How do you calculate the energy needed for a change of state?

To calculate the energy needed for a change of state, use the formula: Energy = Mass × Specific Latent Heat.

When a substance changes state, such as from solid to liquid or liquid to gas, it requires energy. This energy is called latent heat. The specific latent heat is the amount of energy needed to change the state of 1 kilogram of a substance without changing its temperature. There are two types of specific latent heat: latent heat of fusion (for melting/freezing) and latent heat of vaporisation (for boiling/condensing).

The formula to calculate the energy required for a change of state is:
\[ \text{Energy (E)} = \text{Mass (m)} \times \text{Specific Latent Heat (L)} \]

Here, the energy (E) is measured in joules (J), the mass (m) in kilograms (kg), and the specific latent heat (L) in joules per kilogram (J/kg).

For example, if you want to calculate the energy needed to melt 2 kg of ice (specific latent heat of fusion for ice is 334,000 J/kg), you would use the formula:
\[ E = 2 \, \text{kg} \times 334,000 \, \text{J/kg} = 668,000 \, \text{J} \]

This means 668,000 joules of energy are required to melt 2 kg of ice.

Similarly, if you need to find the energy required to boil 1.5 kg of water (specific latent heat of vaporisation for water is 2,260,000 J/kg), you would calculate:
\[ E = 1.5 \, \text{kg} \times 2,260,000 \, \text{J/kg} = 3,390,000 \, \text{J} \]

This tells you that 3,390,000 joules of energy are needed to turn 1.5 kg of water into steam. Understanding these calculations is crucial for analysing energy changes in various physical processes.