Understanding the difference between chemical and physical changes is a core KS3 chemistry skill, introduced in Year 7 and built on throughout Years 8 and 9. The key question is always the same: has a new substance been formed, or has the same substance simply changed its state or appearance?
What is a physical change?
A physical change is a change in which no new substance is formed. The material may change its shape, size, state or appearance, but its chemical identity stays the same. Physical changes are usually reversible — you can get the original substance back.
Examples of physical changes
- Melting ice — water changes from solid to liquid, but it is still water (H₂O). Freeze it again and you recover the ice.
- Boiling water — water changes from liquid to gas (steam), but it is still H₂O. Condense it and you get liquid water again.
- Dissolving salt in water — the salt crystals disappear, but the sodium chloride molecules are still present, spread through the water. Evaporate the water and the salt reappears unchanged.
- Cutting paper — the paper changes shape, but the paper molecules are unaltered.
- Bending a metal wire — the shape changes; the metal is unchanged.
What is a chemical change?
A chemical change (also called a chemical reaction) is a change in which one or more new substances are formed. The atoms in the starting materials are rearranged to make different compounds. Chemical changes are usually irreversible — you cannot easily recover the original substances.
Examples of chemical changes
- Burning wood — wood reacts with oxygen to produce carbon dioxide, water vapour, and ash. The wood is gone; you cannot unburn it.
- Rusting iron — iron reacts slowly with oxygen and water to form iron oxide (rust). Iron oxide is a completely different substance from iron.
- Baking a cake — eggs, flour and other ingredients undergo chemical reactions in the oven. The cake cannot be converted back to its raw ingredients.
- Neutralisation — an acid and an alkali react to form a salt and water. The acid and alkali no longer exist as such.
- Burning magnesium — magnesium reacts with oxygen to produce magnesium oxide, a white powder with very different properties from the silvery magnesium ribbon.
How do you recognise a chemical change?
When a chemical reaction occurs, one or more of the following signs may be observed:
| Sign | Example |
|---|---|
| Colour change | Iron turns orange-brown as it rusts |
| Gas produced | Bubbles when zinc is added to dilute acid |
| Temperature change | Hand-warmers get hot (exothermic); some reactions cool down (endothermic) |
| Precipitate formed | A solid forms when two clear solutions are mixed |
| Light produced | Burning magnesium emits a bright white light |
| Irreversibility | The change is difficult or impossible to reverse |
Not all these signs are required — a reaction might show only one. Equally, a temperature change alone does not prove a chemical reaction (dissolving ammonium nitrate in water cools the solution but is a physical change).
Comparison table: physical vs chemical changes
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance formed? | No | Yes |
| Usually reversible? | Yes | No |
| Arrangement of atoms | Unchanged | Rearranged to form new compounds |
| Energy change? | Small (state changes) | Often large (heat, light released or absorbed) |
| Examples | Melting, dissolving, cutting | Burning, rusting, neutralisation, baking |
Worked examples
Example 1: rusting iron (chemical change)
An iron nail left outside in damp conditions slowly turns orange-brown. This is a chemical change: iron has reacted with oxygen and water to form iron oxide (rust), which has different properties from iron. The change is slow and irreversible — you cannot convert rust back to iron simply by removing the water. The word equation is:
iron + oxygen + water → iron oxide (rust)
Example 2: melting ice (physical change)
An ice cube placed in a warm room melts into liquid water. This is a physical change: the water molecules are the same in solid and liquid form; they have just gained enough energy to move more freely. The change is easily reversed — place the liquid water back in a freezer and it becomes ice again. No new substance is formed.
Clearing up the dissolving misconception
A very common misconception is that dissolving is a chemical change, because the solid seems to disappear and the water changes appearance. In fact, dissolving is a physical change.
When salt (sodium chloride) dissolves in water, the salt ions spread throughout the water — but they are still sodium ions and chloride ions. No new substance has been made. If you evaporate the water (heat the solution gently), the salt reappears unchanged. The original substance is recoverable, which is a hallmark of a physical change.
Compare this to burning magnesium: after burning, you have magnesium oxide — an entirely new compound with different physical and chemical properties from magnesium. You cannot recover magnesium simply by removing the heat.
The key test question to ask yourself is: Can I get the original substance back? If yes, the change is almost certainly physical. If a new substance has been made and the process is hard to reverse, it is chemical.
According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should learn about the differences between chemical and physical changes, and reversible and irreversible reactions, as part of their chemistry programme.
BBC Bitesize KS3 Chemistry covers physical and chemical changes, signs of a chemical reaction, and reversible reactions as core content for Year 7 and Year 8 chemistry.
Frequently asked questions
How do you tell if a change is chemical or physical?
Ask two questions: (1) Has a new substance been formed with different properties? (2) Is the change difficult or impossible to reverse? If both answers are yes, the change is chemical. If the same substance is present throughout and the change can be undone (e.g., by changing temperature), the change is physical. You can also look for signs of a chemical reaction: colour change, gas production, temperature change, precipitate, or light — though these alone are not conclusive.
Is dissolving a physical or chemical change?
Dissolving is a physical change. When a solid dissolves in water, its particles spread out through the liquid but are chemically unchanged. The original substance can be recovered by evaporating the water — for example, dissolving salt in water then heating the solution gently causes the water to evaporate, leaving the salt behind. Because no new substance is formed and the change is reversible, dissolving is classified as a physical change, despite appearances.
Is burning a physical or chemical change?
Burning is a chemical change. When a substance burns, it reacts with oxygen to produce entirely new substances — for example, burning wood produces carbon dioxide, water vapour and ash. These products have very different properties from the original wood, and the change cannot be reversed. The energy released as heat and light during burning is a strong indicator that a chemical reaction has taken place and new bonds have been formed.
Can chemical changes be reversed?
Most chemical changes are very difficult or impossible to reverse under everyday conditions. For example, you cannot unburn a piece of paper or convert rust back into iron simply by removing water. However, some chemical changes can technically be reversed with enough energy or the right conditions — for example, the electrolysis of water produces hydrogen and oxygen from water (which is itself the product of burning hydrogen). In chemistry, we say chemical changes are generally irreversible, but this is not an absolute rule at the atomic level.
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