Thermodynamics Lecture: Pure Substances and Phase Changes

Pure Substances

• Definition: A substance with a fixed composition.
  • Can be composed of multiple elements (e.g., water = hydrogen + oxygen).
  • Can exist in multiple phases (e.g., liquid water and water vapor).

Piston-Cylinder System

• Heat Addition: Causes volume to increase but pressure remains constant (isobaric process).
• Temperature Changes:
  • Water at 20°C, heated to 100°C with minimal volume change.
  • Density decreases slightly as temperature increases.
  • Further heating causes phase change, producing a liquid vapor mixture at 100°C.

Phases of Water

• States:
  • Compressed Liquid
  • Saturated Liquid
  • Saturated Liquid Vapor Mixture
  • Saturated Vapor
  • Super-heated Vapor
• Phase Changes:
  • Volume expands dramatically when water becomes vapor.
  • Further heating increases temperature and vapor volume.

Property Diagrams

• T-v Diagram: Plots temperature vs. specific volume.
  • Important for visualizing phase changes at constant pressure.

Saturation Concepts

• Saturation Temperature (Tsat): Temperature for phase change at constant pressure.
• Saturation Pressure (Psat): Pressure for phase change at constant temperature.
  • Example: Boiling point changes with altitude due to pressure variations.

Pressure Cookers

• Higher pressure raises boiling temperature, speeding up cooking.

Saturation Dome

• Regions:
  • Inside: Saturated liquid vapor.
  • Left: Compressed liquid.
  • Right: Superheated vapor.
  • Top: Critical point.

Property Tables

• Provide data for water's states at different pressures and temperatures.
• Categories:
  • Compressed Liquid
  • Superheated Vapor
  • Saturated Liquid Vapor (Two tables: pressure-first and temperature-first)
• Properties: Pressure, Temperature, Specific Volume, Internal Energy, Enthalpy, Entropy.

Interpolation

• Used when exact values are not in tables.
• Linear interpolation is common but less precise than other methods.
• Software tools provide more accurate results.

Subscripts in Properties

• f: Saturated Liquid
• g: Saturated Vapor
• fg: Difference between saturated liquid and vapor (useful for calculations).

Identifying Phases

• Example Problems:
  • Using T-v diagrams to determine state/region.
  • Interpreting tables for specific properties.
• Identify phase based on T, P, and specific volume.

Conclusion

• Understanding of phase changes is crucial for thermodynamics.
• Further study includes learning about enthalpy and entropy.
• Recommended to use tables and software tools for precise calculations.

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Additional Resources: Check links for more complex examples and further lectures.