics of the ideal gas law has to say and how it applies to situations you have observed. You will be doing lots of this in the lab! See you there.
