Reversible and irreversible changes

Introduction

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

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.

Mixing

Mixing substances can cause an irreversible change. For example, when vinegar and bicarbonate of soda are mixed, the mixture changes and lots of bubbles of carbon dioxide are made. These bubbles, and the liquid mixture left behind, cannot be turned back into vinegar and bicarbonate of soda again.

Burning

Burning is an example of an irreversible change. When you burn wood you get ash and smoke. You cannot change the ash and smoke back to wood again.

Reversible changes

A reversible change is a change that can be undone or reversed.

A reversible change might change how a material looks or feels, but it doesn’t create new materials.

Melting

Melting is an example of a reversible change. For example melted chocolate can be changed back into solid chocolate by cooling.

Freezing

Freezing is an example of a reversible change. For example we can freeze orange juice to make ice lollies. The ice lollies can be changed back into orange juice by heating.

Boiling. evaporating and condensing

Boiling, evaporating and condensing are all examples of reversible changes. For example, if you could capture all the steam that is made when a kettle boils, you could turn it back to water by cooling it.

Dissolving

Dissolving is an example of a reversible change. For example, when salt is mixed with water it disappears because it dissolves in the water to make salty water. But we can get the salt can back again by boiling off the water. That leaves the salt behind.

Dissolving

Some substances dissolve when you mix them with water. When a substance dissolves, it looks like it disappears. But in fact it has just mixed with the water to make a transparent (see-through) liquid called a solution.

When you mix sugar with water, the sugar dissolves to make a transparent solution. Salt dissolves in water too.

• Substances that dissolve in water are called soluble substances.

• Substances that do not dissolve in water are called insoluble substances.

When you mix sand or flour with water, they do not dissolve.

 

Separating mixtures

Sieving

A mixture made of solid particles of different sizes, for example sand and gravel, can be separated by sieving.

Filtering

A mixture of water and an insoluble substance like sand can be separated by filtering.

Evaporating

By dissolving salt in water we make a solution. The salt disappears into the water. We can separate the salt from the water by boiling the solution. The water will evaporate until it is all gone. The salt will be left behind.

If we collect the water vapour that evaporates we can cool it to form water again.