PFB1023_W6-C4.1

Chapter 4: Pure Substance: Pressure, Volume and Temperature (PVT)

Course Overview

• Learning Outcomes: At the end of the lesson, students should be able to:

  • Define the meaning of pure substance

  • Analyze the phases and their changes

  • Articulate the relationship of pressure, volume, and temperature

Pure Substances

• Definition: A pure substance has a fixed chemical composition throughout.

  • Example: Air is a mixture of gases but is treated as a pure substance due to its uniform composition.

  • Can consist of mixed chemical elements or compounds if homogeneous (e.g., CO2).

• Phases: A pure substance remains pure even when it exists in multiple phases as long as the chemical composition is the same (e.g., ice and liquid water).

Phases of Pure Substances

General Characteristics

1. Solid

   • Strong molecular bond

   • Fixed position, oscillating molecules

2. Liquid

   • Intermediate bond strength

   • Molecules can rotate and translate

3. Gas

   • Weakest bond

   • Molecules move randomly and collide

States of Liquid

1. Compressed Liquid (Subcooled Liquid)

   • Not about to vaporize

   • Temperature increases with added heat

2. Saturated Liquid

   • About to vaporize

   • Heat addition causes evaporation

3. Saturated Liquid-Vapor Mixture

   • Liquid and vapor coexist at equilibrium

   • Heat added without temperature change

4. Saturated Vapor

   • About to condense

   • Heat removal causes condensation

5. Superheated Vapor

   • Not about to condense

   • Temperature increases with added heat

Saturation and Phase Change

• Saturation: Mixture of vapor and liquid can coexist at a given temperature and pressure.

• Saturation Temperature (Tsat): Temperature at which a pure substance changes phase at a given pressure.

• Saturation Pressure (Psat): Pressure at which a pure substance changes phase at a given temperature.

Latent Heat

• Definition: Energy absorbed or released during phase changes.

• Latent Heat of Fusion: Energy absorbed during melting.

• Latent Heat of Vaporization: Energy absorbed during vaporization.

• Values at 1 atm:

  • Latent Heat of Fusion: 333.7 kJ/kg

  • Latent Heat of Vaporization: 2256.5 kJ/kg

Phase Diagram Concepts

• Critical Point: Where saturated liquid and vapor states are identical.

• Triple Point: Condition of temperature and pressure where a substance exists in three phases in equilibrium.

Phase Transition and Behavior

P-T Diagram Variations

• Solid, Liquid, Gas Regions: Defined based on temperature and pressure conditions.

• Transition Curves:

  • Sublimation curve (solid/vapor).

  • Fusion curve (solid/liquid).

  • Vaporization curve (liquid/vapor).

PV Diagram Insights

• Provides detailed volume information during phase changes.

• Areas of equilibrium indicate significant volume changes (e.g., V/L, V/S equilibrium regions).

P-V-T Surfaces

• Relationship among P, V, and T expressed as a three-dimensional surface, with PT and PV diagrams as 2D projections.

Conclusion

• Importance of understanding phase changes and behaviors in thermodynamics, particularly in engineering applications.