Can I use a phase diagram to determine states of matter?

When it comes to physics, a state of matter is one of the distinct forms in which matter exists.

It’s understandable that you automatically think of the ‘big three’, which are solids, gasses and liquids.

That may also extend to plasmas, or maybe even Bose-Einstein condensates.

The latter is a state of matter which is usually formed when a gas of bosons (particles) at low densities is cooled to temperatures close to absolute zero.

Phases like these are better-suited to describing what state of matter is present in a system than any single definition.

Identifying the phases of matter

Imagine a single ice cube in a closed system, where no heat light or energy is escaping.

Yes, the ice is solid and, yes, there is gaseous water vapour and liquid water present in the system.