Reversible and Irreversible Changes

Two kinds of change

Things change all the time. Ice melts on a warm day. A candle burns down. Bread turns into toast. But not all changes are the same. Some changes can be undone, and some cannot. Scientists put them into two groups: reversible changes and irreversible changes.

Learning to tell them apart is a big idea in chemistry, and once you can spot the difference, you will see it happening all over your home and kitchen.

Reversible changes — you can undo them

A reversible change is one you can undo to get back what you started with. Nothing new is made — the same stuff just changes its shape or its state.

Here are some everyday examples:

• Melting and freezing. Ice melts into water, and water freezes back into ice. It is the same water all along. Chocolate, butter and wax do the same thing.
• Dissolving. Stir sugar or salt into water and it seems to vanish, but it is still there. Let the water evaporate and the sugar or salt comes back. This links to what you learn in Mixtures and Solutions.
• Changing shape. Squashing clay, bending a paperclip, folding paper or stretching an elastic band all change the shape, but it is the same material. You can usually change it back.

The clue for a reversible change is this: no new substance is made. Melting and freezing are part of the bigger story of The Three States of Water.

Irreversible changes — you cannot undo them

An irreversible change makes a new substance, and once it has happened, you cannot get the original back. These changes are permanent.

Some examples:

• Burning. When wood, paper or a candle burns, it turns into ash, smoke and gases. You cannot turn ash back into wood.
• Cooking and baking. Raw egg becomes solid when fried. Cake batter becomes cake. Bread becomes toast. None of these can be undone.
• Rusting. When iron gets wet and is left in the air, it slowly turns into orange, flaky rust — a new substance.
• Mixing certain things. Add vinegar to baking soda and it fizzes furiously, making a new gas (carbon dioxide). The fizzing is a sign that something new has formed.

How to spot a new substance

How do you know a brand new substance has been made? Look out for these signs of change:

Sign	Example
A new colour	A sliced apple turning brown
Bubbles of gas	Vinegar fizzing on baking soda
A new smell	Toast smelling different from bread
Heat or light given off	A candle flame, a sparkler
Something solid forming	A boiled egg going firm

If you see one or more of these, an irreversible change has probably happened.

Why the difference matters

Knowing whether a change can be undone is genuinely useful. Cooks rely on irreversible changes to turn raw food into a meal. Recyclers melt glass and metal — reversible changes — so the material can be used again and again. Builders mix cement with water in a change that cannot be undone, which is exactly why a wall stays solid. And firefighters understand that burning is irreversible, so they work to stop it before it spreads.

Try this — a safe ice and chocolate test

With a grown-up, do two changes side by side. First, put an ice cube on a plate and watch it melt into water, then pop the water in the freezer and watch it become ice again — a reversible change. Next, ask a grown-up to gently melt a few chocolate chips, then let them cool and set — also reversible, because it is still chocolate. For comparison, look at a piece of toast in the kitchen: the bread became toast and cannot turn back. You have now seen reversible and irreversible changes with your own eyes. Always let an adult handle anything hot.

Reversible or irreversible — every change tells you something about what is happening to the stuff around you.