Chemical Reactions

A chemical reaction is a process where one or more substances change into one or more different substances. The starting substances are called reactants; the new substances they form are called products. The atoms involved do not change (they are the same before and after), but they rearrange themselves into new combinations. Chemical reactions are happening all around you all the time: rust forming on metal, food digesting in your stomach, fuel burning in cars, plants making sugar from sunlight. Every change you see (apart from very simple physical ones) is a chemical reaction.

  • ReactantsStarting substancesOn the left of an equation
  • ProductsNew substances formedOn the right of an equation
  • Conservation of massNo atoms created or destroyedDiscovered by Lavoisier, 1700s
  • Endothermic reactionsAbsorb energyOften cool their surroundings
  • Exothermic reactionsRelease energyHeat their surroundings
  • CatalystsSpeed up reactionsWithout being used up themselves

What makes it a chemical reaction?

You can usually tell a chemical reaction is happening if you see one or more of these signs:

  • A colour change (rust forming on iron).
  • A gas being produced (bubbles or fizzing).
  • A solid forming in a liquid (precipitate).
  • Heat being given off or absorbed.
  • Light being given off (flames, fireworks).
  • A change in smell.

Just changing form (like ice melting) is not a chemical reaction; the substance is still the same (just a different physical state). Chemical reactions actually change what the substance is.

The main types of reaction

  • Combination (synthesis): two or more substances combine. Iron + oxygen = iron oxide (rust).
  • Decomposition: one substance breaks down into simpler ones. Water can be split into hydrogen and oxygen by electricity.
  • Single replacement: one element replaces another in a compound. Zinc + hydrochloric acid = zinc chloride + hydrogen.
  • Double replacement: two compounds swap parts. Silver nitrate + sodium chloride = silver chloride + sodium nitrate.
  • Combustion: a substance burns in oxygen, releasing heat and light. Methane + oxygen = carbon dioxide + water + heat.
  • Acid-base neutralisation: acid + base = salt + water.
  • Oxidation-reduction (redox): electrons are transferred between substances.

Conservation of mass

One of the most important rules in chemistry is the law of conservation of mass: in any chemical reaction, the total mass of the reactants always equals the total mass of the products. No atoms are created or destroyed; they are just rearranged. This was first carefully proved by the French chemist Antoine Lavoisier in the 1770s, and it underpins almost all of modern chemistry.

Fact When a candle burns, it looks like the wax is just disappearing. In fact, the wax is reacting with oxygen from the air to form carbon dioxide and water vapour, which drift away as invisible gases. If you collected all the products and weighed them, they would weigh slightly more than the original wax (because of the oxygen pulled in from the air). The "missing" mass is not missing at all; it is just floating away as gas.

Energy in chemical reactions

  • Exothermic reactions: release energy (usually as heat). Examples: burning, rusting, neutralising acids and bases, your body digesting food.
  • Endothermic reactions: absorb energy from their surroundings. Examples: melting an ice cube, cooking an egg, photosynthesis (which absorbs sunlight).

The energy difference between starting materials and products determines whether a reaction is exothermic or endothermic.

Pick a topic below to find out more.

What Is a Chemical Reaction?A change where atoms rearrange themselves into new substances. Burning, rusting, and baking a cake are all chemical reactions.
CombustionA chemical reaction where something burns in oxygen, giving off heat and light. Candles, campfires, and rocket engines all use combustion.
Oxidation and RustingSlow reactions with oxygen. Rust on iron and tarnish on silver are both kinds of oxidation.
PhotosynthesisThe reaction plants use to make food from sunlight, water, and carbon dioxide, giving off oxygen as a side-product.
CatalystsSubstances that speed up a reaction without being used up themselves. Enzymes in your body are natural catalysts.
Endothermic and ExothermicTwo types of reaction: endothermic ones soak up heat from around them, exothermic ones give heat off.
Acids Reacting with MetalsA common reaction in chemistry lessons: drop a piece of zinc into hydrochloric acid and you get hydrogen gas fizzing off.