Mixtures and separating

A carton may promise pure orange juice, but to a chemist the juice is anything but. It is water with sugars, acids and hundreds of other substances mixed in. In chemistry, pure means one substance only, with nothing else in it. By that standard almost nothing around you qualifies. Tap water carries dissolved minerals. Even clean, dry air is a mixture: about 78 per cent nitrogen, 21 per cent oxygen and 1 per cent other gases, mostly argon.

How can anyone tell whether a sample is pure? One test needs only gentle heat and a thermometer. A pure substance melts at one sharp temperature, as ice does at 0 °C. A mixture softens gradually over a range of temperatures, usually starting lower. So chemists melt a little of a new product to check its purity.

Stir a spoonful of sugar into tea and the grains vanish. They have not gone anywhere. Water particles tug the sugar apart, one particle at a time, until its particles are scattered evenly among the water’s. The result is a solution: a mixture blended so finely that it looks like a single clear liquid. Its particles are far too small to see, to settle to the bottom or to be caught by a filter.

Every solution has two parts. The substance that gets dissolved, here the sugar, is the solute. The substance that does the dissolving, here the water, is the solvent, and it is usually the larger part. Water dissolves so many things, from salt to the oxygen that fish breathe, that it is often called the universal solvent.

Has dissolved sugar been destroyed? Put it to the test. Weigh 100 grams of water and 10 grams of sugar, stir until the sugar disappears, and weigh again. The balance reads 110 grams. Nothing was lost. The sugar’s particles are all still in the cup. They have only spread out through the water, too finely to see, and a sip confirms it. Boil the water away and the same 10 grams come back.

How fast sugar dissolves is another matter, and three things hurry it along. In hot water the particles move faster and knock the sugar apart sooner. Stirring carries fresh water to each grain. And fine sugar beats a sugar cube of the same weight, because dissolving happens only where water touches sugar, and many small grains offer far more surface area than one lump.

Keep spooning salt into a glass of water and a point comes when no more will dissolve. Extra grains sink and stay, however long you stir. The solution is now saturated: it holds all the solute it can. The amount that takes, usually given as grams of solute in 100 grams of water at a stated temperature, is the substance’s solubility.

Solubility differs enormously from one substance to the next. At room temperature, 100 grams of water will take about 36 grams of table salt before it is saturated. It will take about 200 grams of sugar, twice the water’s own weight, and only about 10 grams of baking soda. Chalk barely dissolves at all, however long it is stirred.

Does temperature change how much water can hold? For most solids it does, and upward. At 20 °C, 100 grams of water dissolve about 200 grams of sugar. Near boiling the figure is about 480 grams. Sweet makers rely on this. They dissolve a great deal of sugar in hot water, and as the syrup cools it can no longer hold it all.

Gases behave the other way round. A gas becomes less soluble as water warms, because its fast-moving particles escape from the liquid more easily. That is why a fizzy drink left in a warm room goes flat sooner than one kept cold. It matters to fish as well. Fresh water at 0 °C can hold nearly 15 milligrams of oxygen in each litre, but at 30 °C it holds only about half as much.

Muddy water is a mixture of a different kind. Its grains of soil are only hanging in the water, not dissolved, and each grain is many thousands of times wider than a water particle. That gap in size is all a filter needs. In filtration, the mixture is poured through a barrier full of fine holes, such as paper. Water slips through, while the grains are too big and stay behind. A coffee filter, a tea strainer and the sand beds of a waterworks all sort by size in the same way.

The method has a firm limit. Pour salt water through the finest filter paper and it comes out just as salty. The salt is dissolved, so its particles are about as small as the water’s own and pass through the holes alongside them. An ordinary filter can only catch what never dissolved.

How do you recover something that has dissolved? Take away the solvent. On hot, dry coasts people have done this for thousands of years in salt pans, shallow ponds of sea water left open to the Sun and wind. Sea water is about 3.5 per cent dissolved salt by weight, some 35 grams in every kilogram. As the water leaves by evaporation, turning to vapour and drifting off, the same salt is crowded into less and less water. Once the solution is saturated, the salt can no longer all stay dissolved, and it begins to come out as solid crystals.

This growth of crystals from a solution is called crystallisation. The particles stack in a regular pattern as they settle, which is why salt forms neat cubes. Slow evaporation gives large, well-shaped crystals, and fast boiling gives many tiny ones.

Evaporation saves the solid and throws the liquid away into the air. What if the liquid is what you want? Then the vapour has to be caught. In distillation, a mixture is boiled in a closed vessel and its vapour is led away through a cooled tube. On the cold surface the vapour turns back into liquid, a change called condensation, and the drops run down into a separate container. The whole apparatus is called a still.

Distil sea water and its two parts end up in different places. The water boils off, condenses and collects as fresh water, fit to drink. The salt does not boil at anything like 100 °C, so it stays behind in the first vessel. The method works whenever the parts of a mixture turn to vapour at very different temperatures.

Who first built a still? Nobody knows. Greek-speaking alchemists in Roman Egypt had them by about the year 300, when Zosimos of Panopolis described the apparatus. The chemists of the early Islamic world used the still widely and put the craft in writing. The many works under the name of Jābir ibn Ḥayyān report distilling one substance after another. The Book of the Chemistry of Perfume and Distillations, attributed to the ninth-century Baghdad scholar al-Kindī, gives 107 recipes for perfume and describes distilling roses for their scent. Around 900 the physician al-Rāzī, in his Book of Secrets, listed a laboratory’s equipment and its standard operations, among them distillation and filtration.

The words travelled with the craft. The cap of the still, where the vapour gathers, was al-anbīq in Arabic, a word borrowed from Greek. Latin Europe made it alembic. Chemistry itself is named through alchemy, from the Arabic al-kīmiyāʾ.

Salt and water are easy to part, because salt hardly turns to vapour at all. Two liquids are harder, since both evaporate. The way in is that every liquid has its own boiling point. Warm a mixture slowly and the liquid that boils at the lowest temperature turns to vapour first and can be collected first, then the next. Separating liquids in this order is called fractional distillation.

Industry does it on a vast scale. An oil refinery sorts crude oil into petrol, diesel and road tar this way. Even air is distilled. Chilled to about minus 200 °C, air becomes a pale blue liquid. As it warms, nitrogen boils off first, at minus 196 °C, and oxygen follows at minus 183 °C. Most of the oxygen in hospital tanks was separated from the air like this.

Black ink looks like one colour. Put a dot of it near the bottom of a strip of absorbent paper, dip the end in water and wait. As the water creeps up the strip it carries the ink along, and the dot smears out into separate bands, perhaps blue, red and yellow. The black was a mixture of dyes. This method is chromatography, from the Greek for colour writing. The botanist Mikhail Tsvet gave it that name in 1906, after using it to split the green of leaves into its green and yellow pigments.

Why do the dyes part company? Each one is pulled two ways. It dissolves in the moving water, and it clings to the paper. A dye that is more soluble and clings less is carried further, so each ends up at its own height.

A gold panner scoops river gravel into a shallow pan, adds water and swirls. Sand and mud wash over the rim, and the flecks of gold stay put. No filter is involved and nothing dissolves. The sorting is done by density, the mass packed into each cubic centimetre. Gold has about 19 grams in every cubic centimetre and sand under 3, so in moving water the gold sinks fastest and settles underneath.

Left alone, many mixtures sort themselves this way, slowly. Oil floats on water, and mud settles in a pond. A centrifuge hurries the job. It spins the mixture at thousands of turns a minute, which flings the densest part outward to the bottom of the tube. Spin a tube of blood for a few minutes and its red cells, the densest part, are packed at the bottom, with pale yellow plasma above them.

Could a filter be made fine enough to hold back what is dissolved? It can. A membrane is a thin sheet with holes so small that water and other small particles pass while larger ones are stopped. Your kidneys clean your blood with membranes like this, and a dialysis machine does the job when kidneys fail. Blood flows along one side of a membrane and clean fluid along the other. Waste particles, crowded in the blood and scarce in the fluid, spread across by diffusion. Blood cells are too big to follow.

The same idea now supplies drinking water. In reverse osmosis, sea water is pressed against a membrane that lets water through and holds salt back. It takes great pressure, around sixty times that of the atmosphere, but it uses less energy than boiling. About seventy per cent of the world’s desalted water is made this way.