Physical & Chemical Changes (College Board AP® Chemistry): Revision Note

Evidence of Physical Changes

• Matter is constantly experience changes

• A physical change occur when the appearance or state of matter changes

• In physical changes, the internal composition of matter remains the same

  • E.g. All the changes of state of a substance are physical changes because the particles preserve their identity

Changes of state

Interconversions between states of matter are all physical changes

• Another common example of physical change is the formation or separation of a mixture

• A mixture is a combination between different substances in which the internal structure of its components remain unchanged

  • When salt and water are mixed in a beaker, they water molecules and the salt ions interact between them, but their internal structure stays the same

  • The mixture can be separated into its components by a simple distillation. Water can be collected in a separate container, while the salt remains in the round-bottom flask

Separation of a mixture between water and salt

The separation of a mixture is a common physical change because the components do not lose their internal structure

Evidence of Chemical Changes

• A chemical change occur when the internal composition of a substance is changed

• In chemical changes, the atoms from the original substances rearrange to form new substances

• The new substances are chemically different than the original ones

  • E.g. When hydrogen burns with air, water is produced. Water is totally different in its chemical composition compared to hydrogen gas and oxygen gas

• The best indicators for chemical changes are accompanied by a change in color, the formation of a precipitate (insoluble compound in aqueous media), or the production of heat or light

  • E.g. The chemical reaction between nitric acid and a copper penny, will produce a brown gas and a blue green solution

Reaction between nitric acid and a copper penny

Chemical change showing the formation of a brown gas and a blue green solution when nitric acid reacts with a copper penny