Exploring Mixtures and their Separation Class 9 Notes, Numericals and Questions

What Exploring Mixtures and their Separation is about

Weightage: This chapter is part of Unit II, Matter - Its Nature and Behaviour, which carries 25 of the 80 theory marks in the school annual exam, shared with Journey Inside the Atom and Atomic Foundations of Matter.Source: CBSE Science (086) syllabus, Class IX 2026-27, course structure, Unit II Matter - Its Nature and Behaviour

Key concepts

Homogeneous and heterogeneous mixtures

NCERT section 5.1 How Can We Classify Mixtures?

A mixture contains two or more substances that are not chemically combined, so they can be separated by physical methods.

• Homogeneous mixture: the same composition everywhere; you cannot see separate parts. A stirred sugar solution tastes equally sweet in the first and last sip. Examples: salt or sugar solution, vinegar (acetic acid in water), soda (carbon dioxide in water), air, and alloys such as brass.
• Heterogeneous mixture: composition differs from place to place; parts can often be seen. Examples: sand in water, oil and water, iron filings and sulphur, smoke, fog, dust in air.

Gases mix freely, so mixtures of gases (air, hydrogen with oxygen as rocket fuel) are homogeneous. Most solid-solid mixtures are heterogeneous.

Alloys are homogeneous mixtures of metals (or a metal and a non-metal) made by melting them together. They cannot be separated by physical methods. Brass is about 80% copper and 20% zinc; bronze about 80% copper and 20% tin; stainless steel is iron with chromium, nickel, a little carbon and molybdenum.

Worked example

Classify as homogeneous or heterogeneous: air, milk, smoke, brass, muddy water, vinegar. Explain milk.

Try it first, then show the solutionSolution

1. Homogeneous: air (gases mix evenly), brass (alloy), vinegar (solution).
2. Heterogeneous: smoke (solid in gas), muddy water (suspension).
3. Milk looks uniform but is a colloid: fat and protein particles of 1 to 1000 nm are spread through water, and it scatters light. So it is heterogeneous.

Answer: Homogeneous: air, brass, vinegar. Heterogeneous: smoke, muddy water and milk (a colloid).

Concentration of solutions: % m/m, % m/v, % v/v

NCERT section 5.2 Solutions

A solution has a solute (the substance dissolved) and a solvent (the substance that dissolves it). Concentration is the amount of solute in a given amount of solvent or solution. The right proportion matters: ORS, medicines, pesticide sprays and even tea all need correct amounts.

Method	Formula	Used for
Mass by mass (% m/m or % w/w)	$\frac{\text{mass of solute}}{\text{mass of solution}}\times 100$	food labels, milk powder, spice mixes, alloys
Mass by volume (% m/v)	$\frac{\text{mass of solute (g)}}{\text{volume of solution (mL)}}\times 100$	medicines, e.g. 5% glucose, 0.9% saline drip
Volume by volume (% v/v)	$\frac{\text{volume of solute}}{\text{volume of solution}}\times 100$	two liquids, e.g. vinegar, perfumes

Key point: mass of solution = mass of solute + mass of solvent. The denominator is always the solution, not the solvent.

Dilip Mahalanabis, an Indian paediatrician, developed and used ORS for dehydration caused by cholera and diarrhoea; it has saved millions of lives.

Worked example

(i) 25 g of sugar is dissolved in 225 g of water. Find % m/m. (ii) How much salt is in 500 mL of 0.9% m/v saline? (iii) 12 mL of lemon concentrate is made up to 240 mL of drink. Find % v/v.

Try it first, then show the solutionSolution

1. (i) Mass of solution $=25+225=250$ g; % m/m $=\frac{25}{250}\times 100=10\%$.
2. (ii) 0.9 g in 100 mL, so in 500 mL: $0.9\times 5=4.5$ g.
3. (iii) % v/v $=\frac{12}{240}\times 100=5\%$.

Answer: (i) 10% m/m (ii) 4.5 g (iii) 5% v/v.

Solubility, saturated solutions and solubility curves

NCERT section 5.2 Solutions

Solubility is the largest mass of solute that 100 g of solvent can hold at a stated temperature. When no more solute will dissolve at that temperature, the solution is saturated.

• Solubility of most solids in water increases with temperature.
• Solubility of gases in water decreases as temperature rises (warm soda goes flat faster; fish get less oxygen in warm water).

A solubility curve plots solubility (g per 100 g water) against temperature. A steep curve means solubility changes a lot with temperature.

Data from the NCERT table (g per 100 g water):

Salt	20 °C	40 °C	60 °C	80 °C
Potassium nitrate	32	62	106	167
Sodium chloride	36	36.5	37	37
Potassium chloride	35	40	46	54

Potassium nitrate is very temperature-sensitive; sodium chloride hardly changes. That is why common salt is obtained from sea water by evaporation, not by cooling.

Crystals that come out on cooling = solubility at the higher temperature − solubility at the lower temperature (per 100 g water).

Worked example

A saturated solution of potassium nitrate in 100 g of water at 60 °C is cooled to 20 °C. Using the table, how much potassium nitrate crystallises? What mass of potassium chloride saturates 250 g of water at 60 °C?

Try it first, then show the solutionSolution

1. At 60 °C: 106 g dissolves; at 20 °C: only 32 g can stay dissolved.
2. Crystals formed $=106-32=74$ g.
3. Potassium chloride at 60 °C: 46 g per 100 g water, so 250 g water needs $46\times 2.5=115$ g.

Answer: 74 g of potassium nitrate crystallises; 115 g of potassium chloride saturates 250 g of water at 60 °C.

Crystallisation

NCERT section 5.3 Methods of Separation of Homogeneous Mixtures

A crystal is a solid whose particles are arranged in a regular, repeating pattern (rock salt, *mishri*, snowflakes, frost, quartz).

Crystallisation gets a pure solid from a saturated solution, using the fact that solubility changes with temperature. It is used to purify solids (removing small amounts of impurity) and to separate solids.

Activity 5.3: copper sulphate crystals
1. Dissolve copper sulphate in about 25 mL water in a beaker, warming in a water bath and stirring; the teacher adds a drop of dilute sulphuric acid (stops unwanted reactions).
2. Keep adding copper sulphate until no more dissolves: a hot saturated solution.
3. Filter while hot to remove insoluble impurities.
4. Cover with a watch glass and let it cool slowly without disturbance: large, shiny blue crystals form. Rapid cooling gives smaller, less regular crystals.
5. Filter, rinse with a little cold water and dry.

Safety: copper sulphate is toxic; do not touch it; sulphuric acid is handled by the teacher.

Why not just evaporate to dryness? Soluble impurities stay with the solid, and some solids decompose or char on strong heating (sugar). Crystallisation leaves soluble impurities behind in the liquid.

Coastal communities in India made *panga* salt by boiling concentrated brine and *karkatch* salt by evaporating sea water.

Worked example

Why is crystallisation better than evaporating to dryness for getting pure copper sulphate from an impure sample?

Try it first, then show the solutionSolution

1. Evaporating to dryness leaves every dissolved substance behind, so soluble impurities stay mixed with the product.
2. Strong heating may also remove the water that is part of the blue crystals and spoil them.
3. In crystallisation, only the main substance (present in large amount) comes out as crystals on cooling; impurities in small amounts remain dissolved.

Answer: It leaves soluble impurities in the solution and avoids damage by heating, giving pure, well-formed crystals.

Distillation and fractional distillation

NCERT section 5.3 Methods of Separation of Homogeneous Mixtures

Distillation = heating a liquid mixture so that the liquid with the lower boiling point turns to vapour, then cooling (condensing) the vapour back to liquid in a separate container.

It is used to:

• separate two miscible liquids whose boiling points differ by at least about 25 °C (acetone 56 °C and water 100 °C);
• recover a liquid (solvent) from a solution of a solid, e.g. pure water from salt water.

Set-up: distillation flask with the mixture, on wire gauze and tripod over a burner; a thermometer at the side arm; a water condenser with cold water entering at the lower end and leaving at the upper end; the pure liquid (distillate) collected in a conical flask.

Fractional distillation: used when boiling points differ by less than 25 °C, with a fractionating column. Crude petroleum is separated in refineries into petroleum gas, petrol, kerosene (aviation fuel), diesel, lubricating oil and bitumen. LPG is the gas fraction liquefied under pressure.

In Kannauj, the 'perfume capital of India', *Mitti ka Ittar* (earthy fragrance) is made by the traditional Deg-Bhapka distillation method.

Evaporation versus boiling: evaporation happens at the surface at any temperature; boiling happens throughout the liquid at a fixed temperature (its boiling point).

Worked example

Which pairs can be separated by simple distillation? (a) water (100 °C) and acetone (56 °C) (b) alcohol (78 °C) and chloroform (61 °C) (c) water and salt (d) alcohol (78 °C) and benzene (80 °C)

Try it first, then show the solutionSolution

1. (a) Difference $=100-56=44$ °C ≥ 25 °C: yes.
2. (b) Difference $=78-61=17$ °C < 25 °C: no, needs fractional distillation.
3. (c) Salt does not vaporise, so water distils off and salt stays: yes.
4. (d) Difference $=2$ °C: no, simple distillation cannot separate them.

Answer: (a) and (c).

Paper chromatography

NCERT section 5.3 Methods of Separation of Homogeneous Mixtures

Chromatography (Greek *chroma* = colour, *graphein* = to write) separates substances dissolved in the same solvent because they move up the paper at different rates: each component sticks to the paper and dissolves in the solvent to a different extent.

Activity 5.5:
1. Draw a pencil line about 2 cm from the bottom of a strip of chromatography (or filter) paper. Pencil, because ink would itself separate.
2. Put a small spot of black sketch-pen ink on the line.
3. Hang the strip in a jar with a little water, so the water level is below the spot (otherwise the spot dissolves into the water in the jar).
4. As water rises, the ink separates into coloured bands.

Uses: separating colours in ink and food colours, pigments in spinach leaves or petals (may need alcohol as solvent), detecting drugs in blood, and checking purity of substances.

A pure substance gives one spot; a mixture gives several.

Worked example

A student's chromatogram of a green food colour shows a yellow spot high up and a blue spot lower down. What does this tell you? Give two mistakes that would spoil the experiment.

Try it first, then show the solutionSolution

1. Two spots means the green colour is a mixture of at least two dyes, yellow and blue.
2. Yellow travelled further, so it moves faster with the solvent (sticks less to the paper).
3. Mistakes: drawing the start line in ink; dipping the paper so the solvent covers the spot.

Answer: The green colour is a mixture of a yellow and a blue dye; avoid an ink start line and keep the solvent below the spot.

Separating heterogeneous mixtures: separating funnel and sublimation

NCERT section 5.4 How Can We Separate the Components of Heterogeneous Mixtures?

Immiscible liquids (do not mix, form layers), such as mustard oil and water, are separated with a separating funnel, based on their different densities.
1. Pour the mixture in and let it stand: two layers form; the less dense liquid (oil) is on top.
2. Open the stopcock to run off the lower layer (water).
3. Close it just before the boundary; discard the small mixed portion.
4. Run the upper layer (oil) into a separate container.

Uses: separating oil from water; in extracting iron, the lighter slag is removed from molten iron in a similar way.

Sublimation: some solids change directly from solid to vapour on heating (below their melting point), and the vapour turns directly back to solid on cooling (deposition). Camphor, naphthalene, ammonium chloride, iodine and dry ice (solid carbon dioxide) sublime.

Activity 5.7: heat camphor and sand gently in a china dish covered by an inverted funnel with a cotton plug. Camphor deposits on the cool inner wall of the funnel; sand stays in the dish. The cotton plug stops vapour escaping.

Used to separate a sublimable solid from a non-sublimable one: ammonium chloride from common salt (CBSE practical), naphthalene from sand.

Worked example

A 500 mL bottle of coconut oil has a mass of 460 g. If the oil is shaken with water and left to stand, which layer is on top, and how would you separate them?

Try it first, then show the solutionSolution

1. Density of oil $=\frac{460\text{ g}}{500\text{ mL}}=0.92$ g mL${}^{-1}$; water is 1 g mL${}^{-1}$.
2. Oil is less dense and does not mix with water, so it floats as the upper layer.
3. Use a separating funnel: run off the lower water layer, discard the mixed band, then collect the oil.

Answer: Oil on top (0.92 g mL${}^{-1}$ < 1 g mL${}^{-1}$); separate with a separating funnel.

Suspensions, centrifugation and coagulation

NCERT section 5.4 How Can We Separate the Components of Heterogeneous Mixtures?

A suspension is a heterogeneous mixture in which solid particles (larger than 1000 nm) do not dissolve but stay spread through a liquid for a while. Particles are visible, settle on standing and can be removed by filtration. Examples: sand or chalk in water, muddy water, sawdust or tea leaves in water.

Very fine particles in muddy water may stay suspended and pass through filter paper. Two extra methods help:

Centrifugation: spinning the mixture very fast in tubes. The tubes swing outwards and the heavier particles move outwards to the bottom of the tube, leaving the lighter liquid on top (like feeling pushed outwards in the *phugadi* or *kikli* spinning game). Used to separate blood into plasma and cells, cream from milk, and in industries. The paperfuge, a hand-spun paper and string device, does this without electricity and helps test blood for malaria and anaemia in remote areas.

Coagulation: adding a coagulant such as alum (*fitkari*) makes fine suspended particles clump together. The heavier clumps then settle (sedimentation) and the clear water is poured off (decantation) or filtered. Making paneer is also coagulation: lemon juice or vinegar makes milk proteins clump.

Sewage treatment uses sedimentation, coagulation and filtration.

Worked example

After a flood, village well water stays cloudy even after standing and being filtered through cloth. Suggest a cheap method to clear it and explain each step.

Try it first, then show the solutionSolution

1. Add a little powdered alum and stir: alum makes the fine particles clump together (coagulation).
2. Leave it undisturbed: the larger clumps settle at the bottom (sedimentation).
3. Pour off the clear water gently (decantation) or filter it.
4. Boil or chlorinate before drinking, since separation does not kill germs.

Answer: Coagulation with alum, then sedimentation and decantation or filtration, then disinfection.

Colloids and the Tyndall effect

NCERT section 5.5 Tyndall Effect

A colloid has particles of 1 to 1000 nm: too small to see or to settle, but large enough to scatter light. Examples: milk, blood, tomato sauce, ice cream, fog, clouds, smoke, butter, starch in water.

Property	Solution	Colloid	Suspension
Nature	homogeneous	heterogeneous (looks uniform)	heterogeneous
Particle size	< 1 nm	1 to 1000 nm	> 1000 nm
Visible to the eye?	no	no	yes
Passes through filter paper?	yes	yes	no (residue left)
Settles on standing?	no	no (stable)	yes
Tyndall effect?	no	yes	yes
Example	salt water	milk	chalk in water

Tyndall effect: the scattering of a beam of light by particles, which makes the path of the beam visible. Named after John Tyndall. Seen in sunbeams through a dense tree canopy or a small window in a dusty room, floodlights in a stadium, and car headlights in fog. In Activity 5.1, a laser beam is invisible in salt water but visible in chalk water and diluted milk.

Parts of a colloid: the dispersed phase (particles, like a solute) and the dispersion medium (in which they are spread). In blood, cells and proteins are the dispersed phase and the watery plasma is the medium.

Emulsions: colloids of two liquids. Oil-in-water: milk, vanishing cream. Water-in-oil: butter, cold cream. Emulsifying agents (soap, milk proteins) keep them stable.

Worked example

A laser beam is passed through three beakers: copper sulphate solution, starch in water, and chalk powder in water. In which is the beam visible, and which leaves a residue on filtering?

Try it first, then show the solutionSolution

1. Copper sulphate solution: particles < 1 nm, no scattering; beam not visible; no residue.
2. Starch in water: colloid; beam visible (Tyndall effect); passes through filter paper.
3. Chalk in water: suspension; beam visible; chalk is left on the filter paper.

Answer: Beam visible in starch and chalk mixtures; only the chalk suspension leaves a residue.

Science in real life: Exploring Mixtures and their Separation

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Concentration and Solubility of Solutions
• Separation Methods, Suspensions, Colloids and the Tyndall Effect

Exploring Mixtures and their Separation: formulas and key facts

Mass of solution

$Massofsolution=massofsolute+massofsolvent.$

Mass by mass percentage

$\%\text{ m/m}=\frac{\text{mass of solute}}{\text{mass of solution}}\times 100$

Mass by volume percentage

$\%\text{ m/v}=\frac{\text{mass of solute (g)}}{\text{volume of solution (mL)}}\times 100$

Volume by volume percentage

$\%\text{ v/v}=\frac{\text{volume of solute}}{\text{volume of solution}}\times 100$

Crystals on cooling

$Masscrystallised(per100gwater)=solubilityathighertemperature-solubilityatlowertemperature.$

Particle sizes

$Solution<1nm;colloid1to1000nm;suspension>1000nm.$

Distillation rule

Simple distillation: boiling points differ by about 25 °C or more; less than 25 °C: fractional distillation.

Which method

Crystallisation	pure solid from solution
Distillation	miscible liquids or solvent recovery
Chromatography	dyes
Separating funnel	immiscible liquids
Sublimation	sublimable solid
Centrifugation	fine heavy particles
Coagulation	fine suspended particles with alum

Useful values

Saline drip 0.9% m/v; glucose drip 5% m/v; vinegar about 5% v/v acetic acid; brass about 80% Cu, 20% Zn.

Exploring Mixtures and their Separation: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Exploring Mixtures and their Separation

The question patterns to expect, and the method that works for each.

Tricks to remember Exploring Mixtures and their Separation

Short memory aids. Say each one aloud once, then use it on a question the same day.

• One, thousand

  Solution under 1 nm; colloid 1 to 1000 nm; suspension over 1000 nm.

  Use it when: Particle-size questions.

• Solution in the bottom line

  In every percentage formula, the bottom is the whole solution, never just the solvent.

  Use it when: Avoiding the most common numerical slip.

• Light shows the crowd

  A beam is visible only when there are particles big enough to bump into: colloids and suspensions, not solutions.

  Use it when: Tyndall effect.

• CANNIDI sublimes

  Camphor, Ammonium chloride, Naphthalene, Iodine, Dry Ice.

  Use it when: Recalling substances that sublime.

• Spin, clump, drop

  Centrifuge spins heavy particles out; alum clumps fine particles; clumps drop (sediment) and clear water is poured off.

  Use it when: Cleaning muddy water and separating blood.

Exploring Mixtures and their Separation: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Exploring Mixtures and their Separation

The slips examiners see most often, and what to write instead.

• ✗Mistake: Dividing by the mass of solvent instead of solution

  The question gives solute and solvent masses.

  Wrong working

  20 g sugar in 80 g water: % m/m $=\frac{20}{80}\times 100=25\%$.

  Right working

  Mass of solution $=100$ g; % m/m $=\frac{20}{100}\times 100=20\%$.

  ✓Do this: Add solute and solvent first.

  Usually costs: The answer mark.

• ✗Mistake: Saying colloids are homogeneous

  Milk and blood look uniform.

  Wrong working

  Milk is a homogeneous mixture because it looks the same everywhere.

  Right working

  Milk is a colloid, which is heterogeneous: its particles (1 to 1000 nm) scatter light.

  ✓Do this: If it shows the Tyndall effect, it is not a true solution.

  Usually costs: 1 mark in classification MCQs.

• ✗Mistake: Using simple distillation for liquids with close boiling points

  Students remember 'distillation separates liquids' without the 25 °C rule.

  Wrong working

  Alcohol (78 °C) and benzene (80 °C) can be separated by simple distillation.

  Right working

  Their boiling points differ by only 2 °C; simple distillation will not separate them (fractional distillation is needed for differences under 25 °C, and very close ones are hard even then).

  ✓Do this: Subtract the boiling points before choosing.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Putting the solvent above the spot in chromatography

  Students think the spot must be wetted.

  Wrong working

  Dip the strip so the ink spot is under water.

  Right working

  The solvent level must be below the spot so the solvent rises through it and carries the colours up.

  ✓Do this: Spot above solvent; start line in pencil.

  Usually costs: 1 mark in procedure and diagram questions.

• ✗Mistake: Confusing evaporation, crystallisation and sublimation

  All involve heating and a solid at the end.

  Wrong working

  Crystallisation and evaporation are the same; camphor evaporates.

  Right working

  Evaporation removes all solvent; crystallisation forms pure crystals on cooling a saturated solution; sublimation is solid to vapour directly.

  ✓Do this: Learn one defining phrase for each.

  Usually costs: 1 to 2 marks.

NCERT exercise guide: Exploring Mixtures and their Separation

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Exploring Mixtures and their Separation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which of these is a heterogeneous mixture?
   (a) Brass
   (b) Vinegar
   (c) Fog
   (d) Air

   Hint

   Droplets in air.

   Answer

   (c) Fog

2. Q2 · 1 mark · MCQExam level

   Camphor can be separated from sand by
   (a) filtration
   (b) sublimation
   (c) distillation
   (d) chromatography

   Hint

   Solid to vapour.

   Answer

   (b) sublimation

3. Q3 · 1 mark · Assertion-reasonExam level

   Assertion (A): Alum helps muddy water become clear.
   Reason (R): Alum makes fine suspended particles clump together so they settle.
   (a) A and R are both true, and R explains A correctly.
   (b) A and R are both true, but R does not explain A.
   (c) A is true, R is false.
   (d) A is false, R is true.

   Hint

   Coagulation.

   Answer

   (a)

4. Q4 · 2 marks · Very short answerExam level

   Define solubility. How does it change with temperature for (i) a solid like potassium nitrate (ii) a gas like oxygen?

   Hint

   Maximum per 100 g water.

   Answer

   Solubility is the maximum mass of solute that dissolves in 100 g of solvent at a given temperature. (i) It increases with temperature. (ii) It decreases with temperature.

5. Q5 · 2 marks · Very short answerHarder exam level

   A doctor needs 250 mL of 5% m/v glucose solution. How much glucose is required, and how should it be made?

   Hint

   5 g per 100 mL.

   Answer

   $\frac{5}{100}\times 250=12.5$ g glucose; dissolve it in some water and make the total volume up to 250 mL.

6. Q6 · 2 marks · Very short answerHarder exam level

   Why do we filter the copper sulphate solution while it is hot, and why is slow cooling preferred?

   Hint

   Think of what would crystallise early.

   Answer

   Filtering hot removes insoluble impurities before any copper sulphate crystallises on the filter paper. Slow cooling gives particles time to arrange regularly, forming larger, purer crystals.

7. Q7 · 3 marks · Short answerHarder exam level

   Methanol (boiling point 65 °C) and water (100 °C) are mixed. Name the method to separate them, say which liquid collects first in the receiver, and explain how the thermometer reading tells you when the first liquid has finished distilling.

   Hint

   Difference is 35 °C.

   Answer

   Simple distillation, since the boiling points differ by 35 °C (more than 25 °C). Methanol vaporises first, condenses and collects. The thermometer stays near 65 °C while methanol distils; when it starts rising towards 100 °C, the methanol is nearly all collected and the receiver should be changed.

8. Q8 · 3 marks · Short answerHarder exam level

   Suggest the best separation method for each, with a reason: (i) a cooperative making salt in sunny coastal pans (ii) a refinery getting pure sugar crystals from cane juice (iii) a perfume maker in Kannauj collecting the fragrance of rose petals with steam.

   Hint

   Keep the solid, get pure crystals, or collect a vapour.

   Answer

   (i) Evaporation: sodium chloride's solubility hardly changes with temperature and sunlight removes the water cheaply. (ii) Crystallisation: cooling a concentrated syrup gives pure crystals and leaves impurities in the liquid; evaporating to dryness would char the sugar. (iii) Distillation (the Deg-Bhapka method): the fragrant vapour is carried off and condensed.

9. Q9 · 3 marks · Short answerHarder exam level

   A student says 'milk is a solution because it passes through filter paper and does not settle'. Is the student right? What test would settle the question?

   Hint

   Shine a beam.

   Answer

   No. Colloids also pass through filter paper and do not settle. Shine a laser or torch beam through diluted milk: its path becomes visible (Tyndall effect), so milk is a colloid, not a solution.

10. Q10 · 5 marks · Long answerHarder exam level

    Describe how you would separate a mixture of sand, iron filings and common salt. Give the order of steps and the property used in each.

    Hint

    Magnet first.

    Answer

    1. Move a magnet over the dry mixture: iron filings are attracted (magnetic property). 2. Add water and stir: salt dissolves, sand does not (solubility). 3. Filter: sand stays on the paper (particle size). 4. Evaporate or crystallise the filtrate to get salt (volatile solvent). Wash and dry the sand.

11. Q11 · 3 marks · Short answerHOTS / competency

    Chloroform (density 1.49 g mL⁻¹) and water do not mix. In a separating funnel, which liquid forms the upper layer and which is run out first? Why is this the opposite of oil and water?

    Hint

    Compare densities with water.

    Answer

    Chloroform is denser than water (1.49 > 1 g mL⁻¹), so it sinks and forms the lower layer; it is run out first through the stopcock, and water stays on top. Oil is less dense than water, so with oil the water is the lower layer and is run out first.

12. Q12 · 3 marks · Short answerHOTS / competency

    A salt works gets sodium chloride from sea water by evaporation, while a fertiliser plant gets potassium nitrate by cooling hot solutions. Use solubility data (sodium chloride 36 to 37 g, potassium nitrate 21 to 167 g per 100 g water between 10 °C and 80 °C) to explain why each uses a different method.

    Hint

    How steep is each change?

    Answer

    Sodium chloride's solubility hardly changes with temperature (about 1 g), so cooling would give almost no crystals; the water must be evaporated. Potassium nitrate's solubility falls by about 146 g per 100 g water from 80 °C to 10 °C, so cooling a hot saturated solution gives a large crop of crystals.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Give two examples of homogeneous mixtures.Show answer

   Salt solution and air (also vinegar, brass, soda).

2. 2.Define concentration of a solution.Show answer

   The amount of solute present in a given amount of solvent or solution.

3. 3.What does 0.9% m/v saline mean?Show answer

   0.9 g of sodium chloride in every 100 mL of solution.

4. 4.What is a saturated solution?Show answer

   One that cannot dissolve any more solute at that temperature.

5. 5.How does temperature affect the solubility of gases in water?Show answer

   It decreases as temperature rises.

6. 6.What minimum difference in boiling points suits simple distillation?Show answer

   About 25 °C.

7. 7.Name the process by which a vapour turns directly into a solid.Show answer

   Deposition.

8. 8.Which coagulant is used to clear muddy water?Show answer

   Alum (fitkari).

9. 9.What is the Tyndall effect?Show answer

   Scattering of a beam of light by colloid or suspension particles, making its path visible.

10. 10.Name the dispersed phase and dispersion medium in blood.Show answer

    Dispersed phase: blood cells and proteins; dispersion medium: plasma (mostly water).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you classify ten everyday mixtures as homogeneous or heterogeneous?
• ☐Can you solve % m/m, % m/v and % v/v problems in both directions?
• ☐Can you find the mass that crystallises from a solubility table?
• ☐Can you list the steps of crystallising copper sulphate and say why each matters?
• ☐Can you draw and label a distillation set-up?
• ☐Can you apply the 25 °C rule to pairs of liquids?
• ☐Can you explain how paper chromatography separates dyes?
• ☐Can you name five substances that sublime?
• ☐Can you explain centrifugation and coagulation with examples?
• ☐Can you fill the solution-colloid-suspension table from memory?

Exploring Mixtures and their Separation: frequently asked questions