PHASE DIAGRAM

Intermolecular Forces of Liquids and Solids

  • Phase Diagram

Learning Objectives

  1. Describe the components of a phase diagram

  2. Use phase diagrams of pure substances to determine their phase at given temperature and pressure

  3. Interpret the phase diagrams of water and carbon dioxide

  4. Describe how changes in temperature and pressure can change the state of matter

  5. Construct the phase diagram of a substance from given data

Phase Diagram Definition

  • A phase diagram is a graphical representation of the physical states of a substance under different conditions of temperature and pressure.

Features of a Phase Diagram

A. The Three Areas

  • Three distinct areas are marked: solid, liquid, and vapor (gas).

  • Under specific conditions on the diagram, a substance can exist in its solid, liquid, or gas phase.

  • Labels indicate the stable states of a system in equilibrium.

B. Three Lines (Curves)

  1. Green Line

    • Divides solid and liquid phases.

    • Represents melting (solid to liquid) and freezing (liquid to solid) points.

  2. Blue Line

    • Divides liquid and gas phases.

    • Represents vaporization (liquid to gas) and condensation (gas to liquid) points.

  3. Red Line

    • Divides solid and gas phases.

    • Represents sublimation (solid to gas) and deposition (gas to solid) points.

C. Two Important Points

1. The Triple Point

  • The point in a phase diagram where all three phases of matter coexist in equilibrium.

  • Represents a unique combination of pressure and temperature where solid, liquid, and gas can all exist simultaneously.

2. The Critical Point

  • Terminates the liquid/gas phase line.

  • Represents the temperature and pressure at which the liquid and gaseous phases merge into a single phase known as a supercritical fluid.

  • The temperature at this point is called the critical temperature, and the pressure is referred to as critical pressure.