Each physical homogeneous type (solid, liquid orvapour) is a phase.
Two or three phases may coexist. e.g. Ice and water at 0 oC.
Mixtures of gases (e.g. Air) can only be treated as pure substances if phase changes are excluded. This is because phase changes of constituents occur at different temperatures. Oxygen evaporates at -183 oC,Nitrogen at -194 oC.