Final answer:
A phase has uniform composition and properties and can be solid, liquid, or gas, but it is not always a pure substance as it can also be a homogeneous mixture.
Step-by-step explanation:
A phase is any part of a sample with uniform composition and properties. Phases can exist in different states including solids, liquids, or gases. A phase does not necessarily have to be a pure substance; it can also be a homogeneous mixture that maintains uniformity throughout. However, a characteristic that is not true for all phases is being always a pure substance; phases can consist of mixtures as well.