A phase is another term for a state of matter. There are four phases - solids, liquids, gases, and plasma.
Solid[]
Solids have the least temperature of the other states of matter. However, it also possesses the highest density because its molecules are closely packed together with no wiggle room. Therefore, if the molecules gain heat and energy, they would expand and so experience decrease in density. Some examples include ice - the solid form of water - which melts after gaining temperature. A solid has definite volume, shape and mass.
Liquid[]
Liquid molecules are more spread apart than solids with enough room to move up to a certain extent. As a result, liquids take the shape of any container they fill. As temperature increases the energy between molecules decreases and draws the molecules closer together, freezing the liquid into a solid. The only two elements that are liquid at room temperature are mercury and bromine. Liquids have definite volume and mass.
Aqueous[]
Aqueous molecules are a subcatgory of liquids often used in Solutions where the solvent is liquid water.
Gas[]
Gas does not possess a definite shape or volume. As a result, gas molecules have a wide range of mobility. Examples of gases include hydrogen and helium.
Plasma[]
Plasma occurs when gas is heated. Gas molecules become excited and move at a much faster pace, causing atoms to collide and shaking electrons loose.