Reversible vs Irreversible Changes
A reversible change is one that can be undone to get back the original material, while an irreversible change cannot easily be undone because new substances have formed. In brief: melting ice is reversible, since cooling the water gives ice again, but burning a candle or baking a cake is irreversible, because you cannot turn ash and smoke back into wax or cake batter.
The idea of reversibility is a helpful first test when classifying changes. Most reversible changes are physical changes, and most irreversible changes are chemical changes. The rule is useful but has exceptions, so this article explains it carefully, with examples, a comparison table, a worked classification exercise and typical pitfalls.
What is a reversible change?
In a reversible change, the substance keeps its chemical identity, and the original material can be recovered by reversing the conditions. Changes of state are the classic examples: heating ice gives water, and cooling the water gives ice again. Dissolving salt in water is also reversible, because evaporating the water leaves the salt behind.
The mass of the material stays the same during a reversible change, and no new substance appears. Stretching a spring, bending a ruler slightly and inflating a balloon are other examples, as long as the material returns to its original form.
What is an irreversible change?
An irreversible change creates one or more new substances with different properties, and there is no simple way to return to the starting materials. Burning wood gives ash, carbon dioxide and water vapor. Frying an egg makes the proteins permanently change their structure, so the egg never becomes liquid again.
Signs of an irreversible change include a color change that stays, a gas being produced, a new solid forming, heat or light being released and a new smell. The next articles in this series examine these signs in more detail.
Reversible and irreversible changes compared
| Feature | Reversible change | Irreversible change |
|---|---|---|
| New substance formed? | No | Yes |
| Can the original be recovered easily? | Yes | No |
| Usual type of change | Physical | Chemical |
| Typical energy effect | Small, only for state changes | Often large heat or light |
| Example | Melting wax | Burning wax |
Examples sorted into two groups
| Change | Reversible? | Reason |
|---|---|---|
| Ice melting | Yes | Cooling gives ice again |
| Sugar dissolving in tea | Yes | Evaporating the water recovers sugar |
| Water boiling | Yes | Steam condenses to water |
| Iron rusting | No | Rust is a new substance |
| Wood burning | No | Ash and gases form |
| Milk turning sour | No | New acids form |
| Baking a cake | No | Ingredients react |
| Crushing a can | Not easily | Shape changes, but material is unchanged |
Worked example: classifying changes
Decide whether each change is reversible, and explain why.
- Dissolving 10 g of salt in 100 mL of water. Heat the solution until the water evaporates. A white solid with a mass of 10 g remains. The salt is recovered, so the change is reversible.
- Heating a piece of magnesium ribbon in air. It burns with a bright white flame and leaves a white powder, magnesium oxide. This is a new substance, so the change is irreversible.
- Freezing orange juice in an ice tray. Leaving the cubes at room temperature returns the juice. The change is reversible.
- Frying an egg. The runny white becomes solid, and cooling does not restore it. The change is irreversible.
For each answer, the question to ask is whether a new substance has formed.
When is the rule not enough?
The rule “reversible means physical, irreversible means chemical” is only a guide. Some chemical changes can be reversed under the right conditions. For example, water can be split into hydrogen and oxygen by electrolysis, and hydrogen and oxygen can react to form water again. Some physical changes are practically hard to reverse: it is not easy to un-mix a thoroughly stirred jar of sand and sugar, yet the substances are unchanged.
For this reason, scientists decide the type of change by asking whether the chemical identity of the substances has changed, not only whether the change can be undone.
Why does reversibility matter?
- Recycling: metals and glass can be melted and reshaped many times, because melting is reversible.
- Cooking: chocolate can be melted and set again, but a baked cookie cannot be returned to dough.
- Safety: chemical changes such as burning cannot be undone, so they need careful control.
- Industry: chemists design reactions and separations by knowing whether a step can be reversed.
How did scientists study reversibility?
In the 1700s, Antoine Lavoisier showed that burning is a reaction with oxygen and that the total mass is conserved, which helped to explain why some changes create new substances. In the 1800s, the study of heat engines led to the idea of reversible and irreversible processes in thermodynamics, and to the second law of thermodynamics. In that more advanced meaning, a process is irreversible if it increases the total disorder of the universe.
Which mistakes are common?
Mistake 1: Thinking that a change is reversible only because it looks similar afterwards
Cooked food sometimes looks like the raw food but has changed chemically.
Mistake 2: Saying that dissolving is irreversible because the solid disappears
The solid is still present in the solution and can be recovered by evaporation.
Mistake 3: Believing that all physical changes are easy to reverse
Breaking glass is a physical change, but you cannot easily put the pieces back together.
Mistake 4: Assuming that all irreversible changes are dangerous
Cooking, digestion and baking are irreversible and safe. The word describes the permanent nature of the change, not its danger.