Prepare a vacuum pump, vacuum table, and bell jar so that a high degree of vacuum can be attained. In a clean watch glass place a few table spoons of tap water (temperature about 65°F) and support the watch glass over a small container of concentrated sulfuric acid (to help the pump absorb water vapor), all on the vacuum table. Put the bell jar over the apparatus, using vacuum wax for a good seal (see figure below).

The triple-point experiment. At a very low pressure liquid water boils to a vapor as it simultaneously is freezing to a solid. (Note: Do not repeat this demonstration frequently, as the water vapor drawn into the vacuum pump may damage it.) There must be no leaks or the desired result will not occur. The vacuum table should be equipped with a good vacuum gauge, and a thermometer should be suspended from the bell jar so its bulb will be in the water. Start the pump, and take simultaneous pressure and temperature readings every 15 sec. First, dissolved air comes out of the water in very small bubbles. Then, at a point (approximately) where p = 0.52 in. Hg and t = 60°F, boiling of the water begins. Boiling continues as both pressure and temperature drop. After a few minutes the temperature will have dropped to 32°F and pressure will be about 0.18 in. Hg. Bubbling will have slowed down or almost ceased. After another minute or so (watch closely for it) the water surface loses its sheen and the water suddenly freezes to ice. Ice formed, liquid was present, and vapor was being

given off’ all at the same time! This condition point (p = 0.18 in. Hg, t = 32°F; or 4.57 mm Hg, 0°C, respectively) is known as the triple point for water, and this demonstration is commonly known as the triple-point experiment (see figure below).

Phase diagram for water, showing the triple point. At the triple point P, water exists in equilibrium among all three states-solid, liquid, and vapor.Water was boiled, not by heating, but by pulling a vacuum on it, and ice was frozen from boiling water. No flame was required to furnish heat to boil the water. No refrigerant was used to freeze the water. Can you explain the processes involved in this experiment? If you have really thought through the kinetic molecular theory of heat, the Maxwellian distribution idea, the discussion of sensible and latent heat, the vapor-pressure explanation of boiling, and the real meaning of adiabatic processes, you can.Water is used in the above examples for two reasons:

1. It is the most common medium of heat exchange and is always present in the air to be conditioned, and

2. the changes of state which we desire to show can be easily demonstrated for water with relatively simple equipment within a range of temperatures and pressures easily obtainable in the laboratory.

It will be shown later that the same basic principles apply to the substances we call refrigerants.