Physical and chemical changes

Physical changes

Changes of state are examples of physical changes. No new substances are made, and the change is often easily reversed. For example:

  • liquid water can be cooled down until it forms solid water (ice)
  • ice can be heated until it forms liquid water again

Chemical changes

Chemical changes are different from physical changes. New substances are made in chemical changes, and the change is often not easily reversed. Chemical changes involve chemical reactions and include:

  • methane burning in air, forming carbon dioxide and water
  • magnesium reacting with dilute hydrochloric acid, forming magnesium chloride and hydrogen

In a chemical change, particles separate from one another and then join together in different ways to before.

This diagram shows what happens to iron and sulfur particles in a chemical reaction to form iron:

Iron atom symbols are added to sulfur atom symbols to produce the compound iron sulfide, which contains both atom symbols mixed up

Explain how the particle diagram shows that a chemical change happens.

To begin with, iron particles are only joined to other iron particles, and sulfur particles are only joined to other sulfur particles. After the chemical change, iron particles and sulfur particles are joined to each other.

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