Changes of Phase. Phase. “Phase” refers to the state of matter a material is in: solid, liquid or gas. (we will ignore other states like plasma for now since they make life a bit complicated)
Solid Liquid Gas
Evaporation of a liquid occurs at any temperature
How quickly a liquid evaporates depends on the temperature
It occurs ONLY at the surface of the liquid
Boiling is different:
It occurs at one temperature
It occurs throughout the liquid
It occurs when the pressure of the evaporating liquid equals or is greater than the air pressure pushing down on the surface of the liquid
(Vapor Pressure > Atmospheric Pressure)
As more and more gas molecules evaporate, the pressure increases.
But as more and more molecules are in the gas phase, some will begin to condense.
Eventually, the rate of molecules evaporating and condensing will be equal to each other
The pressure inside the container will get to a new constant pressure
This is the vapor pressure of this liquid. It will be constant for this temperature
At any one temperature, a liquid will have a constant vapor pressure
Molecules will constantly be moving in and out of the gas phase
But as temperature increases, more molecules will start evaporating, and the vapor pressure will increase
Vapor Pressure depends on the temperature AND the liquid being heated.
Some liquids, like water, evaporate fairly easily and have a moderate vapor pressure
Others, like acetone, evaporate very easily, and have a very high vapor pressure
Others, like olive oil, have a low vapor pressure, because it is quite difficult for the molecules to evaporate
Compare the vapor pressure at any one temperature (say room temperature)
Or compare the temperatures needed to reach atmospheric pressure