Unit 4 Pure substances and mixtures
Key unit competence: Distinguish between pure substances and mixtures and apply appropriate methods to separate the components of mixtures.
Key ideas
- A pure substance contains only one kind of particle (an element or a compound) and has a fixed melting point and boiling point.
- A mixture contains two or more substances not chemically joined; its composition can vary and its parts keep their own properties. Homogeneous mixtures are uniform (salt solution, air); heterogeneous mixtures are not (sand and water).
- A solution = solute + solvent. A suspension has insoluble particles that settle; a colloid has very small particles that do not settle (milk, fog).
- Separation methods: filtration (insoluble solid from liquid), evaporation and crystallisation (soluble solid from solution), simple distillation (solvent from solution), fractional distillation (miscible liquids with different boiling points).
- More methods: separating funnel (immiscible liquids, e.g. oil and water), chromatography (dyes or pigments), magnetic separation (iron from sulfur), sublimation (iodine or ammonium chloride from salt), decantation and sieving.
- Purity can be checked by melting or boiling point: a pure substance melts and boils sharply at a fixed temperature; impurities lower the melting point and raise the boiling point.
Common misconceptions
- "A clear liquid must be pure." No: A clear liquid can be a solution, such as salt water.
- "Filtration can separate salt from salt water." No: Dissolved salt passes through filter paper; use evaporation or crystallisation.
- "Mixtures and compounds are the same thing." No: Compound parts are chemically joined in fixed ratio; mixture parts are not.
Worked examples
Example 1. Describe how to obtain pure dry salt from rock salt (salt mixed with sand).
Solution
1. Crush the rock salt and add water; stir so the salt dissolves.
2. Filter: sand stays as residue, salt solution passes as filtrate.
3. Heat the filtrate in an evaporating dish until crystals start to form, then leave to cool and crystallise.
4. Dry the crystals between filter papers.
Example 2. A spot moves 3.6 cm while the solvent moves 9.0 cm. Calculate Rf.
Solution
Rf = distance of spot ÷ distance of solvent front
= 3.6 ÷ 9.0
= 0.40.
Practice questions
Define a mixture.
Show answer
Two or more substances physically combined, not chemically joined, in any proportion.
Common mistake: Saying the substances are chemically joined.
Name the method to separate kerosene and water.
Show answer
Separating funnel.
Common mistake: Choosing filtration.
What is the residue in filtration?
Show answer
The insoluble solid left on the filter paper.
Common mistake: Confusing it with the filtrate.
Give one test to show water is pure.
Show answer
It boils at exactly 100 °C (or freezes at 0 °C) at normal pressure.
Common mistake: Saying it looks clear.
Name two methods used in water treatment and state their purpose.
Show answer
Filtration (removes solids); chlorination (kills germs); sedimentation (settles particles).
Common mistake: Listing chromatography.
Explain why sublimation can separate iodine from sand.
Show answer
Iodine turns directly into vapour on heating and re-forms as crystals on a cool surface; sand does not sublime.
Common mistake: Saying sand sublimes.
Distinguish between a solution and a suspension.
Show answer
Solution: particles dissolved, clear, do not settle, pass through filter. Suspension: particles visible, settle, trapped by filter.
Common mistake: Saying both are clear.
A mixture of 12.0 g contains 4.8 g of iron filings. Calculate the percentage of iron.
Show answer
4.8 ÷ 12.0 × 100 = 40%.
Common mistake: Dividing 12.0 by 4.8.
Why is the separating funnel tap opened slowly near the end?
Show answer
To stop exactly at the boundary so the two layers do not mix again.
Common mistake: Saying it prevents the funnel from breaking.
Spots from an orange drink: Rf 0.25 and 0.62. Known dyes: A (0.25), B (0.48), C (0.62). Which dyes are in the drink?
Show answer
A and C.
Common mistake: Including B because it lies between the two.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. In salt solution, the salt is the: a. Filtration 2. Which method is used to obtain pure water from salt water? b. solute 3. Which method separates sand from water? c. Using a separating funnel 4. Which method separates oil from water? d. Simple distillation Show answer
1 → b (solute) · 2 → d (Simple distillation) · 3 → a (Filtration) · 4 → c (Using a separating funnel)
- The solute is the substance that dissolves; water is the solvent.
- Water boils off and is condensed; salt remains.
- Sand is insoluble and is trapped by filter paper.
- Oil and water are immiscible and form two layers that can be run off separately.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
salt has no effect · a fixed melting point · colloid · impurities raise the boiling point- A pure substance has .
- Adding salt to water changes its boiling point to about 101 °C. This shows that .
- Milk is an example of a .
Show answer
1: a fixed melting point · 2: impurities raise the boiling point · 3: colloid
- Pure substances melt sharply at one temperature.
- Dissolved impurities raise the boiling point of a liquid.
- Milk has tiny fat droplets spread in water that do not settle.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The liquid that passes through filter paper is called the: a. Filtration, then evaporation of the filtrate 2. A sample of 'pure' aspirin melts between 128 °C and 133 °C. Pure aspirin melts at 136 °C. This shows the sample: b. is impure 3. Which order of methods would separate a mixture of sand, salt and water? c. filtrate 4. Which mixture can be separated by sublimation? d. Ammonium chloride and sodium chloride Show answer
1 → c (filtrate) · 2 → b (is impure) · 3 → a (Filtration, then evaporation of the filtrate) · 4 → d (Ammonium chloride and sodium chloride)
- The filtrate passes through; the residue stays on the paper.
- Impurities lower the melting point and spread it over a range.
- Filter out the sand first, then evaporate the filtrate to get salt.
- Ammonium chloride sublimes on heating; sodium chloride does not.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
fractional distillation · a separating funnel · a magnet · separate liquids with close boiling points- Ethanol (b.p. 78 °C) and water (b.p. 100 °C) are separated by .
- In fractional distillation, the fractionating column helps to .
- A mixture of iron filings and sulfur powder can be separated using .
Show answer
1: fractional distillation · 2: separate liquids with close boiling points · 3: a magnet
- They are miscible liquids with different boiling points.
- Repeated condensation and evaporation on the column's surface separates the liquids.
- Iron is magnetic; sulfur is not.