|MadSci Network: Chemistry|
Interesting question. When a gas is cooling, why does the temperature stop falling during condensation? To start with, it must be realized that heat is molecular motion. This is a rather subtle concept, and previously heat was thought to be some kind of "caloric" fluid substance. It wasn't until 1798, that Benjamin Thompson, (American born, with British sympathies and living in Bavaria), was watching a cannon being bored and he reasoned that the heat produced was "excited and communicated" by motion. Temperature is literally a measure of the speed of molecular motion. The higher the temperature the faster the molecules are traveling. And as something cools, the molecules will be traveling slower and slower. Thus, as that gas you mentioned cools, the molecules will be traveling slower and slower. Incidently, at room temperature, the average air molecule is traveling about a thousand miles an hour! Molecules have attractive forces between them resulting from charge, dipole effects, van der Waals forces, etc. As the temperature drops, the molecules will travel slower and slower until the attractive force gets the upper hand. This would be at 100° C for steam. At this point, a very drastic thing happens. There is a "change of state" and the steam begins to condense into water. As the steam, at 100° C, condenses into water, still at 100° C, the bonds that hold liquid water together start forming. When these bonds form, heat is released. This heat must must be removed for the water to form. Therefore, heat must continue to be extracted until all the steam has condensed into water. Only then does the water begin to cool below 100° C. Now of course, I am speaking of an ideal system with instant mixing of the heat, so the whole system remains at 100° C. In the real world, different parts of a vessel can be different temperatures, but the principles are still valid. It turns out that you must remove 2300 joules worth of heat to turn one gram of steam (at 100° C) into one gram of water (also at 100° C). Conversely, you must add back the 2300 joules if you want to turn that gram of water back into steam. These 2300 joules are called the heat of evaporation. Water forms rather strong bonds between itself, thus it has a high heat of evaporation and a rather high boiling point. Nitrogen has much weaker intermolecular attractions and has a heat of evaporation of 200 joules per gram. And although nitrogen is a heavier molecule than water, nitrogen boils at -196° C. Similarly, as heat continues to be removed from the liquid, it will cool until its attractive forces are strong enough to hold it together as a solid. In other words, it will freeze. Water freezes at 0° C. And the liquid water will stay at 0° C until it is completely frozen. One must remove 330 joules per gram to convert liquid water to ice. Only after that, will the temperature fall below 0° C. This means that the temperature of any ice/water system at equilibrium, whether there's just a little ice or it's almost completely ice, will be 0° C. Any good physics or general chemistry text should have a discussion about changes of state and about the heats of evaporation and melting. Mike Conrad.
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