A candle burning, a nail rusting, a cake cooking — all involve chemical reactions.
A chemical reaction is when two or more elements or compounds meet and interact to form new compounds or separate out some of the elements.
The chemicals involved in a chemical reaction are called the reactants. The results are called the products.
The products contain exactly the same atoms as the reactants but in different combinations.
The products have exactly the same total mass as the reactants. This is called conservation of mass.
Some reactions are reversible, which means the products can be changed back to the original reactants. Others, such as making toast, are irreversible.
Effervescence is a reaction in which gas bubbles form in a liquid, turning it fizzy.
A catalyst is a substance that speeds up, slows down, or enables a chemical reaction to happen but remains unchanged at the end.
Nearly all reactions involve energy. Some involve light or electricity. Most involve heat. Reactions that give out heat are called exothermic. Those that draw in heat are called endothermic.
Oxidation is a reaction in which oxygen combines with a substance. Burning is oxidation; as the fuel burns it combines with oxygen in the air. Reduction is a reaction in which a substance loses its oxygen.