Chemical Reactions

Light a match, and in a fraction of a second wood and chemicals become smoke, ash, heat and light. Leave a bike out in the rain, and over months shiny steel turns into crumbly orange rust. Bake a cake, and runny batter becomes a sponge. Each of these is a chemical reaction, a process in which substances change into entirely new substances.

Atoms swapping partners

Everything around you is made of atoms joined together by chemical bonds. In a reaction, some of those bonds break and new ones form, so the atoms end up arranged in a different way. The starting substances are called reactants and the new ones are called products.

When methane, the main gas in a kitchen hob, burns, each methane molecule reacts with two oxygen molecules. The result is one molecule of carbon dioxide and two of water. Every carbon, hydrogen and oxygen atom that went in comes out again; they have only changed partners. This is the law of conservation of mass: atoms are never created or destroyed in a chemical reaction. That is why chemists can write reactions as balanced equations, with the same number of each kind of atom on both sides.

Signs that a reaction has happened

Unlike melting ice, which is still water afterwards, a chemical reaction makes something new. Common clues are a change of colour, bubbles of gas, a new smell, a solid appearing from a liquid, or a change in temperature. Vinegar poured on baking soda fizzes because the reaction produces carbon dioxide gas. Milk going sour, leaves turning brown in autumn and toast darkening are all reactions too.

Energy in, energy out

Breaking bonds takes energy, and forming new bonds releases it. If the new bonds release more energy than the old ones took to break, the reaction gives off heat. Burning, the reaction in a hand warmer and the respiration that powers your cells all release energy. If the reverse is true, the reaction absorbs heat and feels cold, like the chemical cold packs used for sports injuries. Photosynthesis is a reaction that stores the Sun's energy in the bonds of sugar.

Even reactions that release energy usually need a push to get started. A match will not light until you strike it. This starting push is called the activation energy: the bonds have to be shaken loose before new ones can form.

Fast and slow

Some reactions take years; others are over in a millionth of a second. Several things change the speed. Heating things up makes molecules move faster and collide harder, so more collisions lead to a reaction. Higher concentrations mean more collisions. Cutting a solid into smaller pieces exposes more surface, which is why flour dust can explode while a bag of flour just sits there.

A catalyst speeds a reaction up without being used up itself. It gives the reaction an easier route with a lower energy barrier. The catalytic converter in a car uses platinum and other metals to turn harmful exhaust gases into less harmful ones. Living things rely on their own catalysts, called enzymes. Without enzymes, digesting a meal would take years instead of hours.

Why it matters

Chemical reactions run the world. They power engines and bodies, make medicines, plastics, fertilisers and steel, and cause problems such as rust and pollution. Understanding which reactions happen, how fast, and how much energy they involve lets us cook food safely, build longer-lasting bridges, make cleaner fuels and design new materials.