Physical & Chemical Changes (College Board AP® Chemistry): Study Guide

Evidence of physical changes

Physical changes

• Matter constantly undergoes changes

• A physical change occurs when a substance’s properties change but its composition does not, e.g. a change of state

• 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 of two or more substances that are not chemically joined, so each keeps its own composition

  • When salt and water are mixed, the water molecules and the salt ions attract each other, but the particles themselves are unchanged

  • Dissolving an ionic solid can also be argued to be a chemical process, because ionic bonds break (see Physical & Chemical Processes)

  • 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

Chemical changes

• A chemical change occurs when substances are transformed into new substances, typically with different compositions

• 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

• Possible evidence that a chemical change has occurred includes the production of heat or light, the formation of a gas, the formation of a precipitate (an insoluble solid that forms in a solution) and a color change

  • E.g. when nitric acid reacts with a copper penny, a brown gas and a blue-green solution form

  • These are clues, not proof: boiling also produces a gas and dissolving can release heat, so check whether a new substance has formed

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