Coaching Centre
Physical and chemical change
The difference between physical and chemical changes in terms of particles, new substances, reversibility and energy, with everyday examples and the particle model.
Toast is a one-way trip. You can melt butter and let it set again, freeze water and melt it again, but no amount of cooling will turn burnt toast back into bread. Something different happened to the bread.
Physical changes
In a physical change, a substance changes its size, shape or state, but it is still the same substance. Melting chocolate, crushing a can, cutting paper, dissolving sugar in tea and boiling water are all physical changes. The melted chocolate is still chocolate; the steam is still water.
At the particle level, the particles move around, spread out or pack together, but the particles themselves don't change. When ice melts, each water molecule is still one oxygen atom bonded to two hydrogen atoms. No bonds inside the molecules are broken.
Physical changes are often easy to reverse: freeze the water again, or evaporate the water to get the salt back. But not always. A smashed glass is still glass (a physical change), yet you'd have to melt it down to reshape it.
Chemical changes
In a chemical change, one or more new substances are made, with different properties from the substances you started with. Another name for a chemical change is a chemical reaction. Burning, rusting, cooking an egg, baking a cake, digesting food and fireworks exploding are all chemical changes.
At the particle level, the bonds between atoms break and new bonds form. The atoms rearrange into new particles. When methane (natural gas) burns, the carbon and hydrogen atoms in methane molecules break apart and join with oxygen atoms to make carbon dioxide and water molecules.
Chemical changes are usually hard to reverse. You can't un-burn wood or un-cook an egg. Some can be reversed, but only with another chemical reaction: for example, electricity can split water back into hydrogen and oxygen.
- Play Ice melting and Water boiling. Do any atoms change partners?
- Play Methane burning. Which bonds break? Which new molecules form?
- Check the atom count at the top right before and after each change. What do you notice?
- Is dissolving salt a physical or chemical change? Explain using the particles.
Comparing the two
| Physical change | Chemical change | |
|---|---|---|
| New substance? | No: same substance, new shape, size or state | Yes: one or more new substances form |
| Particles | Same particles, rearranged or spread out | Bonds break and re-form; atoms join up in new ways |
| Reversibility | Often easy to reverse | Usually hard to reverse |
| Energy change | Usually small | Often large: heat, light or sound |
| Mass | Conserved | Conserved |
| Examples | Melting, freezing, boiling, dissolving, cutting, crushing | Burning, rusting, cooking, digesting, fermenting |
Notice that mass is conserved in both. Atoms are never created or destroyed in a chemical reaction; they are only rearranged. You'll look at this more closely in 5C.
Many everyday events involve both. When a candle burns, the wax melts (physical) and the wax vapour burns (chemical). When you cook an egg, the heat makes the egg proteins change shape and bond together in new ways, and the clear runny white becomes solid and white: a chemical change.
Test yourself
Which is a chemical change?
Burning makes new substances, such as carbon and carbon dioxide.
The best evidence that a chemical change has happened is that
A new substance is the defining feature.
When water boils, the water molecules
Boiling is a change of state, a physical change.
In a chemical reaction, what happens to the atoms?
Bonds break and re-form; the atoms themselves stay the same.
Which physical change is hard to reverse?
Still wood, so physical, but it can't be easily rejoined.
Frying an egg is a chemical change because
A cooked egg can't be uncooked.
Label the chemical change.
Physical or chemical?
In a physical change, no new is formed. In a chemical change, between atoms break and new ones form. Another name for a chemical change is a chemical . Physical changes are often easy to . In both kinds of change, is conserved.
Compare melting butter with baking a cake. Explain which is a physical change and which is a chemical change, referring to particles and reversibility.
Melting butter is a physical change. Heating gives the butter's particles more energy so they move more freely and the solid becomes a liquid, but the particles themselves are the same. No new substance forms, and putting the butter in the fridge reverses the change.
Baking a cake is a chemical change. In the oven, the baking powder reacts to make carbon dioxide gas, the egg proteins change and bond together, and sugars brown. Bonds between atoms break and form in new ways, making new substances with different properties: the soft batter becomes a spongy, brown solid with a new taste. You can't turn a cake back into batter, so the change is not reversible.