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.
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Analogy
A reaction is like taking a Lego house apart and building a car from the same bricks.
Example
magnesium + oxygen -> magnesium oxideReactants: magnesium, oxygenProduct: 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.