Change of State

Change of state refers to the transition of matter from one physical form to another (solid, liquid, or gas) without a change in chemical composition. This process occurs at constant temperature when heat energy is either absorbed or released.

1. Latent Heat and Specific Latent Heat

Latent Heat: This is the total amount of heat energy required to change the state of a substance at a constant temperature. Unlike 'sensible heat' which changes the temperature, latent heat is used to break or form intermolecular bonds.

• Specific Latent Heat of Fusion (L_f): The amount of heat required to change 1 kg of a substance from solid to liquid state at its melting point. For ice, this is approximately 3.36 x 10⁵ J/kg.
• Specific Latent Heat of Vaporization (L_v): The amount of heat required to change 1 kg of a substance from liquid to gaseous state at its boiling point. For water, this is approximately 2.26 x 10⁶ J/kg.

Formula: Q = mL, where Q is heat energy (Joules), m is mass (kg), and L is specific latent heat (J/kg).

2. Melting, Evaporation, and Boiling

These are distinct processes involving state changes:

• Melting: Transition from solid to liquid. It occurs at a fixed temperature called the melting point.
• Boiling: Rapid evaporation throughout the bulk of a liquid. It occurs only at a specific temperature (boiling point) where the vapor pressure equals the atmospheric pressure.
• Evaporation: Slow transition from liquid to gas occurring at the surface of the liquid. It can happen at any temperature.

Differences between Evaporation and Boiling

• Evaporation: Surface phenomenon, occurs at all temperatures, causes cooling, silent.
• Boiling: Bulk phenomenon, occurs at a specific temperature, temperature remains constant, involves bubble formation throughout.

3. Influence of Pressure and Impurities

Effect on Boiling Point

• Pressure: An increase in external pressure increases the boiling point (e.g., in a pressure cooker). A decrease in pressure (at high altitudes) lowers the boiling point.
• Impurities: Dissolved substances (like salt) increase the boiling point of a liquid.

Effect on Melting Point

• Pressure: For substances that expand on freezing (like ice), increased pressure lowers the melting point (Regelation). For substances that contract on freezing, increased pressure raises the melting point.
• Impurities: Dissolved substances (like salt on icy roads) lower the melting point of a solid.

4. Applications

Change of state principles are applied in:

• Refrigerators/Air Conditioners: Utilize the cooling effect of evaporation of volatile liquids (refrigerants).
• Pressure Cookers: Use high pressure to raise the boiling point of water, cooking food faster.
• Thermal Regulation: Humans sweat to cool down via evaporation.