Phases-Changes-Heating-and-Cooling-Curves-and-Phase-Diagrams

Phase Changes

• Phase change refers to the transition of a substance from one state of matter (solid, liquid, gas) to another.

Energy and Molecular Order Changes

• Phase changes depend on:

  • Nature of the substance

  • Temperature and pressure of the environment

• Phase change involves:

  • Gain/absorption of heat (endothermic) or loss/release of heat (exothermic).

Processes Associated with Phase Changes

• Specific processes of phase change shown in diagrams include:

  • Sublimation: solid to gas

  • Fusion: solid to liquid

  • Vaporization: liquid to gas

• Accompanying energy changes during phase changes:

  • Heat energy is absorbed during the transition between solid-liquid and liquid-gas.

Reverse Phase Changes

• Reverse processes include:

  • Solidification: liquid to solid

  • Condensation: gas to liquid

  • Deposition: gas to solid

• In these cases, heat energy is released.

Heating Curve

• Plotting heat energy against temperature gives rise to a heating curve, providing critical information:

  1. Initial increase in temperature with heat energy input indicates kinetic energy increase.

  2. Constant temperature indicates a phase change is occurring.

  3. Example phase changes and temperatures:

     • Solid to liquid (melting or fusion)

     • Liquid to gas (boiling or vaporization)

• Heat of fusion (Hf) and heat of vaporization (Hv):

  • Hf is added without a temperature change during melting.

  • Hv is larger than Hf, indicating more energy is needed to convert liquid to gas due to breaking attractions among liquid particles.

Cooling Curve

• Removal of heat results in a cooling curve:

  • Phase changes occur at the same temperatures as their corresponding heating curve processes.

Phase Diagram

• Different states exist at varying temperatures and pressures.

• Highlights:

  1. Triple Point: All three phases coexist at a specific temperature and pressure.

  2. Critical Point: Above this temperature and pressure, only the gas phase exists.

Phase Diagrams of Water and Carbon Dioxide

• Water Phase Diagram:

  • Triple point at (4.58 torr, 0.01°C), basis for Kelvin scale (273.15 K).

  • Solid-liquid line tilt indicates liquid water's higher density than ice.

• Carbon Dioxide Phase Diagram:

  • Solid to gas transition at 1 atm pressure.

  • Liquid CO2 requires pressure greater than 5.11 atm.

Section Assessment

• Understanding phase diagrams helps answer questions regarding vapor pressure, triple and critical points, and phase equilibrium.