How Is the Vapor Pressure of a Liquid Related to Temperature?
- The temperature of a liquid is a measure of the average amount of energy possessed by the molecules in that liquid. Any individual molecule may possess significantly less energy than average, or significantly more. Some of these highly energetic molecules can escape from the liquid into the air. This is why a liquid evaporates even if it is well below its boiling point.
- If you heat the liquid in a closed environment, those highly energetic molecules that escaped from the liquid are not free to wander, but are trapped in the confined space above the surface of your liquid. Some of them will re-enter the liquid phase, while other molecules are simultaneously escaping from it. The molecules in the vapor phase exert a pressure on the sides of the container. This is the vapor pressure.
- A liquid held at a fixed temperature below its boiling point can reach an equilibrium in which the number of molecules escaping equals the number re-entering the liquid phase. If the temperature is low, the number of molecules in the vapor phase above the surface will be low when this equilibrium is established. At a higher temperature, there are more molecules in the vapor phase at equilibrium. The vapor pressure increases as the temperature increases.
- Water boils at 212 degrees Fahrenheit at one standard atmosphere of pressure; it boils at a lower temperature if the pressure is lower. The boiling point of a liquid is the temperature at which the vapor pressure equals the atmospheric pressure. If you wanted to make your cup of tea on the top of Mount Everest, you would have some problems. The pressure is much lower at high altitudes, so when you heat the water, its vapor pressure equals the surrounding atmospheric pressure at a lower temperature than it would at a lower altitude and your tea won't be as hot. You need to drink it quickly before it cools off even more.