Holywell Primary and Nursery School

Ch Ch Ch Ch Changes

Reversible changes

Evaporation and condensation are changes of state:

  • evaporation involves a liquid changing to a gas
  • condensation involves a gas changing to a liquid.

Evaporation is the reason why damp clothes dry on a washing line. Condensation is the reason why windows become foggy on a cold day.


The particles in a liquid have different energies. Some will have enough energy to escape from the liquid and become a gas. The remaining particles in the liquid have a lower average kinetic energy than before, so the liquid cools down as evaporation happens. This is why sweating cools you down. The sweat absorbs energy from your skin so that it can continue to evaporate.


The particles in a gas have different energies. Some may not have enough energy to remain as separate particles, particularly if the gas is cooled down. They come close together and bonds form between them. Energy is released when this happens. This is why steam touching your skin can cause scalds: not only is the steam hot, but energy is released into your skin as the steam condenses.

Factors affecting the rate of condensation and evaporation

The rate of condensation increases if the temperature of the gas is decreased. On the other hand, the rate of evaporation increases if the temperature of the liquid is increased.

Libby used a tea towel to hold a cold plate above a boiling kettle.  As the kettle boiled, the water inside it began to evaporate. As this evaporated water (steam) hit the cold surface of the plate, it condensed back into water. As it dripped of the plate, we caught the water in a mug.     This is a good example of a reversible change.

Irreversible changes

A change is called irreversible if it cannot be changed back again. For example you cannot change a cake back into its ingredients again.

Irreversible changes are permanent. They cannot be undone.

In an irreversible change, new materials are always formed. Sometimes these new materials are useful to us.


Heating can cause an irreversible change. For example you heat a raw egg to cook it. The cooked egg cannot be changed back to a raw egg again.

When you cook a egg the change is irreversible


Burning is an example of an irreversible change. When you burn paper you get ash and smoke. You cannot change the ash and smoke back to paper again.  However, Miss Beckford did try?

Yet again, scientific facts proven by the 5MH science team.

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