Specific latent heat calculator: E = mL

• Specific latent heat of fusion: solid to liquid (melting) or back. For water (ice), about 334,000 J/kg.
• Specific latent heat of vaporisation: liquid to gas (boiling) or back. For water, about 2,260,000 J/kg.

Vaporisation needs much more energy because the particles must be separated completely. Use the value the question gives: some textbooks round these figures.

• Boiling water: turning 0.5 kg of water at 100 °C into steam. $E = 0.5 \times 2260000 = 1130000$ J (1.13 MJ).
• Melting ice: 200 g of ice at 0 °C. Mass = 0.2 kg, so $E = 0.2 \times 334000 = 66800$ J.
• Finding L: 34,000 J melts 0.1 kg of a solid at its melting point. $L = 34000 \div 0.1 = 340000$ J/kg.

On a graph of temperature against time while a substance is heated, the flat sections are changes of state. Energy is still going in, but it breaks bonds between particles instead of raising the temperature. The sloping sections are where the temperature rises: use $\Delta E = mc\Delta\theta$ there, with our specific heat capacity calculator.

To turn ice at −10 °C into water at 20 °C you need three amounts of energy: heating the ice to 0 °C ($mc\Delta\theta$ with ice's specific heat capacity), melting it ($mL$), then heating the water to 20 °C ($mc\Delta\theta$ with water's). Add them up. Set the working out in three lines so each mark is easy to see.

• Using $mc\Delta\theta$ for a change of state (there's no temperature change).
• Mixing up fusion and vaporisation values.
• Leaving mass in grams.

Practise with our specific latent heat questions, and see our guide to specific heat capacity.

• Sweating: evaporation takes energy (latent heat of vaporisation) from your skin, cooling you down.
• Steam burns: steam at 100 °C releases its latent heat as it condenses on skin, so it burns worse than water at the same temperature.
• Ice packs: melting ice takes in energy from a drink without its own temperature rising, keeping the drink cold until it has all melted.
• Refrigerators: a refrigerant evaporates inside the fridge (taking in energy) and condenses outside (giving it out).