Reversible and irreversible changes

Changes that can be made to substances can be split into two main groups:

A reversible change is a change that can be undone or reversed such as:

• dissolving
• melting
• freezing
• evaporation
• mixing

If you can get back the substances that you started the reaction with, then that's a reversible reaction. For example, water can be frozen and turned into ice, and if you heat the ice then it melts and turns back into water.

A reversible change might change how a material looks or feels.

Did you know?

Some substances can go through more than one reversible change and still return to how they were before.

Water can be frozen to make ice, melted to make water again, evaporated to make steam and condensed back to water again, and it will still be the original substance.

An irreversible change means that once something has been changed, it cannot be returned to its former state. This is usually because a chemical reaction has happened and a new product has been made.

There are lots of examples of this.

Did you know?

Irreversible changes happen all around us all the time, and without them our lives would be very different.

Lots of food that we make is a result of irreversible changes; the cakes we bake and eat, the bread we toast, and the eggs we scramble.

Other examples of irreversible changes are cement, which is used to construct buildings, and petrol which is burned as fuel in cars.

• You can make your own volcano using irreversible change. Mixing vinegar and bicarbonate of soda creates a chemical reaction that produces lots of carbon dioxide bubbles, as well as water and a salt. Be sure you ask an adult to help you to make it safely.

• Water can be a solid, a liquid or a gas. Water can go through all three states as part of the water cycle.

• We can see irreversible changes when we cook. When you bake a cake it's impossible to turn it back into its original form of flour, eggs, butter and sugar.

• When wood is burned, it turns to ash which is an irreversible change and can't be returned to its original form.

• Dissolving is a reversible change.

• On average, an evaporated drop of water spends around 10 days in the air before condensing and falling as rainfall.

• Evaporation accounts for about 90% of the moisture in the Earth's atmosphere.

• Ice melts and becomes water at 0°C, and water boils and becomes steam at 100°C.

Chemical reaction – The way in which two or more chemicals react to create a different chemical.

Dissolve – When a solid substance has mixed with a liquid to make a transparent (see-through) liquid.

Evaporation – When a liquid turns into a gas slowly, at temperatures below the normal boiling point.

Freezing – The process that sees a liquid change state from a liquid to solid as it cools.

Irreversible change – A change that cannot be undone or reversed.

Melting – When something changes state from a solid to a liquid due to heat.

Mixture – Something that is created when two or more substances are combined together.

Product – A substance formed out of a chemical reaction.

Reversible change – A change that can be undone or reversed.

Substance – Any solid, liquid or gas with its own properties.

Investigate some changes (with a grown up to help) and classify them as reversible or irreversible.

You could try:

• stirring some sand into water
• toasting bread
• burning a candle
• adding some vinegar to bicarbonate of soda
• melting an ice cube