Specific Latent Heat

Definition of Specific Latent Heat

Specific latent heat is the amount of energy needed to change the state of 1 kilogram of a substance without changing its temperature. This energy is used to break or form the bonds between particles during a change of state, such as melting or boiling.

During a change of state, the temperature remains constant even though energy is being transferred. This is because the energy goes into changing the arrangement of particles rather than increasing their kinetic energy.

There are two main types of specific latent heat:

• Latent heat of fusion: energy needed to change 1 kg of a substance from solid to liquid (melting) or liquid to solid (freezing).
• Latent heat of vaporisation: energy needed to change 1 kg of a substance from liquid to gas (boiling) or gas to liquid (condensing).

Calculating Energy for State Changes

The energy required to change the state of a substance can be calculated using the formula:

$$Q = m \times L$$

• $Q$ = energy transferred (in joules, J)
• $m$ = mass of the substance (in kilograms, kg)
• $L$ = specific latent heat (in joules per kilogram, J/kg)

This formula applies whether the energy is being absorbed (melting or boiling) or released (freezing or condensing).

For instance, if you want to find the energy needed to melt 2 kg of ice with a latent heat of fusion of 334,000 J/kg, you multiply:

$Q = 2 \times 334,000 = 668,000 \text{ J}$

This means 668,000 joules of energy are needed to melt 2 kg of ice at 06C without changing its temperature.

Types of Specific Latent Heat

Latent heat of fusion is the energy required to change a substance between solid and liquid states. For example, melting ice to water or freezing water to ice.

Latent heat of vaporisation is the energy required to change a substance between liquid and gas states. For example, boiling water to steam or condensing steam to water.

Both latent heats are specific to each substance. For water:

• Latent heat of fusion = 334,000 J/kg
• Latent heat of vaporisation = 2,260,000 J/kg

This means it takes much more energy to boil water into steam than to melt ice into water.

Practical Applications

Understanding specific latent heat helps explain everyday processes such as:

• Melting ice: When ice melts, it absorbs energy from the surroundings without rising in temperature, which cools the environment (used in ice packs).
• Boiling water: When water boils, it absorbs a large amount of energy to become steam, which can be used in heating systems or steam engines.
• Energy transfer during phase changes: Energy is transferred during melting, boiling, freezing, and condensing without temperature change, important in weather phenomena and industrial processes.

For example, when boiling a kettle, the water temperature stays at 1006C while it changes from liquid to steam, even though energy is still being supplied.

The energy absorbed or released during these changes is crucial in many natural and technological systems, such as climate regulation, refrigeration, and heating.

Example:

Calculate the energy needed to boil 0.5 kg of water at 1006C. The latent heat of vaporisation of water is 2,260,000 J/kg.

Using $Q = m \times L$:

$Q = 0.5 \times 2,260,000 = 1,130,000 \text{ J}$

This means 1,130,000 joules of energy are needed to turn 0.5 kg of water into steam at 1006C.