What is a phase diagram? Draw the phase diagram of water and mark its triple point. What is special about the triple point?

1. 1 · Define phase and phase diagram. A phase is a state of matter: solid, liquid or gas. A phase diagram is a P-T graph showing which phase is stable.
2. 2 · Draw the diagram. P-T axes; fusion, vaporisation and sublimation curves meeting at one point; regions marked ice, water and steam.
3. 3 · Name the curves.
   • Fusion curve (ice line): ice and water together.
   • Vaporisation curve (steam line): water and steam together.
   • Sublimation curve (hoarfrost line): ice and vapour together.
4. 4 · Mark the triple point. Where the three curves meet: 273.16 K and about 611 Pa.
5. 5 · Explain why it is special. Only there do ice, water and vapour coexist. It is exact, so it fixes the kelvin scale.
6. 6 · Real life. A pressure cooker works because higher pressure raises the boiling point, as the steam line shows.
7. 7 · Close in one line. The triple point is the one state where all three phases of water meet.

Scoring tip: Label all three curves and all three regions. A diagram with only the triple point marked gets part marks.