Materials can be changed in two ways, these are chemical or irreversible changes and physical or reversible changes.
Physical or Reversible
Physical changes alter the appearance or other physical properties of a material. Whilst the actual substance within the material remains fundamentally the same it may look different. Changes in the state of matter – melting, freezing, boiling, condensing and evaporating – are all examples of physical changes. When water changes and becomes a solid, a liquid or a gas, a physical change has occurred. However, since there are no chemical reactions involved, no new substance is formed and the changes that take place are reversible. This is true of most physical changes.
Chemical or Irreversible
When a chemical change occurs, new substances are formed as the molecular structure of the combined substances are broken down and recombined. The bonds between particles of the substances concerned are broken and, whilst some can be reversed, most chemical changes are irreversible. For example, when a cake is baked a chemical change takes place as a result of the baking process. The cooked cake looks and tastes different to the un-cooked cake and this process cannot be reversed. A new substance has been formed.
