73. Specific Latent Heat

GCSE Physics: Specific Latent Heat

Specific latent heat is the amount of energy required to change the state of 1 kilogram of a substance without changing its temperature.

1. Temperature and State Changes

Normally, heating a substance increases the kinetic energy of its particles, which raises the temperature. However, during a change of state (melting or boiling), the temperature remains constant even though energy is still being added.

• Breaking Bonds: During melting or boiling, the energy supplied is used to weaken or break the intermolecular forces holding the particles together, rather than increasing their internal kinetic energy.

• Forming Bonds: During cooling (condensing or freezing), new bonds form between particles. This process releases energy, which counteracts the cooling and keeps the temperature constant until the state change is complete.

2. Types of Specific Latent Heat

There are two distinct types of specific latent heat depending on which state change is occurring:

• Specific Latent Heat of Fusion ($L_f$): The energy change when a substance transitions between a solid and a liquid (melting or freezing).

• Specific Latent Heat of Vaporization ($L_v$): The energy change when a substance transitions between a liquid and a gas (evaporating or condensing).

3. The Specific Latent Heat Equation

The energy required or released during a state change can be calculated using the formula:

\text{Energy (E)} = \text{Mass (m)} \times \text{Specific Latent Heat (L)}

• Energy (E): Measured in Joules (J).

• Mass (m): Measured in kilograms (kg).

• Specific Latent Heat (L): Measured in Joules per kilogram (J/kg).

4. Example Calculation

Question: How much energy is required to completely boil 2.5 kg of water at 100°C? (Specific latent heat of vaporization for water = 2,260,000 J/kg).

1. Identify values: m = 2.5kg, L = 2,260,000J/kg.

2. Apply formula: E = 2.5 x 2,260,000$.

3. Result: 5,650,000 Joules (or 5.65 MJ).

5. Summary Table