Properties of Pure Substances

Pure Substances

  • A pure substance has a fixed composition throughout.

  • It can be a single element, a compound, or a homogeneous mixture of elements or compounds.

  • Key examples:

    • Water in solid, liquid, and vapor phases.

    • Mixture of liquid water and water vapor.

    • Gases like $ ext{CO}2$ (carbon dioxide) and $ ext{N}2$ (nitrogen), as long as no phase change occurs.

/

/

Phases of Pure Substances

  • Pure substances exist in various phases under different conditions, for example:

    • Copper is solid at room temperature.

    • Mercury is liquid at room temperature.

  • The three principal phases are:

    • Solid: Molecules are tightly bound, forming a regular pattern.

    • Liquid: Molecules have intermediate intermolecular bonds and can flow.

    • Gas: Molecules have weak intermolecular bonds, move rapidly, and cannot stick together.

Phase Change Processes

  • Two phases of a pure substance can coexist in equilibrium. Example: water in a boiler where liquid and vapor phases coexist.

  • Solid to Liquid (Melting) and Liquid to Gas (Vaporization) are common phase change processes.

  • Different phases have varying intermolecular forces:

    • Strong in solids, intermediate in liquids, and weak in gases.

Property Diagrams

  • Diagrams such as T-v (Temperature vs. specific volume) and P-v (Pressure vs. specific volume) help visualize phase changes:

    • Critical Point: The temperature and pressure above which liquid and vapor phases can't be distinguished.

    • Saturation Line: Represents equilibrium between liquid and vapor phases.

    • Regions in diagrams:

    • Compressed Liquid Region

    • Saturated Liquid-Vapor Region

    • Superheated Vapor Region

Saturation

  • Saturation occurs when liquid and vapor phases exist together at specific temperature and pressure.

  • The relationship between saturation pressure and saturation temperature is direct; higher pressure results in higher saturation temperature.

  • Whence saturation pressure and temperature can be graphed on a vapor pressure curve.

Quality of a Mixture

  • Quality (x) defines the state of a mixture of liquid and vapor at saturation:

    • Quality is calculated as the ratio of the mass of vapor to the total mass of both vapor and liquid.

    • For example, if the mass of vapor is 0.2 g and the mass of liquid is 0.8 g, the quality is:
      x = \frac{0.2}{0.2 + 0.8} = 0.2 \text{ or } 20\%

Latent Heat

  • Latent heat refers to the energy involved during phase changes:

    • Latent Heat of Fusion: Energy absorbed during melting (equivalent energy released during freezing).

    • Latent Heat of Vaporization: Energy absorbed during vaporization (equivalent energy released during condensation).

    • Examples include:

      • Latent heat of fusion for water: 333.7 kJ/kg

      • Latent heat of vaporization for water: 2256.5 kJ/kg

Important Definitions

  • Critical Point: Condition above which liquid and vapor phases cannot be distinguished.

  • Triple Point: Condition where solid, liquid, and vapor coexist in equilibrium.

  • Sublimation: Solid to vapor.

  • Vaporization: Liquid to vapor.

  • Condensation: Vapor to liquid.

  • Fusion: Solid to liquid.

Moisture Content

  • Moisture content is the opposite of quality:

    • It measures the ratio of liquid mass to the total mass of liquid and vapor.

Ideal Gas Equation of State

  • The ideal gas behavior can be expressed as: PV = nRT where:

    • P = Pressure,

    • V = Volume,

    • n = Number of moles,

    • R = Ideal gas constant (specific to the gas).

  • Relationships between pressure, volume, and temperature can determine state changes in closed systems, such as increasing volume or changing temperatures after adjusting conditions.