Methods of Separating Mixtures

Introduction: Why We Separate Mixtures

Almost nothing in nature comes ready-made for our use. Sea water has to give up its salt; crude petroleum from the ground has to be split into petrol, diesel, kerosene and bitumen; cream has to be lifted out of milk; sand and other dirt has to be cleared from drinking water. Each of these is a separation problem. The trick is to look at the mixture, find a property in which the components differ — particle size, density, boiling point, solubility, ability to be dragged by a moving solvent — and exploit that difference. This part walks through the seven separation techniques covered in NCERT Chapter 5 and ends with the most fundamental split of all: elements vs compounds.

5.12 Evaporation — Salt from Sea Water

Evaporation is used when the solute is a solid that does not decompose on heating, dissolved in a volatile liquid (usually water). The mixture is heated; the liquid escapes as vapour and the solid is left behind. Salt from sea water, sugar from sugarcane juice and dried inks all use this principle. In the laboratory we use a china dish on a wire gauze.

5.13 Centrifugation — Cream from Milk, Components of Blood

Centrifugation is used when the particles are so small they will not settle out by themselves — for example fine cream droplets in milk, or red blood cells suspended in plasma. The mixture is spun in a centrifuge. The denser particles are flung outwards and pile up at the bottom of the tube; the lighter components float at the top.

5.14 Separating Funnel — Two Liquids that Do Not Mix

If two liquids are immiscible — that is, they refuse to mix — they form two clear layers. A separating funnel lets us drain the lower layer through the stopcock, then close the tap when the upper layer reaches it. Oil and water, kerosene and water, and the mixture of an organic solvent with an aqueous extract in the chemistry lab are all separated this way.

5.15 Chromatography — Splitting the Dyes in an Ink Spot

Chromatography uses the fact that different components of a mixture travel at different speeds when carried by a solvent across a stationary medium. In paper chromatography:

1. A pencil line is drawn on a strip of filter paper near the bottom.
2. A small spot of black ink is placed on the line.
3. The strip is dipped (with the spot just above the liquid) into water in a beaker.
4. Water rises up the paper and carries the dyes upward; different dyes travel different distances and form coloured bands.

5.16 Distillation — Separating Two Miscible Liquids by Boiling Point

Two miscible liquids that have very different boiling points (a difference of at least 25°C) can be separated by simple distillation. The mixture is heated. The liquid with the lower boiling point boils first, its vapour passes into a water-cooled condenser, turns back to liquid (the distillate) and is collected. Acetone (b.p. 56°C) and water (b.p. 100°C) are easily separated this way.

5.17 Fractional Distillation — Many Liquids, Close Boiling Points

When the boiling points of two liquids differ by less than 25°C (or when many liquids are mixed), simple distillation cannot give pure components. We use fractional distillation. A long column packed with glass beads is fitted between the flask and the condenser. As vapours travel up, they cool, condense and revapourise many times. Each time the vapour gets purer in the more volatile component. By the time it reaches the top, only the most volatile liquid passes into the condenser.

Industrial uses include: petroleum refining (separating crude oil into petrol, diesel, kerosene, lubricants and bitumen) and obtaining oxygen, nitrogen and argon from liquefied air.

5.18 Crystallisation — Pure Copper Sulphate

Crystallisation gives purer solids than simple evaporation. Steps to purify impure copper sulphate:

1. Dissolve impure copper sulphate in the minimum amount of hot water in a china dish.
2. Add a few drops of dilute sulphuric acid to make the solution clear.
3. Filter the hot solution to remove insoluble impurities.
4. Cover the filtrate and let it cool slowly. Beautiful blue crystals of pure copper sulphate form.
5. Filter the crystals and dry them between sheets of filter paper.

Why is crystallisation better than evaporation? Because some solids decompose on heating, evaporation may give an impure solid contaminated by soluble impurities. Crystallisation lets impurities stay in the mother liquor.

5.19 Quick Reference — Which Method for Which Mixture?

5.20 Pure Substances Revisited — Elements and Compounds

Pure substances themselves come in two kinds.

Element

An element is a pure substance built from only one kind of atom. Iron (Fe), copper (Cu), oxygen (O₂), hydrogen (H₂), gold (Au) and carbon (C) are all elements. Elements can be metals (iron, copper, gold), non-metals (oxygen, sulphur, carbon) or metalloids (silicon, germanium).

Compound

A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion. Water (\(\text{H}_2\text{O}\)) always has hydrogen and oxygen in the ratio 1:8 by mass. Common salt (NaCl) always contains sodium and chlorine in a fixed ratio. The properties of a compound are completely different from those of the elements that built it.

Competency-Based Questions L4 Analyse

Assertion & Reason Questions L5 Evaluate