Question
What are reversible and irreversible changes? Give three examples of each and explain how you can identify which type a change belongs to.
Solution — Step by Step
A reversible change is one that can be undone — the original substance can be recovered. No new substance is formed. The change in properties (shape, size, state) can be reversed by simple means such as cooling, removing pressure, or adding/removing energy.
Key indicator: the change does not create a new substance. The atoms and molecules rearrange or the state changes, but the chemical identity remains the same.
1. Melting of ice: Ice (solid water) melts to form liquid water when heated. Simply cool the water below 0°C and it becomes ice again. The substance (water, H₂O) has not changed — only the state changed.
2. Stretching a rubber band: Pull a rubber band and it stretches. Release it and it returns to its original shape. The rubber polymer chains extended and then contracted — no new substance formed.
3. Dissolving sugar in water: Add sugar to warm water, stir, and it dissolves (appears to “disappear”). Evaporate the water and the sugar reappears. The sugar molecules dispersed throughout the water but were not chemically changed — you can recover them.
An irreversible change is one that cannot be undone by simple physical means. Usually a new substance with different properties is formed. The original substance is permanently changed. These are almost always chemical changes.
Key indicators: colour change, gas evolved, change in smell, heat/light produced, precipitate formed — one or more of these usually signals an irreversible chemical change.
1. Burning of wood: Wood + oxygen → carbon dioxide + water vapour + ash. The original wood cannot be recovered from the ash. New substances (CO₂, H₂O, ash) are formed.
2. Cooking an egg: Raw egg → cooked egg. The egg proteins denature (change their structure permanently) due to heat. No matter how you cool the cooked egg, it will not return to raw. New chemical bonds formed during cooking cannot be reversed.
3. Rusting of iron: Iron + oxygen + water → iron oxide (rust). Rust has different properties from iron — different colour (orange-brown), brittle, non-magnetic. While the rust can be chemically removed, simply reversing the conditions (removing water or oxygen) does not restore the iron.
Ask three questions:
- Is a new substance formed with different properties? If yes → irreversible (chemical change)
- Can the original material be recovered easily? If yes → reversible (physical change)
- Are there signs of chemical reaction (heat, light, gas, smell, colour change, precipitate)? If yes → likely irreversible
Most physical changes (change of state, shape, size) are reversible. Most chemical changes (reactions) are irreversible.
Note: Some chemical changes can be reversed but require a different chemical process — for example, rust can be removed from iron, but it requires a chemical reaction (acid treatment), not simply cooling or removing pressure. So it is still classified as irreversible in common usage.
Why This Works
Reversibility depends on whether the change is merely physical (rearranging molecules/atoms without breaking chemical bonds) or chemical (breaking and forming new chemical bonds). Breaking and re-forming chemical bonds requires specific conditions and often produces very different substances.
Physical changes involve only intermolecular forces (weaker). Changing state (solid↔liquid↔gas) just overcomes these without breaking covalent or ionic bonds. So state changes are easily reversible.
Chemical changes involve breaking covalent or ionic bonds and forming new ones. The energy landscape is fundamentally different — you can’t simply reverse the conditions and expect the old bonds to reform spontaneously.
Alternative Method — Classification by Change Type
| Change | Type | Reversible? |
|---|---|---|
| Melting/freezing | Physical (state) | Yes |
| Dissolution | Physical | Yes |
| Stretching | Physical (deformation) | Yes (if elastic) |
| Combustion | Chemical | No |
| Cooking | Chemical | No |
| Rusting | Chemical | No |
| Tearing paper | Physical | No (but not chemical — it’s physically irreversible) |
Note that tearing paper is physically irreversible (you can’t untear it) but is not a chemical change. So physical changes can sometimes be irreversible too, though they don’t form new substances.
Common Mistake
Students sometimes say “all physical changes are reversible and all chemical changes are irreversible.” This is mostly true but not perfectly so. Some chemical reactions are reversible (e.g., dissolving CO₂ in water to form carbonic acid — remove CO₂ and the reaction reverses). Some physical changes are effectively irreversible (tearing paper, breaking glass).
The more accurate rule: reversible changes don’t form new substances; irreversible changes usually form new substances with different properties.
For CBSE Class 6 exams: If the question says “give examples of reversible changes,” focus on state changes (melting, boiling, freezing) and shape changes (stretching rubber). For irreversible, focus on burning, cooking, and rusting. These 6 examples reliably appear in exam marking schemes.