Definition · Plain-language
Chemical reaction
A chemical reaction is a process in which substances, the reactants, are rearranged into new substances, the products.
The step most authors miss
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Reactants becoming products
In a chemical reaction, the starting substances — the reactants — are converted into new substances — the products. This is shown by a chemical equation, with reactants on the left, an arrow indicating the direction of change, and products on the right. What actually happens is that bonds in the reactants are broken and new bonds form, rearranging the same atoms into different combinations. Because the products are chemically different from the reactants, a chemical reaction is a chemical change, not merely a physical one.
Conservation of mass
A chemical reaction rearranges atoms but never creates or destroys them, so the total mass of the products always equals the total mass of the reactants. This is the law of conservation of mass, and it is the reason chemical equations must be balanced — every atom on the reactant side must appear on the product side. If a reaction appears to lose or gain mass, it is because a gas has escaped or been taken in; accounting for every substance, including gases, restores the balance.
Signs and energy of a reaction
Several observable signs suggest a chemical reaction has occurred: a change in colour, the production of a gas (bubbling), the formation of a solid precipitate when solutions are mixed, the release of light, or a change in temperature. Reactions also involve energy: breaking bonds absorbs energy and forming bonds releases it, so a reaction is exothermic (releasing energy overall) or endothermic (absorbing it). The activation energy is the minimum energy needed to start a reaction, which is why some reactions need a spark or heat to begin.
Key facts
At a glance
- Definition: a process turning reactants into new products
- What happens: bonds break and new bonds form, rearranging atoms
- Reactants: the starting substances (left of the equation)
- Products: the new substances formed (right of the equation)
- Conservation: total mass of products equals total mass of reactants
- Signs: colour change, gas, precipitate, light, temperature change
Common misconceptions
What people often get wrong
Often heard: A chemical reaction can create or destroy atoms.
Actually: Atoms are only rearranged, never created or destroyed. The total mass of reactants equals the total mass of products — the law of conservation of mass.
Often heard: Any visible change, like boiling or dissolving, is a chemical reaction.
Actually: Boiling and most dissolving are physical changes — no new substance forms. A chemical reaction produces chemically different products, shown by signs such as a colour change or a new gas.
Often heard: If mass seems to be lost in a reaction, mass has been destroyed.
Actually: Mass is conserved. An apparent loss usually means a gas has escaped; accounting for all substances, including gases, shows the mass is unchanged.
