Thermo January 20

Chemical Composition and Pure Substances

  • Definition of Pure Substance

    • A pure substance has a consistent chemical composition throughout, allowing it to be characterized uniformly in a specific area.
    • Examples discussed include water (H2O) and air.

  • Examples of Pure vs Mixture

    • Water System:

    • State: In both liquid and vapor forms, the chemical composition is H2O.

    • Conclusion: Water is a pure substance as it maintains the same composition in both phases.

    • Air System:

    • Contains multiple components (e.g., nitrogen, oxygen, CO2) in different proportions, thus varying in composition between liquid and vapor phases.

    • Conclusion: Air is not a pure substance due to its varying compositions.

Phase Coexistence

  • Two-Phase Systems
    • Mixtures can consist of liquid and vapor phases.
    • Most common practical application: Refrigeration systems that cycle between liquid and vapor states.
    • Importance of examining coexistence, especially for water, in thermodynamics, where power generation often involves phase changes of water at different states.

Water System and Thermodynamics

  • Introduction to the Water System:

    • The water system is crucial in thermodynamics due to its role in power production.

  • Phase Change Process of Pure Substances:

    • Starting with water at room temperature (20°C), under normal atmospheric pressure (101 kPa).
    • Specific Volume (v): Indicated as a small variable defining the volume per unit mass.
    • Liquid water at room temperature is identified as a "compressed liquid."

  • Heating Process:

    • When heat is added:
    • Temperature increases, reaching 50°C and later 100°C.
    • During heating, the pressure is maintained at atmospheric pressure (1 atm).
    • The piston in the cylinder allows for volume expansion due to increased temperature, raising specific volume until vaporization (boiling) point at 100°C.

Phases of Water Under Pressure

  • States of Water:

    • Compressed Liquid:

    • All liquid phase; temperature can increase without phase change until the saturation point.

    • Saturated Liquid:

    • Begins to vaporize as it reaches 100°C, while pressure is held constant.

    • Vapor bubbles form within the liquid phase.

    • Saturated Liquid-Vapor Mixture:

    • As heat continues to be added, a mixture of liquid and vapor occurs, with the pressure remaining constant.

    • Saturated Vapor:

    • Contains 100% vapor phase post complete vaporization of liquid.

    • Superheated Vapor:

    • Further heating increases temperature and specific volume while having 100% vapor phase.

Pressure and Temperature Effects

  • Critical Point:

    • The point in a phase diagram where the saturated liquid and vapor states become indistinguishable due to sufficiently high pressure and temperature.
    • Distinguishing factors:
    • Critical Temperature: The temperature at which the properties of liquid and vapor phases converge.
    • Critical Pressure: The minimum pressure required to maintain the vapor phase at a critical temperature.

  • Example of Water's Critical Point:

    • Specific volume at the critical point for water is 0.003155 m³/kg, representative of the phase transition where liquid and vapor cannot be differentiated.

Saturated State Characteristics

  • Saturated Temperature:

    • Temperature at which a pure substance changes phases at a given pressure.

  • Saturated Pressure:

    • Conversely, the pressure at which a pure substance changes phases at a specific temperature.

  • Importance of the relationship between temperature and pressure:

    • Higher pressure increases boiling points (e.g., critical points), crucial for applications like cooking at altitude where pressures are lower.

Latent Heat

  • Definition:

    • The energy absorbed or released during a phase change, with two principal forms:
    • Latent Heat of Vaporization: Energy absorbed when transitioning from liquid to vapor.
    • Latent Heat of Fusion: Energy absorbed when transitioning from solid to liquid.

  • Practical Example of Latent Heat:

    • Melting 1 kg of ice at 0°C requires 334 kJ of energy to become liquid water.

Note: A firm understanding of the phase changes of water and the associated thermodynamic properties is essential for effective application in engineering and technological processes.