1. Phase Diagrams Zumdahl, page 774 -

2. Consider water which exists in three familiar phases: Solid Liquid Gas Each of the phases has distinct physical properties and each of the phases can co-exist with the others. We can observe phase boundaries such as the meniscus between a liquid and gas.

3. Consider water which exists in three familiar phases: Solid Liquid Gas Each of the phases has distinct physical properties and each of the phases can co-exist with the others. At atmospheric pressure: Low T ---> 0 o C ---> 100 o C ---> high T solid | liquid | gas

4. You also know that, as you change the pressure, the temperatures of the phase transitions will change. For example, as you reduce the pressure on the liquid, the boiling point will be reduced. ( e.g. up a mountain ) As you increase the pressure on a sample of ice, you lower the melting point. ( e.g. under a skate blade )

5. You also know that, as you change the pressure, the temperatures of the phase transitions will change. For example, as you reduce the pressure on the liquid, the boiling point will be reduced. ( e.g. up a mountain ) As you increase the pressure on a sample of ice, you lower the melting point. ( e.g. under a skate blade ) We can summarize these Pressure vs. Temperature relationships on a Phase Diagram.

6. P 100 KPa T 373K273K Log scale

7. P 100 KPa T 373K273K Liquid Gas SolidSolid

8. P 100 KPa T 373K273K Liquid Gas SolidSolid T C Zumdahl, page 780

9. Two special points: Triple Point solid, liquid, and gas ALL co-exist Critical Point The phase boundary between liquid and gas vanishes.

10. Two special points: Triple Point solid, liquid, and gas ALL co-exist Critical Point The phase boundary between liquid and gas vanishes. For water, the triple point is at: 273.16K and 611 Pa compared to the freezing point of 273.15 at 101 325 Pa. The critical point is at 647.1K and 22.05 MPa.

11. Two special points: Triple Point solid, liquid, and gas ALL co-exist Critical Point The phase boundary between liquid and gas vanishes. For the triple point, both T and P are fixed and this is the second fixed point for the Kelvin scale. At temperatures greater than the critical temperature a material is called a Supercritical Fluid.

12. Note that, at pressures below the triple point, water will not melt but will sublime instead. It transforms from solid to gas without a liquid being formed as the temperature is increased.

13. Strictly speaking, I am being a bit careless with the use of the word phase for the solid. The solid STATE actually shows about ten different PHASES that only exist at high pressure. Ice III for example exists at about 270K at a pressure of about 200 megapascals. ( 2000 atmospheres ) The different PHASES have different arrangements of the water molecules.

14. The gaseous STATE is a single homogeneous PHASE. The liquid STATE is a single homogeneous PHASE. The SOLID state may contain two or more crystalline PHASES in equilibrium with each other.