Thermodynamics of Pure Substances: Phases and Properties

1. Phases of Pure Substances

• Solid: Molecules are closely packed and vibrate in place due to strong intermolecular forces.
• Liquid: Molecules are closer together but remain free to move with weaker intermolecular forces.
• Vapor: Molecules are far apart and possess very high kinetic energy.

2. Phase Change Processes

• Compressed Liquid: A liquid that is not about to vaporize (e.g., water at 20°C and 1 atm).
• Saturated Liquid: A liquid that is about to vaporize (e.g., water at 100°C and 1 atm).
• Saturated Vapor: Vapor that is about to condense (e.g., steam at 100°C and 1 atm).
• Superheated Vapor: Vapor that is far from condensing (e.g., steam at 150°C and 1 atm).

3. Latent Heat

• Latent Heat of Fusion: The energy required to change 1 kg of solid into liquid.
• Latent Heat of Vaporization: The energy required to change 1 kg of liquid into vapor.
• Example: For water at 100 kPa, the latent heat of vaporization is 2258 kJ/kg.

4. Saturation Terms

• Saturation Temperature (T_sat): The temperature at which a substance changes phase at a given pressure.
• Saturation Pressure (P_sat): The pressure at which a substance changes phase at a given temperature.
• Quality (x): The ratio of the mass of vapor to the total mass in a saturated mixture, defined as: x = m_vapor / m_total.

5. Property Diagrams

• T-v Diagram: Illustrates phases at various temperatures and specific volumes.
  • Critical Point: The state where liquid and vapor phases are indistinguishable.
  • Superheated Region: The area to the right of the saturated vapor line.
  • Compressed Liquid Region: The area to the left of the saturated liquid line.

6. Property Tables

• Saturated Tables: Provide properties for substances in saturated conditions.
• Superheated Tables: Used for states beyond the vapor dome.
• Compressed Liquid Tables: Used when the liquid is below the saturation temperature.

7. Ideal Gas Law

• Applies to gases at low densities.
• Equation: Pv = RT (where v = specific volume and R = gas constant).

8. Specific Heats

• Specific Heat at Constant Volume (C_v): Energy required to raise the temperature of 1 kg of a substance by 1°C at constant volume.
• Specific Heat at Constant Pressure (C_p): Energy required to raise 1 kg of a substance by 1°C at constant pressure.