Physical & Chemical Changes (Cambridge (CIE) O Level Chemistry): Revision Note

Physical & Chemical Changes

Physical change

• Physical changes (such as melting or evaporating) do not produce any new chemical substances

• These changes are often easy to reverse

• Making a mixture from 2 or more substances or dissolving a solute in a solvent are examples of physical changes as no new substances are produced and are usually relatively easy to separate

Chemical change

• During chemical changes (usually referred to as chemical reactions), new chemical substances are formed that have very different properties to the reactants

• There may be signs that a new substance has formed, such as:

  • A colour change

  • A precipitate being formed

  • Bubbles of gas being produced

• Most chemical reactions are very difficult to reverse

• Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic)

  • The majority of chemical reactions are exothermic with only a small number being endothermic