A phase is a region of space where all the material properties are the same.
- Water in a glass is formed by one phase. The water with the glass is formed of two phases: a liquid phase and the solid phase. - A clear broth is composed of two liquid phases: the fat lenses supernatant and the broth itself.
A sample of material is - homogeneous, if it is formed of one phase - heterogeneous, if it it consists of several phases.
Clean air is homogeneous. Smoke is heterogeneous, since it consists of solid particles and air.
Milk microscopically: A heterogeneous mixture consisting of two phases