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  1. Describe melting and boiling in terms of energy input without a change in temperature.

This graph shows the temperature change as you supply heat energy to a solid (assuming the solid doesn’t undergo sublimation – which is when it changes phase directly from solid to gas). You’ll have the same curve when cooling a gas until it becomes a solid. Note that when it comes to cooling, energy will be lost from the substance to its surroundings, instead of being gained – this is why the cooling curve won’t be a mirror image.

As you heat a solid, its temperature rises until it reaches its melting point. As more energy is supplied, the solid melts and the temperature doesn’t change. It is now a liquid.

When you supply heat energy to a liquid, its temperature will rise until it reaches its boiling point. As the liquid vaporises, its temperature doesn’t change. It is now gas.

Heating a gas will increase its temperature.


2. Distinguish between boiling and evaporation.



  1. Describe condensation and solidification.

Condensation is when a gas changes to a liquid.

As a gas loses heat energy, they also lose kinetic energy, causing them to move slower. Their intermolecular forces of attraction become more significant, pulling the molecules together and allowing them to form weak bonds until they form a liquid.

A pretty common example of condensation is when your mirror steams up after a shower.

Solidification, or freezing, is when a liquid changes to a solid.

As liquids lose energy, their molecules slow down and form more intermolecular bonds with one another. They become locked in place, and thus, they form a solid.


  1. Use the terms latent heat of vaporisation and latent heat of fusion, and give a molecular interpretation of latent heat.

Latent heat of vaporisation is the energy supplied when a substance changes its physical state from liquid to gas. It is called the latent heat of vaporisation because it is the energy supplied when the liquid vaporises.

Latent heat of fusion is the energy supplied when a substance changes from solid to liquid. It is called the latent heat of fusion because a lot of solids are fused together by melting them and combining them, e.g. in the creation of alloys.

You might also be interested to know that the word latent is an adjective that describes something that exists, but is not yet developed or manifested or hidden or concealed.

We call it latent heat because we can’t measure this using a thermometer.

The temperature does not change when melting or boiling, because this latent heat is used to break the bonds between the molecules.

The latent heat of vaporisation is usually greater than the latent heat of fusion because normally, more bonds have to be broken when boiling than when melting.




Notes submitted by Sarah

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