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Heating houses

Changing state

A substance must absorb heat energy so that it can melt or boil. The temperature of the substance does not change during melting, boiling or freezing, even though energy is still being transferred.

A heating curve is a graph showing the temperature of a substance plotted against the amount of energy it has absorbed. You may also see a cooling curve, which is obtained when a substance cools down and changes state.

Water changes from solid ice to liquid water at zero degrees celcius. Water turns from liquid water to water vapour gas at 100 degrees celcius.

A heating curve for ice

Notice that the temperature stays the same during a change of state, melting or boiling, even though heat energy is still being absorbed.

The temperature also stays the same while a liquid freezes, even though heat energy is still being released to the surroundings.

Specific latent heat

The specific latent heat of a substance is a measure of how much heat energy is needed to melt or boil it. It is the energy needed to melt or boil 1 kg of the substance.

Different substances have different specific latent heats. The specific latent heat of a given substance is different for boiling than it is for melting. The table shows some examples.

Latent heats of substances

substancespecific latent heat of melting kJ/kg specific latent heat of boiling kJ/kg

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