Fundamental Definitions

The following are some fundamental definitions pertinent to the study of phase diagrams.

1. System: Any particular portion of the universe in which we are interested.

2. Closed System: A system isolated from the rest of the universe with respect to matter entering or leaving the system. Can however be heat (energy) transfer across boundaries of such a system.

3. Open System: A system not isolated from the rest of the universe in which matter can move in and/or out of the system.

4. Component: The smallest number of independent variable chemical constituents necessary to define any phase in the system.

5. Phase: Any particular part of a system physically homogeneous in itself, but which can be separated mechanically from any other phase of the system.

6. Homogeneous Reaction: Occurs within a single phase in a system.

7. Heterogeneous Reaction: Occurs within more than one phase in a system.

8. Equilibrium: The condition of minimum energy for the system such that the state of a reaction will not change with time provided P & T are kept constant.

9. Stability: The condition of minimum energy.

10. Instability: The condition where energy is not at a minimum.

11. Metastability: The condition where the energy of the system exceeds that of one or more other possible configurations of the system.

12. Degrees of Freedom (or Variance of a System): The number of independent variables which must be arbitrarily fixed in order to define a system completely. In petrology T, P and X (concentration) are most important.