Reversible and Irreversible Changes KS2

What is a reversible change?

A reversible change is a change that can be undone. It might change how a material looks or feels, but no new materials are made.  

Examples of reversible changes:

Melting

• Melting ice is a reversible change - water can be frozen to form ice again.
• Melted chocolate and melted wax, once cooled, will return to being a solid, although their shape might be different.

Freezing

• Freezing water is a reversible change - ice can be melted back to water.
• Juice can be frozen to make ice lollies, but, as they warm up once out of the freezer, the lollies will melt and change back into juice.

Dissolving 

• Soluble substances like salt can be dissolved in water. Even though it looks like it has disappeared, it has just mixed with the water. This mixture is called a solution. Salt that has dissolved can be returned back to salt by evaporating the water.

Evaporation

• When water gets hot, for example when a kettle boils or when you have a hot shower in the bathroom, the water can change state from a liquid to a gas. When the gas cools down or touches something cold, it turns back into a liquid. This is called condensation.

Water being heated in a pan and turning into steam (a gas).

What is an irreversible change?

An irreversible change is a change that cannot be undone. It is permanent. New materials are formed during the change.

Examples of irreversible changes:

Heating

• Heating can cause irreversible changes. Once you cook a raw egg, you cannot reverse this change to make it a raw egg again. Whilst the temperature change is reversible (the cooked egg will cool down), the changes to other properties of the egg are irreversible.

Burning

• When wood is burnt, ash and smoke are produced. These new materials cannot be changed back into wood again.

Mixing

• Mixing substances can cause irreversible changes. When you mix the ingredients to make bread together, they cannot be separated again.
• Mixing water with plaster of Paris is an irreversible change - the water reacts with the plaster to create a new, hard material.

Doctor wrapping plaster of Paris bandages around a patient's foot to create a cast.

When should children learn about reversible and irreversible changes?

According to the National Curriculum Science objectives, children should learn about reversible and irreversible changes in KS2. It is assigned as compulsory learning in Year 5 under the 'Properties and changes of materials' strand of learning.

Pupils should be taught to:

• demonstrate that dissolving, mixing and changes of state are reversible changes
• explain that some changes result in the formation of new materials, and that this kind of change is not usually reversible, including changes associated with burning and the action of acid on bicarbonate of soda.

These objectives, and others, are covered in this ready-to-teach Year 5 scheme of work, Properties and Changes of Materials.