An addition reaction is a type of chemical reaction where two or more substances combine to form a single product. This often occurs with unsaturated compounds, like alkenes and alkynes, which have double or triple bonds. When these compounds react with other molecules, such as hydrogen or halogens, the multiple bonds are broken, allowing new atoms to be added to the molecule, resulting in a more saturated compound. These reactions are important in organic chemistry and are commonly used in the production of various plastics, fuels, and pharmaceuticals. For example, the addition of hydrogen to ethylene (an alkene) produces ethane, a key component in many industrial processes.