Properties of Pure Substances

Properties of Pure Substances

Introduction to Pure Substances

• Definition: A pure substance is a material with a uniform and invariable chemical composition.
• Phases: Can exist in one or more phases (solid, liquid, gas) with the same composition.
• Examples: Water, nitrogen, carbon dioxide.

Key Characteristics:

• Homogeneous Composition: Uniform within the material.
• Fixed Thermodynamic Properties: Properties remain stable at a given state.
• Phase Changes: Can undergo various phase transitions (e.g., melting, vaporization).

Phases and Phase Changes

Main Phases of a Pure Substance:

1. Solid:
   • Fixed shape and volume.
   • Strong intermolecular forces.
2. Liquid:
   • Fixed volume but takes the shape of the container.
3. Gas/Vapor:
   • No fixed shape or volume.
   • Weaker intermolecular forces compared to solids and liquids.

Phase Change Processes:

• Melting (Fusion): Solid to liquid
• Vaporization: Liquid to vapor
• Sublimation: Solid directly to vapor
• Condensation: Vapor to liquid
• Freezing: Liquid to solid
• Deposition: Vapor directly to solid

Triple Point and Critical Point

Triple Point:

• Definition: The unique condition where solid, liquid, and gas phases coexist in equilibrium.
• For Water:
  • Temperature (T) = 0.01°C (273.16 K)
  • Pressure (P) = 0.6117 kPa

Critical Point:

• Definition: The state beyond which liquid and gas phases become indistinguishable (supercritical fluid).
• For Water:
  • Temperature (T) = 374°C (647 K)
  • Pressure (P) = 22.064 MPa
• Beyond this point, no distinct boiling occurs.

Properties of Steam and Steam Tables

Definitions of Key Terms:

• Saturated Liquid (Compressed Liquid): Water at boiling point in liquid form.
• Saturated Vapor: Steam at boiling point with no liquid.
• Wet Steam: Mixture of saturated liquid and saturated vapor.
• Dry Steam (Saturated Steam): Steam with no liquid droplets.
• Superheated Steam: Steam heated beyond saturation temperature.

Steam Tables:

• Provide thermodynamic properties of water and steam at varying temperatures and pressures.
• Includes:
  • Saturation Temperature (Tₛₐₜ): Boiling point at a given pressure.
  • Enthalpy (h): Total heat content (kJ/kg).
  • $h_f$: Enthalpy of saturated liquid
  • $h_{fg}$: Latent heat of vaporization
  • $h_g$: Enthalpy of saturated vapor
  • Entropy (s): Measure of disorder (kJ/kg·K).
  • Specific Volume (v): Volume per unit mass (m³/kg).

Temperature-Enthalpy (T-h) Diagram

• Purpose: Plots temperature (T) against enthalpy (h). Helps visualize heat addition during phase changes.
• Key Regions:
  • Subcooled Liquid (Compressed Liquid) Region
  • Saturation Region (Phase Change)
  • Superheated Vapor Region

Temperature-Entropy (T-s) Diagram

• Purpose: Plots temperature (T) against entropy (s). Useful for analyzing thermodynamic cycles (e.g., Rankine cycle).
• Features:
  • Saturation Dome (where liquid and vapor coexist)
  • Critical Point (peak of the dome)
  • Isotherms (constant temperature lines)

Enthalpy-Entropy (Mollier) Diagram

• Purpose: Plots enthalpy (h) against entropy (s). Useful for analyzing steam turbines and compressors.
• Key Features:
  • Constant Pressure Lines (Isobars)
  • Constant Temperature Lines (Isotherms)
  • Quality Lines (Dryness Fraction Lines)

Thermodynamic Properties During Phase Change

• Latent Heat (h_fg): Energy required for phase change without temperature change.
• Dryness Fraction (x): Ratio of mass of vapor to total mass in wet steam:
  x = \frac{m{vapor}}{m{total}} = \frac{m{vapor}}{m{liquid} + m_{vapor}}
• Quality of Steam:
  • $x = 0$ → Saturated liquid
  • $0 < x < 1$ → Wet steam
  • $x = 1$ → Dry saturated steam

Applications of Steam Properties

• Power Plants (Rankine Cycle): Utilize superheated steam in steam turbines.
• Refrigeration & Heat Pumps: Apply phase change principles.
• Chemical Processes: Use steam for heating and reactions.

Conclusion

• Understanding properties of pure substances, especially water and steam, is crucial for thermodynamics and engineering applications.
• Steam tables, T-s, h-s, and Mollier diagrams are essential tools for energy systems analysis.
• The triple point and critical point determine limits of phase behavior, while enthalpy and entropy are vital for efficient thermal system design.

References

• Engineering Chemistry by Prasanta Rath, Cengage India.
• Thermodynamics: An Engineering Approach by Yunus A. Çengel & Michael A. Boles.