Science
What is a chemical reaction?
Quick answer
In a chemical reaction, substances change into new substances with different properties. Burning, rusting, baking and cooking are chemical reactions.
In one sentence
A chemical reaction happens when substances change into new substances by breaking and forming bonds between atoms.
Explained
When you melt an ice cube, you still have water. That is a physical change. When you burn a candle or bake a cake, you get new substances that were not there before. That is a chemical reaction.
fire
candle
magnifying glass
The starting materials are called reactants. What you end up with are the products. We can show it with a word equation:
methane + oxygen → carbon dioxide + water
This is what happens when natural gas burns on a cooker. Inside, the atoms in the reactants come apart and join up in new ways. No atoms are created or destroyed, so the total mass stays the same in a closed container. A scientist would say that mass is conserved.
How can you tell a reaction has happened? Look for signs:
- Colour change: a cut apple turning brown.
- Gas bubbles: vinegar and baking soda fizz, making carbon dioxide.
- Temperature change: hand warmers get hot, instant cold packs get cold.
- Light or sound: fireworks and sparklers.
- A new solid forming in a liquid, called a precipitate.
Some reactions are slow, like iron rusting, and some are very fast, like an explosion. Heat usually speeds reactions up, and so does breaking a solid into smaller pieces. Chemical changes are usually hard to reverse. You cannot turn toast back into bread, while melting wax can be refrozen.
Reactions are everywhere: plants making food by photosynthesis, your body releasing energy from food, and neutralising in acids and bases. The elements involved are arranged in the periodic table.
Examples
| Reaction | Reactants | Products |
|---|---|---|
| Burning | fuel + oxygen | carbon dioxide + water |
| Rusting | iron + oxygen + water | iron oxide (rust) |
| Fizzing | vinegar + baking soda | carbon dioxide + other substances |
| Photosynthesis | carbon dioxide + water | glucose + oxygen |
Common mistakes
- Confusing physical and chemical changes. Melting, freezing and dissolving can be reversed and make no new substance.
- "Matter is destroyed when burned." It changes into other substances, mostly gases.
- "A change in colour always means a reaction." Mixing paint changes colour but makes no new substance.
Quick quiz
Chemical reactions
Check what you know with 4 quick questions.
Question 1 of 4
Which is a sign of a chemical reaction?
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Related topics
Atoms
An atom is the tiny building block of all matter. It has a nucleus of protons and neutrons in the middle with electrons around it. The number of protons decides which element it is.
Acids and bases
Acids are sour substances with a pH below 7, like lemon juice. Bases are slippery or bitter with a pH above 7, like soap. When an acid and a base react they neutralise each other.
The periodic table
The periodic table is a chart of all 118 known chemical elements, arranged in order of atomic number. Elements in the same column have similar properties.
Black holes
A black hole is a place in space where gravity is so strong that nothing, not even light, can escape. Most form when a very massive star collapses at the end of its life.
Cells
A cell is the smallest living unit. Every living thing is made of one or more cells, each with a membrane, cytoplasm and, in plants and animals, a nucleus that controls it.
Evolution
Evolution is the way living things change over many generations. Individuals with features that help them survive tend to have more young, so those features become more common over time.