Australian Curriculum v9 / ACiQ Year 8 Science - Unit 2 - Elements, compounds and chemical change
Elements, Compounds and Chemical Change | Year 8 Science
Use particle reasoning and evidence to tell elements, compounds, mixtures and chemical changes apart.
Updated 2026-06-15 - 4 min read
Chemistry starts with a simple question: what is the substance made of? In Year 8, you need to tell apart elements, compounds and mixtures, then use evidence to decide whether a change is chemical or physical.
Elements, compounds and mixtures
An element is a pure substance made from one type of atom. Oxygen, carbon and iron are elements.
A compound is a pure substance made when atoms of different elements are chemically joined in a fixed ratio. Water is a compound because each water particle contains hydrogen and oxygen atoms joined together.
A mixture contains two or more substances that are together but not chemically joined. Salt water is a mixture because the salt and water can be separated by physical processes.
The most useful test is not whether the substance looks simple. The question is whether the particles are one type of atom, chemically joined atoms, or separate substances mixed together.
Physical change versus chemical change
A physical change changes the form, state, size or position of particles without making a new substance. Melting ice is physical because water is still water.
A chemical change produces one or more new substances. Signs can include:
- gas forming
- colour change that is not just mixing colours
- heat or light being released or absorbed
- a solid precipitate forming from solutions
- a new smell
These signs are evidence, not automatic proof. You still need to ask whether a new substance formed.
Atoms do not disappear
In a chemical reaction, atoms are rearranged. They do not vanish, and they do not turn into energy. This is the early version of conservation of mass.
For example, when magnesium burns in oxygen, magnesium atoms and oxygen atoms rearrange to form magnesium oxide. The product is different from the starting substances, but the atoms are still present.
Use evidence carefully
Chemical-change evidence is strongest when several observations point in the same direction.
One clue by itself can be misleading. Bubbles may mean gas from a reaction, but they may also mean boiling. A colour change may mean a new substance formed, but it may also come from mixing two coloured substances. Heat may be released by a reaction, but temperature can also change because something was warmed by the surroundings.
A stronger explanation links observations to particles:
- What were the starting substances?
- What changed during the process?
- Is there evidence of a new substance?
- Could the change be reversed by a physical separation method?
In Year 8, you do not need to write full chemical equations for every example. You do need to reason from evidence rather than guessing from appearance.
Quick check
- Is oxygen gas an element, compound or mixture?
- Is muddy water an element, compound or mixture?
- Melting chocolate changes its shape and texture. Is that usually physical or chemical?
- Burning toast makes new substances. Is that physical or chemical?
Answers:
- Element.
- Mixture.
- Physical change.
- Chemical change.
Transfer task
Choose one everyday example: cooking an egg, dissolving sugar, rusting iron, crushing a can or lighting a candle. Decide whether the main change is physical, chemical or both. Support your answer with particle reasoning and evidence.