What defines a phase in thermodynamics?

A phase in thermodynamics is defined as a form of matter that is uniform in composition and state. This means that within a given phase, the physical and chemical properties are consistent throughout, which allows for distinct phases to be identified even within the same system. For example, in a glass of water, the liquid phase is uniform in both composition (H₂O) and state (liquid) throughout.

This uniformity can be in terms of temperature, pressure, and concentration of substances present. Phases can include solid, liquid, and gas forms, and each has distinct physical properties. For instance, ice (solid phase), liquid water, and steam (gas phase) are all different phases of H₂O due to their differences in energy, structure, and molecular interaction.

The other options describe characteristics that do not align with the definition of a thermodynamic phase. For instance, a form of matter with variable chemical composition or a mixture of different compounds does not imply a uniform state, which is essential for defining a phase. Similarly, while a physical state that does not react chemically might refer to a particular condition, it does not capture the necessary criteria of uniform composition and state that defines phases in thermodynamics.