Chemistry · Fundamentals of chemistry · Grade 7

Pure substances and mixtures

How can pure table salt be separated from cloudy rock salt?

The situation

In the Kugitang mountains of Surkhandarya lies the Khojaikon rock salt mine. Rock salt dug out of the mountain is grey and cloudy: sand and clay are mixed into it. The white table salt in the shop, on the other hand, is almost a pure substance. How can clean salt be separated from cloudy salt? More generally, how do we know whether a substance is pure or a mixture, and by what methods are mixtures separated?

Topic

Pure substances and mixtures: methods of separation. Purifying contaminated rock salt: dissolving, filtering and evaporating Khojaikon (Surkhandarya) salt; separating iron filings with a magnet and spirit by distillation

Key ideas

Homogeneous and heterogeneous mixtures
Filtration and evaporation
Yield, %

Formulas

η = \frac{m_{pure}}{m_{sample}} \cdot 100 %

Yield (fraction of purified substance): the ratio of the mass of pure salt obtained to the mass of the sample taken.

s (NaCl) = 35.9 g / 100 g water (20 °C)

Solubility: the greatest mass of salt that can dissolve in 100 g of water (at 100 °C — 39.0 g).

ρ = \frac{m}{V}

Density is a physical property for identifying a substance: iron 7.87, sand 2.65, table salt 2.17, ethanol 0.789 g/cm³.

R_{f} = \frac{ℓ_{spot}}{ℓ_{solvent}}

In chromatography: the ratio of the distance travelled by the spot from the start line to the distance travelled by the solvent front.

p_{water} + p_{organic} = p_{atm}

Steam distillation: a mixture boils below 100 °C when the sum of the vapour pressures of the two liquids equals atmospheric pressure.

Theory

Substances and their properties

A substance is what bodies are made of: a nail — of iron, a glass — of glass. Each substance is recognised by its properties: colour, smell, state of matter, density, melting and boiling points, solubility in water, electrical and thermal conductivity, attraction to a magnet. For example, iron is grey, has a density of 7.87 g/cm³, melts at 1538 °C and is attracted by a magnet; sulfur is yellow, insoluble in water, and because it is not wetted its powder floats on the surface of the water; table salt is white, melts at 801 °C, and 35.9 g of it dissolve in 100 g of water at 20 °C. Try it in “Properties” mode.

Pure substances and mixtures

A pure substance consists only of identical particles and has constant properties: distilled water boils at exactly 100 °C at normal atmospheric pressure and freezes at 0 °C. A mixture is a simple combination of several substances without chemical bonding between them: each substance keeps its own properties, so they can be separated by physical methods. In a homogeneous mixture (salt solution, air, alcohol and water) the parts cannot be told apart even under a microscope. In a heterogeneous mixture the parts can be seen by eye, with a lens or under a microscope: sandy water (a suspension), milk (an emulsion — fat droplets ≈ 1–10 µm), iron and sulfur powder.

Methods of separating mixtures

Settling — heavy solid particles sink to the bottom of the liquid. Filtration — undissolved solid particles stay on the filter paper, while the solution (the filtrate) passes through. Evaporation — water evaporates from the solution and the dissolved substance crystallises. A magnet separates iron from sulfur. Distillation is based on the difference in boiling points: alcohol (78.4 °C) evaporates before water (100 °C), and the vapour condenses in the condenser and falls into the receiver as a liquid. In chromatography dyes rise along the paper with the solvent at different speeds. The method is chosen according to which property of the substances differs.

Practical: purifying contaminated table salt

The sample (10.00 g) is weighed and dissolved in 30 mL of water (up to 10.8 g of salt dissolves in 30 g of water at 20 °C). Sand and clay do not dissolve — the solution is filtered, and the filter paper is rinsed with a little water (otherwise salt solution stays behind in the damp paper). The filtrate is evaporated in a porcelain dish; at the end the crystals are stirred and the flame is lowered so that they do not spatter. The dry salt is weighed and the yield is calculated: $η = m_{pure} / m_{sample} \cdot 100 %$ . Safety: hold the hot dish and tripod with tongs, do not lean over the dish being evaporated, and wear safety goggles.

University: purification and physical constants

Recrystallisation: the substance is dissolved completely in a hot solvent; on cooling, the solubility falls and pure crystals separate, while the impurities stay in the solution. Benzoic acid dissolves in water to 5.63 g/100 mL at 100 °C, but only 0.17 g/100 mL at 0 °C. Melting point is a criterion of purity: an impurity lowers the melting point and widens the melting range ( $Δ T \approx K x$ , $K = R T_{m}^{2} / Δ_{fus} H \approx 72$ K for benzoic acid). Distillation: in an ideal mixture $p = x_{1} p_{1}^{0} + x_{2} p_{2}^{0}$ (Raoult's law), and the vapour is enriched in the more volatile component. Under vacuum the boiling point falls (toluene ≈ 62 °C at 20 kPa); heat-sensitive substances are distilled this way. Steam distillation: aniline + water boils at 98.4 °C, and the distillate contains ≈ 0.31 g of aniline per 1 g of water. Sublimation: a solid evaporates without melting and crystallises on a cold surface (naphthalene, benzoic acid).

Model and sources

Solubilities, densities and temperatures — CRC Handbook and NIST WebBook; vapour pressures — the Antoine equation (Lange's handbook); ethanol–water equilibrium — the measurement table in Perry's handbook. Model: the processes are shown with time speeded up (distillation ×10, evaporation ×10, crystallisation and melting ×20); heat loss is not taken into account; the contraction in volume when liquids are mixed is ignored; acetone–toluene, chloroform–toluene and benzene–aniline are taken as ideal (Raoult); the “onset” of melting is the moment when 25 % of the sample has melted; losses in the practical (solution left in the wet filter, spattering) are qualitative estimates. The dyes in the chromatogram and their $R_{f}$ values are illustrative; the composition of ink differs with the brand.

Did you know?

Chromatography was invented at the beginning of the 20th century by the Russian botanist Mikhail Tsvet to separate plant pigments; the name comes from the Greek “chroma” — colour — and “grapho” — I write.
The average salinity of ocean water is ≈ 35 ‰: every kilogram of seawater contains ≈ 35 g of dissolved salts, most of which is sodium chloride. In many countries table salt is obtained by evaporating seawater in the sun.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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