When a chemical change or reaction occurs, new substances are formed as the molecular structure of the combined substances are broken down and recombined into the new substance. The bonds between particles, either atoms or molecules, of the substances concerned (either elements or compounds) are broken and, whilst some can be reversed, most chemical changes are irreversible. Chemical changes can be a:
- breakdown: where the bonds become so unstable that they break. This is often cause by the application of heat.
- combination: where particles combine to make new substances.
- re-arrangement: where particles break away and are then combined with other substances.
For example, when a cake is baked there is a chemical change. The baked cake looks and tastes different to the un-cooked cake and the cooking cannot be reversed. A new substance has been formed. The new substance formed during a chemical change has not been created; rather it is the result of combining and changing other substances. The mass of the new substance therefore is equal to the total mass of the substances (reactants) involved in the chemical change or reaction. For example, the mass of the baked cake is equal to the mass of all the ingredients of the cake. Its volume may be different but its mass remains the same.
For a chemical reaction to take place needs the involvement of energy. When bonds are broken, there is a need for energy to be taken in, while making a bond actually releases energy. Endothermic reactions occur when heat is taken in order for the chemical to bond to create new substances. Exothermic reactions are those where energy is released (e.g. when adding water to plaster of Paris).