AC9S7U06 • Homework
Pure Substances, Mixtures & Separation Techniques
Use particle models and physical-property differences to justify each classification or separation method. For multi-step mixtures, explain why the order matters.
Part A — 2-mark questions
- Describe the difference between a pure substance and a mixture using a particle model. (2)
- Identify the solute and solvent in saltwater and explain how you know. (2)
- Explain why filtration can separate sand from water but cannot remove dissolved salt from saltwater. (2)
- Oil forms a layer above water. Explain why density is relevant to this observation and name one suitable separation method. (2)
- Sieving is one separation technique identified in the curriculum in First Nations Australian food-processing contexts. State the physical property that makes sieving work and explain it. (2)
Part B — 5-mark questions
- Draw or describe particle diagrams for (a) a pure compound and (b) a mixture. Explain how the diagrams show the difference in composition. (5)
- Use particle theory to explain what happens when sugar dissolves in water. Include the roles of solute and solvent and explain why the final solution can look uniform. (5)
- Compare filtration, evaporation, distillation and chromatography. For each method, identify the physical property or interaction that enables separation and give a suitable example. (5)
- Describe three real-world separation methods drawn from home, recycling and water-treatment contexts. Explain what property difference each method uses. (5)
- Using reliable source-specific examples, explain how separation techniques such as winnowing, yandying or steam distillation can be analysed scientifically. For each example you use, identify the relevant physical property or process. (5)
Part C — Enrichment
11. Analyse how particle theory links classification and separation. (8–10)
In your response, evaluate how particle diagrams distinguish pure substances from mixtures; how solute–solvent interactions create solutions; how particle size, density, solubility, volatility and other physical properties guide separation choices; why complex mixtures may require several techniques; and how modern and First Nations Australian separation examples can be compared without treating all communities or technologies as identical.
Part D — Experiment
12. Separating sand, salt and water.
You are given a mixture containing sand, dissolved salt and water. Design an investigation that recovers all three components. Include:
- a labelled particle-level description of the starting mixture;
- the sequence of separation techniques;
- the property used at each stage;
- what is collected as residue, filtrate and distillate;
- safety controls for heating and glassware;
- evidence you would use to judge whether each component was successfully separated.
Answers and marking guidance
1
Pure substance: one chemical substance represented by the same particle type throughout. Mixture: two or more substances/particle types physically present together.
2
Salt is the solute; water is the solvent. Salt particles are dispersed through the water when dissolved.
3
Sand is insoluble and its suspended particles can be trapped by filter pores. Dissolved salt particles pass through ordinary filter paper with the water.
4
Oil is commonly less dense than water and the liquids are immiscible, so they form layers. Careful decantation or a separating funnel can separate them.
5
Particle size. Openings allow smaller material through while larger particles are retained. A specific First Nations practice should be taught with reliable community/source information.
6
Pure compound: every drawn particle is the same compound particle, even if each contains more than one element. Mixture: at least two different particle types are present. Award for clear model plus explanation.
7
Sugar is solute and water solvent. Sugar particles separate from the crystal and become dispersed among moving water particles through solute–solvent attractions. The particles remain present even when too small to see, producing a homogeneous solution.
8
Filtration: suspended particle size; e.g. sand/water. Evaporation: volatility of solvent; e.g. recovering salt. Distillation: boiling-point/volatility difference plus condensation; e.g. collecting water from saltwater. Chromatography: different affinity for mobile and stationary phases; e.g. separating dyes.
9
Examples: straining pasta by size; magnetic/density separation in recycling; settling/filtration in water treatment. Full marks require three contexts and correct property links.
10
Answers vary by sourced example. Winnowing can exploit different aerodynamic responses related to mass/density/size/shape; yandying uses controlled movement and differences in physical behaviour such as density/size; steam distillation uses volatility and condensation. Do not generalise a specific practice to all First Nations communities.
11
High-scoring responses connect particle composition to pure/mixture classification, explain solutions, match several separation methods to physical-property differences, justify multi-step sequences, and discuss modern and First Nations examples accurately and source-aware.
12
Filter first: sand is residue and saltwater is filtrate. Distil the filtrate: water vaporises, condenses and is collected as distillate; salt remains in the flask and can be dried/crystallised. Explain particle size then volatility/boiling-point differences. Use eye protection, stable heat-resistant apparatus, safe heating and cooling, and do not taste samples. Evidence may include dry recovered sand/salt, collected clear distillate and mass/observation checks.