Chemical reactions

A match flares up, a cake rises in the oven, a bike starts to rust: all around you, substances turn into other substances. These are chemical reactions, and once you understand them, you see everyday life with new eyes.

New substances form

In a chemical reaction, substances with new properties are formed. That is the big difference from melting or evaporating: melted ice is still water, just liquid instead of solid, and in the freezer it turns back into ice. But when wood burns, ash, smoke and CO₂ form, and those never turn back into wood. Melting only changes the state, a reaction changes the substance itself.

Reactants, products and word equations

The substances that go into a reaction are called reactants or starting substances. What comes out are the products. Chemists write this as a word equation with an arrow meaning 'reacts to form': hydrogen + oxygen → water. The reactants stand to the left of the arrow, the products to the right. The arrow shows the direction of the transformation, which is why it is not an equals sign.

Energy and telltale signs

Every reaction involves energy. If energy is released, for example as heat and light in a campfire, the reaction is called exothermic. If energy has to be supplied constantly, like when baking a cake, it is called endothermic. How do you spot a reaction at all? Typical signs are: a new substance forms, the colour changes permanently, a gas bubbles up, it gets warm or cold, or it glows.