Sand, salt and ink: five ways to pull a mixture apart

How to use this simulation

1. Pick a method from the list. It starts on filtration with sand and water. Press play and sand collects on the filter paper while clean water fills the beaker below.
2. Move the slider. In filtration it sets how much insoluble solid is in the 100 g sample, and the readout shows residue plus filtrate always adding back to 100 g.
3. Switch to simple or fractional distillation. In the graph underneath, find where the temperature line goes flat: that flat part is the boiling point. Fractional distillation has two plateaus, ethanol first, then water.
4. Choose paper chromatography. As the solvent climbs, the coloured spots separate. Each label shows its Rf, and the brackets on the left give the distances in millimetres.
5. Choose crystallisation. The solution is heated to concentrate it, then cooled, and crystals form on the bottom. Change how much salt was dissolved hot and watch the yield change.

From the kitchen to the refinery

Draining rice, using a tea strainer, pouring dal through a sieve: all filtration. Salt farmers on the coast dry sea water in the sun and collect the salt, which is close cousin to crystallisation. Boiling sugarcane juice down to jaggery and sugar uses the same idea of concentrating a solution until solid appears.

Industry does the same jobs at scale. A refinery separates petrol, diesel and kerosene from crude oil by fractional distillation. Pharmaceutical labs purify compounds with chromatography and crystallisation. Forensic scientists use paper chromatography to match the ink on a document.

Exam questions usually read: "Which method would you use to separate this mixture, and why?" The recipe for a full answer is the same every time: name the property that differs between the two substances, then name the method.

What a mixture is, and why it can be separated

The substances in a mixture are not chemically joined. Each keeps its own properties: salt still tastes salty in salt water, and water still boils at its usual temperature. Those leftover properties are the tools. Breaking a compound needs a chemical reaction, but breaking a mixture needs only physical methods.

Two words help. The residue is what stays behind on the filter or in the flask. The filtrate or distillate is what passes through or boils over.

How to read the animation

The title line names the method and mixture. The controls underneath hold the method, the mixture and one slider. The slider changes with the method: solid mass for filtration, volume for distillation, how far the solvent climbs for chromatography, and salt dissolved hot for crystallisation.

In the distillations the top half of the canvas shows the apparatus and the bottom half plots temperature against time. The red dot is the temperature now. The thermometer on the flask and the dot on the graph are two views of the same number.

Everything is idealised: steady heating, and a pure liquid that boils at one fixed temperature. Real runs wobble a little, but the idea does not change.

Filtration: a game of particle size

Filter paper has tiny pores. Water molecules and dissolved substances slip through, but grains of sand or chalk are too big and get caught. What is caught is the residue; what runs through is the filtrate. Nothing is lost: a 100 g sample with 20 g of sand gives 80 g of filtrate.

Filtration only works for an insoluble solid in a liquid. Pour salt water through filter paper and the salt walks straight through, because it is dissolved down to single ions. That job belongs to distillation or crystallisation.

Good habits: wet the paper so it sticks to the funnel, pour down a glass rod so nothing splashes, and never fill above the paper edge.

Simple distillation: turn the liquid to vapour and catch it

Heat salt water in a flask and the water boils at 100 °C. The vapour passes through the condenser, cools back to liquid water and drips into the receiver. The salt is not volatile, so it stays in the flask. That is simple distillation: a liquid separated from a dissolved solid.

Take 80 mL of solution in the animation. The thermometer must climb from 25 °C to 100 °C, a rise of 75 degrees, which takes 9.4 time units at steady heating. Then the line goes flat and water distils for 16.0 time units. On the plateau the heat is turning liquid into vapour, not raising the temperature.

Cooling water enters the condenser at the bottom and leaves at the top, so the jacket stays full. The thermometer bulb sits at the side-arm opening because we want the temperature of the vapour, not of the liquid.

Fractional distillation: when both liquids boil

Ethanol boils at 78 °C and water at 100 °C. Both are volatile, so plain distillation would carry over some of each. A fractionating column sits above the flask, packed with glass beads. Rising vapour condenses on the beads, drips back, evaporates again, and each round leaves the vapour richer in the lower-boiling liquid.

With 40 mL of ethanol in 100 mL, the thermometer holds at 78 °C for 8.0 time units while ethanol collects. Then it climbs 22 degrees and holds at 100 °C for 12.0 time units while water collects. The second plateau is 1.5 times as long because there is 60 mL of water against 40 mL of ethanol.

In the animation the delivery tube swings from the first receiver to the second between the plateaus. In a real lab you swap the receiver by hand. A refinery uses the same principle in a very tall column, drawing different fractions off at different heights.

Paper chromatography: a race between colours

Draw a pencil baseline near the bottom of a paper strip and put a drop of ink on it. Stand the strip in solvent, keeping the solvent level below the baseline. The solvent creeps up the paper and drags the dyes with it. A dye that dissolves well in the solvent and clings little to the paper travels far; one that clings stays low.

The distance is summarised as Rf: Rf = distance moved by the spot ÷ distance moved by the solvent front, both measured from the baseline. Rf has no unit and lies between 0 and 1. The black ink here holds 3 dyes: Yellow dye, Red dye, Blue dye. With the solvent front at 80 mm the spots sit at 64, 44 and 20 mm, so Rf is 0.80, 0.55 and 0.25.

With the same solvent and temperature, the same dye gives nearly the same Rf. So you can measure an unknown spot and compare it with known dyes. Change the solvent and the Rf values change, so compare only within one solvent.

Crystallisation: cool it and let it set

Most salts dissolve better hot than cold. Cool a nearly saturated hot solution slowly and the extra salt can no longer stay dissolved, so it forms crystals. The imaginary salt in the animation keeps 20 g dissolved per 100 g of cold water. That is an illustrative value, not a real compound's.

If 50 g was dissolved hot, 20 g stays in solution and 30 g of crystals appear. Yield = 30 ÷ 50 × 100 = 60.0%. The rest stays in the mother liquor, so yield is never 100%.

Slow cooling gives large, well-shaped crystals; fast cooling gives fine powder. Filter the crystals, rinse with a little cold water and dry them between sheets of filter paper.

Which method for which mixture

Do not just memorise this table. Cover the right-hand column and rebuild it.

Try it yourself in the simulation

Write your prediction first, then run the animation.

• Experiment 1: filtration of sand and water, with 5 g and then 40 g of solid. How does the filtrate change? What stays constant?
• Experiment 2: simple distillation at 40 mL and 100 mL. Does the plateau temperature change, or only its length?
• Experiment 3: fractional distillation with ethanol from 10 to 80 mL. Which plateau grows and which shrinks?
• Experiment 4: chromatography with the front at 4 cm, then 9 cm. The spot distances change; does Rf?
• Experiment 5: crystallisation with 30 g and then 80 g dissolved hot. Why does the yield climb?

Worked problems

Every number below comes from the same calculation the animation runs.

Mistakes everyone makes

These cost marks every year.

• Trying to filter salt water. Salt is dissolved to ions and passes straight through the paper.
• Thinking no heat is being used on a plateau. The heat is turning liquid into vapour.
• Dipping the thermometer into the boiling liquid. The bulb belongs at the side-arm to read the vapour.
• Running condenser water the wrong way. It goes in at the bottom and out at the top.
• Drawing the chromatography baseline in pen. The pen ink runs too. Use pencil.
• Starting with the solvent above the baseline. The sample washes away into the jar.
• Writing Rf above 1 or giving it a unit. It is a ratio and always less than 1.
• Expecting a 100% yield from crystallisation. Some salt always stays in the mother liquor.

Exam corner (GCSE, IGCSE, O Level)

Answer without looking, then check.

• Give two differences between simple and fractional distillation.
• A 100 g sample has 20 g of insoluble solid. How much filtrate? Answer: 80 g.
• A spot moves 33 mm while the solvent moves 60 mm. Find Rf. Answer: 0.55.
• 50 g dissolved hot and 20 g stays in cold solution. Mass of crystals? Answer: 30 g.
• Which method separates ethanol and water, and why?

Revision: the whole page in a minute

Read this just before the exam.

• Choosing a method means finding which property of the two substances differs.
• Filtration: insoluble solid plus liquid. Residue + filtrate = sample.
• Simple distillation: liquid plus dissolved solid. Water plateau at 100 °C.
• Fractional distillation: liquids with close boiling points. Ethanol 78 °C first, water 100 °C second.
• Chromatography: Rf = spot distance ÷ solvent distance, between 0 and 1.
• Crystallisation: cool a hot saturated solution slowly; yield = crystals ÷ dissolved hot × 100.

Frequently asked questions