The world often seems unchanging, but in fact, little stays the same for very long. Atoms and molecules are constantly rearranging themselves - breaking down old things and building up new ones. Chemistry helps to explain how this happens through step-by-step changes called chemical reactions.
In a reaction, elements join together to make bigger units called compounds, or compounds split back into their original elements. Many reactions are silent and invisible. Others, like an exploding firework, are energetic and violent. Reactions are the amazing transformations that drive many of the things around us. When candles flicker and cakes rise in your oven, reactions are rearranging atoms into new and different forms.
In a chemical reaction, one ingredient (called a reactant) combines with a second one. During the reaction, the bonds that hold together the atoms or molecules of the reactants split apart. The atoms then rearrange themselves and new bonds form between them to make a different set of chemicals called the product.
Although the products can be very different from the reactants, no atoms are created or destroyed. So, no matter how the reaction takes place, there are always the same number of each kind of atom after a reaction as there were before it. There are three main types of chemical reaction.
Synthesis Reaction: Two or more reactants join together.
Decomposition Reaction: One reactant breaks apart into products.
Displacement Reaction: Atoms of one type swap places with those of another, forming new compounds.
Car engines, power stations, and home heating are three common things powered by a chemical reaction called combustion (burning). The reactants are a fuel (perhaps gas or coal) and oxygen from the air. Adding heat (setting fire to the fuel) provides activation energy that starts the reaction and releases more energy as fire.