Year 6 · Science · Chemical sciences
Reversible and irreversible changes
Students classify changes such as melting, dissolving, cooking, burning and rusting as reversible or irreversible and look for signs that a new substance has formed.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
Learning goals
Learners will
- Explain the difference between reversible and irreversible changes.
- Identify signs that a new substance has been made.
- Classify everyday changes.
Success looks like
- I can give two examples of reversible changes and two of irreversible changes.
- I can list signs that a new substance has formed.
- I can explain my classification using evidence.
The big idea
Materials can change in many ways. Some changes can be undone. These are reversible changes.
Melting and freezing are reversible: melted chocolate can be cooled and set again. Dissolving is reversible too: if salty water is left to evaporate, the salt is left behind.
Other changes make new substances and cannot easily be undone. These are irreversible changes. Cooking an egg, burning wood, baking a cake and rusting iron are all irreversible.
Signs that a new substance may have formed include fizzing (bubbles of gas being produced), a colour change that cannot be reversed, a new smell, or light and heat being given off, as when wood burns.
When bicarbonate of soda and vinegar are mixed, they fizz. The bubbles are carbon dioxide gas, a new substance. You cannot get the bicarbonate of soda and vinegar back, so the change is irreversible.
Bushfires are a powerful Australian example of irreversible change: wood and leaves burn to form ash, smoke and gases.
Worked examples
Getting salt back
Show that dissolving salt in water is reversible.
- Stir a teaspoon of salt into a small amount of warm tap water until it dissolves.
- Pour a little onto a saucer and leave it in a sunny spot for a few days.
- The water evaporates and salt crystals are left behind.
Answer: Dissolving is reversible: evaporation gets the salt back.
The fizzing test
Is mixing bicarbonate of soda and vinegar reversible or irreversible?
- On a tray, put a teaspoon of bicarbonate of soda in a cup.
- Add a tablespoon of vinegar and watch.
- It fizzes, giving off bubbles of carbon dioxide gas.
- A new substance has formed and we cannot get the original materials back.
Answer: Irreversible: a new substance (carbon dioxide gas) is made.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
Which change is reversible?
Melting ice — Melted ice can be frozen again.
Which is a sign that a new substance has formed?
Bubbles of gas being produced when two substances mix — Gas being produced (when nothing is boiling) shows a new substance has formed.
How can you get salt back from salty water?
Let the water evaporate — When the water evaporates, the salt is left behind.
Baking a cake is irreversible because…
New substances form and you cannot get the eggs and flour back — Baking makes new substances; the ingredients cannot be separated again.
Which pair are both reversible changes?
Melting and dissolving — Melting and dissolving can both be reversed.
Which gas makes the bubbles when bicarbonate of soda and vinegar mix?
carbon dioxide — The reaction produces carbon dioxide gas.
Is burning a piece of paper reversible or irreversible?
irreversible — Burning forms ash, smoke and gases; you cannot get the paper back.
Water freezing into ice is a reversible or irreversible change?
reversible — Ice can be melted back into water.
Watch out for
- Students often think dissolving is irreversible because the solid 'disappears'. Evaporation shows it is still there.
- Some think every change involving heat is irreversible. Melting uses heat but is reversible.
- Some think fizzing is boiling. Fizzing in a cold mixture is a new gas being made.
Tips for parents
- Work on a tray and keep paper towel handy for spills.
- Ask 'Can we get the original back? How do you know?' for each change.
- Safety: wear eye protection for the fizzing test, never seal the mixture in a closed bottle, never taste experiment mixtures, and do not use heat sources other than warm tap water and sunlight.
Go further
- Make a 'changes' poster from photos of kitchen activities, labelled reversible or irreversible.
- Leave a steel wool pad (soap-free) damp on a saucer for a week and observe rust forming.
- Find out how bushfire-adapted Australian plants, such as some banksias, release seeds after fire.