Summary
Highlights
When a substance is heated, its particles gain kinetic energy, increasing internal energy and temperature. However, during a change of state (melting or boiling), the energy supplied is used to break the forces holding particles together, not to increase internal energy, thus keeping the temperature constant until the state change is complete. The same principle applies during cooling, where the formation of bonds releases energy, counteracting cooling and maintaining a constant temperature during the state change.
The energy required to change the state of a substance without changing its temperature is known as latent heat, which depends on the substance's type and amount. To standardize this, 'specific latent heat' (SLH) is defined as the energy required to change 1 kilogram of a substance from one state to another without a temperature change. For cooling, it's the energy released during a state change.
There are two types of specific latent heat: specific latent heat of vaporization, for changes between liquid and gas (evaporation or condensation), and specific latent heat of fusion, for changes between solid and liquid (melting or freezing).
Considering 1 kg of water heated from -50°C (ice) to 150°C (steam), the temperature increases until 0°C, where specific latent heat of fusion is applied to melt the ice into water (334,000 J/kg for water). Once melted, the temperature rises again until 100°C, where specific latent heat of vaporization is required to boil the water into steam (2,260,000 J/kg for water). After all the water has become steam, the temperature continues to rise normally. These values represent energy required during heating and energy released during cooling.
The equation for specific latent heat is: Energy (J) = Mass (kg) × Specific Latent Heat (J/kg). An example calculation for boiling 2.5 kg of water at 100°C using the specific latent heat of vaporization (2,260,000 J/kg) yields 5,650,000 J or 5,650 kJ.
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