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Year 7–8 · Year 7–8 Science

Year 7–8 Science — The Chemical World

The chemical world covers the building blocks of matter — elements, compounds and mixtures — and the difference between physical and chemical changes. These concepts underpin VCE Chemistry and should be understood clearly before moving into more complex Year 9–10 chemistry.

Key Concepts & Formulas

  • Element: a pure substance made of only one type of atom; cannot be broken down by chemical means

  • Compound: two or more elements chemically joined in fixed proportions; has different properties from its elements

  • Mixture: two or more substances physically combined; can be separated by physical means

  • Periodic table: elements arranged by increasing atomic number; groups (columns) have similar properties

  • Metals: on the left and centre of the periodic table; good conductors, malleable, lustrous

  • Non-metals: on the right of the periodic table; poor conductors, brittle (if solid)

  • Physical change: no new substance formed; change in state, shape or appearance (e.g. melting, dissolving)

  • Chemical change (reaction): new substance(s) formed; often irreversible; signs include gas production, colour change, precipitate, temperature change, light

  • Reactants → Products; conservation of mass: total mass of reactants = total mass of products

  • Separating mixtures: filtration (solid from liquid), evaporation (dissolved solid from solution), distillation (liquids with different boiling points), chromatography (different solubilities)

Practice Questions

4 questions

Attempt each question before reading the hint. These are styled to match school assessment format.

Q1.Classify each as an element, compound or mixture: (a) gold (b) water (c) air (d) salt water.

2 marks

Q2.Identify each change as physical or chemical and give a reason: (a) melting ice (b) burning wood (c) cutting paper (d) rusting iron.

4 marks

Q3.A student mixes iron filings and sulfur powder. They then heat the mixture. Describe what happens and explain whether the process is a physical or chemical change.

3 marks

Q4.Describe how you would separate a mixture of sand and salt dissolved in water. Name the technique used at each step.

4 marks

Common Mistakes to Avoid

These are the errors that students most frequently make in The Chemical World — and that examiners are specifically watching for.

  • Confusing dissolving with a chemical change — dissolving is a physical change (the solute can be recovered)

  • Saying a compound is a "mixture" of elements — in a compound, elements are chemically bonded in fixed ratios; in a mixture, they are not

  • Thinking mass is lost in a chemical reaction — mass is always conserved; if gas escapes, the system mass decreases but total mass does not

  • Confusing filtration and evaporation — filtration separates a solid from a liquid; evaporation removes liquid from a dissolved solid

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