Vapor Pressure and Boiling
Vapor pressure is the part of the total pressure that is accounted for by the vapor. Ideally, this means that if the total pressure is 12 atm, and the air is 25% vapor, then the vapor pressure is 25% of 12 atm = 3 atm. In a closed system, the amount of vapor in the gas phase will be set by the saturation vapor pressure. The central idea here is that of equilibrium.
Equilibrium is often thought of as the point where movement stops. In chemistry, a better way to think of it is that the rate in one direction and the rate in the other direction are equal and opposite, so there is no net movement.
In the case of the saturation vapor pressure, this means that for every molecule in the liquid that gets enough energy to break free of the liquid and enter the gas phase, there is a molecule already in the gas phase that strikes the surface of the liquid, and lacks the energy necessary to bounce back, hence remaining in the liquid. There is no net change in the ratio of molecules in the gas phase to molecules in the liquid phase.
For pressure in general, one must continue to think of this in terms of the single molecule colliding against others. If the pressure is high, then it will be harder (i.e., requires more energy) for a molecule to break free of the liquid and become a vapor, because the gas phase is crowded with other molecules. Under low pressure, there’s more space, and hence it is easier (i.e., requires less energy) for a molecule to break free of the liquid and become a vapor.
When you boil water, the water will get hot enough to vaporize, but no hotter–because if it got hotter than needed to vaporize, it would have already evaporated. So if the boiling point of water is 100 degrees, and you have boiling water, then you know that the water is 100 degrees–any more and it would vaporize, and any less and it wouldn’t be boiling. When the pressure is low, it takes less energy for a molecule to jump into the empty space above the liquid, meaning that the temperature required to make the water boil is less. In a pressure cooker, the water molecule needs more energy to get into the crowded space above the liquid, so the temperature required to make it boil gets higher.
Pressure is lower at higher altitude for a similar reason: imagine you are under a rug, and then imagine you are under ten rugs. The more layers of carpet on top of you, the higher the pressure is. The atmosphere is no different. There is no air in space, and the only reason that earth has an atmosphere is because gravity pulls the air down to earth. The lower your altitude is, the greater the thickness of the atmosphere above you, and the greater the pressure of the air weighing down on you. Conversely, the higher you go, the closer you are to space, and the less air there is above you weighing down on you.
Since the pressure at high altitude is low, water boils at a lower temperature. That means that the pot of boiling water at the top of Mt. Whitney is significantly cooler than the pot of boiling water at the bottom of Death Valley, and cooler still than the boiling water in the pressure cooker. The pressure cooker makes it possible for the water to remain a liquid at higher temperatures, so you can make boiling water hotter than normally possible (the water can’t vaporize because of the high pressure). Hence, due to the temperature differences, it would take longer to boil an egg at the top of a mountain, and less time to boil an egg in a pressure cooker.
