Phase, in chemistry, is any distinct and uniform physical part of a mixture that is separated from the other parts of the mixture by definite boundaries. For example, in a mixture of ice, liquid water, and water vapor, each form is physically distinct from the others, and the mixture has three phases. Oil and water form two phases with a definite boundary between the phases. Solutions are one-phase systems, because they are homogeneous mixtures—that is, they cannot be separated mechanically.
