Pure substances, mixtures and separation

Today we are learning to

use particle models to distinguish pure substances from mixtures, then choose and justify separation methods using differences in physical properties.

Success criteria

• I can identify one particle type versus two or more particle types.
• I can explain why a solution can look uniform but still be a mixture.
• I can match a separation method to a property difference.
• I can design a multi-step separation sequence and justify each step.

1. Define pure substance: one chemical substance throughout. A pure compound may contain more than one element if every particle is the same compound particle.
2. Define mixture: two or more substances physically combined. Components retain useful physical properties.
3. Model a solution: solute particles are dispersed among solvent particles; dissolved does not mean disappeared.
4. Teach method-property links: sieve—particle size; magnet—magnetism; filtration—insoluble suspended particles; decantation—settling/layers; evaporation/crystallisation—solubility; chromatography—different affinities; distillation—different boiling points/volatility.
5. Emphasise that complex mixtures may require several methods in sequence.

Sand + salt + iron filings

Step 1: use a magnet because iron is magnetic and sand/salt are not.

Step 2: add water because salt dissolves while sand does not.

Step 3: filter because insoluble sand particles are retained and salt solution passes through.

Step 4: evaporate/crystallise to recover salt, or distil if the water must also be collected.

Teacher prompt: “Name the property exploited at every step. If you cannot name the property, your method choice is not justified.”

• A clear liquid leaves residue after evaporation. What does that tell us about purity?
• Why does filtration remove mud but not dissolved salt?
• When would evaporation be preferred to distillation?
• Why might one chromatography spot not absolutely prove purity?
• What method would you choose for two immiscible liquids? What property are you using?

• “Clear means pure.” False — dissolved substances can be invisible.
• “Compounds are mixtures.” False — a pure compound contains one chemical substance with fixed composition.
• “Filtration removes dissolved salt.” False — ordinary dissolved particles pass with the solvent.
• “Dissolved means gone.” False — the solute remains present and can often be recovered physically.

1. Pure sample or mixture: a diagram has identical AB particles only.
2. Choose a method: muddy water after settling.
3. Choose a method: collect clean water from saltwater.
4. Explain why a complex recycling mixture may need more than one method.

Answers: pure compound; decant/filter; distillation; different components require different property-based methods.

Students design a separation flow chart for a novel mixture of pebbles, iron pieces, sand and dissolved salt. They must label the product after each step and write the physical property used. Extension: explain how the plan changes if clean water must also be recovered.