Define Phase.
A phase is a quantity of matter that is homogeneous during in chemical composition and physical structure. Determine if the matter is all gas, all liquid or all solid, it has physical uniformity. Likewise, if chemical composition does not change from one part of the system to another, it has chemical uniformity. Instances of one phase system are a single gas, a single liquid, a mixture of gases or a solution of liquid contained in a vessel. A system containing liquid and gas is known as a two-phase system. Water at triple point exists like water, ice and steam at the same time forms a three-phase system.