In a chemical reaction, atoms and molecules generally split, swap or decompose structures.Chemical reactions also causes atoms and molecules to combine with other compounds that they like, and form a completely new chemical bond thus creating a totally new substance. Many chemical reactions occurr because of the application of energy- that is, the application of heat or cold. For example, when you cook, cooking is actually a chemical reaction. The same is applied when you freeze something, such as water, to make ice cubes. The components of the ice are the same, but it creates a new substance.