A phase is another term for a state of matter. There are three phases:

SolidEdit

Solid is the state of matter with the lowest temperature (the 'coldest'). Also, it has the highest density, because the molecules generally "snuggle" together to preserve energy and heat. Therefore, if the molecules gain heat and energy, they would expand and decrease in density. However, some materials, like ice and bismuth, do the opposite and decrease density when they lose heat.

LiquidEdit

Liquid is the middle phase, where the density, temperature, and other qualities are in between the differences of solid and gas. The only two elements that are liquid at room temperature are mercury and bromine. The molecules are not as compact as a solid, but not spread out either. They move in

GasEdit