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 A change in which atoms are rearranged and joined together differently to form new substances. They often involve energy changes. and include:
- A colourless gas that can do harm to life forms. 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:
Question
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.