Pure Substances and Mixtures: Elements, Compounds and Mixtures — Answers
Answer sheet with model answers and success criteria for the Year 8 mixtures & chemical reactions worksheet.
Success TutoringWorksheetsAnswer sheetYear 8 · Science · Mixtures & Chemical ReactionsPure Substances and Mixtures: Elements, Compounds and Mixtures — Answers
Part A: Element, compound or mixture?
Circle the correct word for each substance.
A pure substance contains only one type of particle. There are two kinds of pure substances. An element is made of only one kind of atom, such as oxygen, iron or gold. A compound is made of two or more different kinds of atoms chemically bonded in a fixed ratio, such as water (H₂O) or carbon dioxide (CO₂).
A mixture contains two or more substances that are not chemically joined. The amounts can vary, and the parts can be separated by physical methods such as filtering or evaporating.
- Aelement
- Bcompound
- Cmixture
Answer: element
It contains only one kind of atom, so it is an element.
- Aelement
- Bcompound
- Cmixture
Answer: compound
Two different elements are chemically bonded in a fixed ratio, so water is a compound.
- Aelement
- Bcompound
- Cmixture
Answer: mixture
Air contains several different substances that are not chemically joined, so it is a mixture.
- Aelement
- Bcompound
- Cmixture
Answer: mixture
Salt is dissolved in water but not chemically joined to it, and the amount of salt can vary — a mixture.
- Aelement
- Bcompound
- Cmixture
Answer: compound
Carbon and oxygen atoms are chemically bonded in a fixed ratio of one to two.
- Aelement
- Bcompound
- Cmixture
Answer: element
Pure gold contains only gold atoms.
Part B: Reading particle diagrams
Use the descriptions of the particle boxes.
Particle boxes (each circle is one atom; joined circles are bonded together)
Box 1: every particle is a single black circle; all particles are the same.
Box 2: every particle is one black circle joined to two white circles; all particles are the same.
Box 3: some particles are single black circles and some are pairs of joined white circles, mixed together.
Box 4: every particle is a pair of joined white circles; all particles are the same.
Answer: Box 1 — all particles are the same and each is a single kind of atom (black).
Answer: Box 2 — all particles are the same, and each contains different atoms (black and white) bonded together.
Answer: Box 3 — it contains two different kinds of particles that are not joined to each other.
- Aan element
- Ba compound
- Ca mixture
- Da solution
Answer: an element
Although the atoms are bonded in pairs, there is only one kind of atom, so it is an element.
Part C: Types of mixtures and testing purity
In a homogeneous mixture the substances are spread evenly, so it looks the same throughout (solutions such as salt water, and air). In a heterogeneous mixture you can see the different parts (sand in water, a salad).
Scientists test purity using melting and boiling points. A pure substance melts and boils at fixed temperatures. Impurities change these temperatures and make the change happen over a range.
- homogeneous mixture
- heterogeneous mixture
- alloy
- melting point
- the different parts can be seen, such as sand and gravel
- looks the same all the way through, such as salt water
- the temperature at which a solid turns into a liquid
- a mixture of a metal with other elements, such as steel
Answer: homogeneous mixture → looks the same all the way through, such as salt water; heterogeneous mixture → the different parts can be seen, such as sand and gravel; alloy → a mixture of a metal with other elements, such as steel; melting point → the temperature at which a solid turns into a liquid
Answer: True
A pure substance has a fixed boiling point. Dissolved salt raises the boiling point of water.
Answer: False
Mixtures usually melt or boil over a range of temperatures. A sharp, fixed melting point is a sign of a pure substance.
Answer: alloy
Part D: Explain it
Model answer: Liquid A is pure water, because a pure substance boils at a fixed temperature and pure water boils at 100 °C at sea level. Liquid B is a mixture, because it boils above 100 °C and over a range of temperatures; dissolved substances raise the boiling point and the temperature rises as the water boils away and the solution gets more concentrated. To show B contains a dissolved solid, I could heat a small sample in an evaporating dish until all the water evaporates. If a solid residue is left behind, B contained a dissolved solid.
Success criteria:
- Identifies A as pure with fixed boiling point reasoning
- Identifies B as a mixture using raised boiling point and boiling range
- Describes evaporation to leave a solid residue
- Uses scientific terms correctly
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
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