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.