# Chemical reaction

A change where atoms are rearranged to make one or more new substances. The starting substances are reactants. The new ones are products.

Chemistry (school) · Part 04: Chemical reactions · Card 1 of 9 · Last updated 2026-10-19

## Analogy

A reaction is like taking a Lego house apart and building a car from the same bricks.

## Example

**Word equation**

magnesium + oxygen -> magnesium oxide
Reactants: magnesium, oxygen
Product: magnesium oxide

## In practice

Burning wood, rusting iron and baking a cake are all chemical reactions.

## Questions people ask

### How can you tell that a chemical reaction has happened?

Signs include a temperature change, fizzing as gas forms, a colour change, a solid forming in a liquid, or a new smell. These show a new substance has been made. Melting ice is not a reaction because it is still water.

### What is the difference between a physical change and a chemical change?

In a physical change, such as melting or dissolving, no new substance forms and you can usually reverse it easily. In a chemical change, atoms rearrange and new substances are made. Burning paper cannot be easily reversed. Melting ice can.

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