A phase diagram is a picture that shows what state of matter (solid, liquid, or gas) a substance will be in depending on the temperature and pressure that is applied to it. For example, if you have a block of ice, it will be a solid. If you increase the temperature the ice will start to melt and turn into a liquid. If you keep increasing the temperature then the liquid will turn into a gas, called water vapor. Pressure can also change the state of matter. For example, if you leave a balloon full of air outside on a cold day it can turn into a solid, called a frost crystal.