Latent heat

When a substance changes its state from a solid to liquid or from a liquid to a gas heat energy is needed. This energy is used not to heat up the substance but to separate the molecules from each other.

This energy is called LATENT HEAT energy.
While a solid is melting and while a liquid is boiling there is no temperature change. The temperature only changes when the change of state is complete.




The units for specific latent heat are joules/kilogram (J/kg).

It may take a different amount of heat to change the solid into a liquid than to change the liquid into a gas and so a substance will have two specific latent heats.

The heat needed is given by the equation:


When a gas condenses and when a liquid freezes latent heat is given out. The amount of heat that was used to melt or boil the substance is released. When steam at 100^oC condenses to water it gives out a lot of heat and we can use this fact to explain why a scald from steam at 100^oC is much more painful than a scald from water at the same temperature.



We can also calculate the mass of water that will be turned to steam if a kettle is left boiling (at 100^oC) for 5 minutes. Will the kettle boil dry?



The large specific heat and specific latent heat of water make it a very good liquid for use as a coolant in many types of machinery. It is also available very cheaply and in large quantity.

Cooling

An object gets colder because it is loosing heat energy to its surroundings. As it cools its temperature obviously drops. This goes on until the object reaches the same temperature as its surroundings. You can tell when this happens because the graph of temperature against time becomes flat. Another reason for the graph flattening out is that the substance might be turning from a gas into a liquid or from a liquid into a solid.