Exploring Mixtures and their Separation

Class 8 explained that a sugar solution is really countless sugar particles occupying the interparticle spaces of water. That raises a very practical question: once two kinds of particles are mixed this thoroughly, how do you ever get them apart again? The answer is that you exploit a difference in some physical property between the two: particle size, boiling point, how strongly a substance clings to a surface, or density. Each technique in this chapter is really a clever way of using one of these differences.

Stir salt into water, stir chalk powder into water, and add a few drops of milk to water: all three look like uniform mixtures at first glance, but they are not the same kind of mixture at all. Shine a narrow beam of light, like a laser pointer, through each from the side: the salt water lets the beam pass through invisibly, but the milky water and the chalky water make the beam's path visible, as a glowing streak, because their larger particles scatter the light. This light-scattering is called the Tyndall effect. Salt water is a true solution, with particles too small to scatter light or be filtered out. Milk in water is a colloid, with particles small enough to stay mixed indefinitely but large enough to scatter light. Chalk powder in water is a suspension, with particles large enough to eventually settle and be caught on a filter paper.

Saying a solution is 'strong' or 'weak' is not precise enough for medicine, cooking or a lab, so chemists express concentration as a percentage in three specific ways, depending on what is easiest to measure. Mass by mass percentage tells you how many grams of solute are present in 100 grams of the total solution, useful for solids and powders, and is exactly what a packaged food label means when it lists how much salt or sugar it contains. Mass by volume percentage tells you how many grams of solute are in 100 millilitres of solution, used wherever measuring a volume is easier than weighing, such as a glucose drip in a hospital. Volume by volume percentage tells you how many millilitres of solute are in 100 millilitres of solution, used when two liquids that mix are involved, such as vinegar (acetic acid in water) or perfume.

The maximum mass of a solute that can dissolve in a fixed amount of solvent, usually 100 grams, at a particular temperature, is called its solubility at that temperature, and a solution that cannot dissolve any more solute at that temperature is called saturated. Solubility is not fixed: for most solids dissolved in a liquid, solubility increases as temperature rises, which is why a solubility curve, a graph of solubility against temperature, climbs upward and lets you read off exactly how much of a solute will dissolve at any given temperature, or predict what happens if a saturated solution is cooled down.

If a solution is saturated at a high temperature and is then cooled slowly, the solvent can no longer hold as much dissolved solute, since solubility has dropped, so the extra solute separates back out as a solid, often growing as neatly shaped crystals, a solid whose particles are arranged in a regular geometric pattern. This process, called crystallization, is used to separate two dissolved solids when one is present in a much smaller quantity, and to purify a solid, since impurities are usually left behind in the remaining liquid. Rock salt, sugar crystals grown while making candy (called mishri), snowflakes, and even the frost that forms on a cold window are all crystals that formed this same way, just outside a laboratory.

A mixture of acetone and water can be separated by distillation because their boiling points differ enough: acetone boils at about 56 degrees Celsius, water at 100 degrees Celsius. On gentle heating, the liquid with the lower boiling point vaporises first, and this vapour is cooled and collected separately, leaving the higher-boiling liquid behind in the flask. This same idea, on a larger industrial scale, is used in a petroleum refinery: crude oil is a mixture of many different liquids, and fractional distillation, using a tall fractionating column, separates liquids whose boiling points differ by less than about 25 degrees Celsius, splitting crude oil into petrol, kerosene, diesel and other products.

Pour mustard oil and water into a separating funnel and leave it undisturbed: rather than blending, they settle into two clear layers, oil floating on top since it is less dense, water sitting below since it is denser. Liquids that behave this way, refusing to dissolve into each other, are called immiscible. Because the layers stay cleanly separate by density, you can open the funnel's stopcock at the bottom to drain off the lower water layer first, close it before any oil escapes, then reopen it to collect the oil separately. This is exactly how oil is separated from water in everyday situations, from a kitchen accident to an oil spill at sea.

Most solids melt into a liquid before they can become a gas, but a few substances, like camphor, ammonium chloride and iodine, can turn directly from solid to gas without ever passing through a liquid state, a process called sublimation, and the reverse, gas straight back to solid, is called deposition. If camphor is mixed with common salt, gently heating the mixture in a covered dish makes the camphor sublime away as vapour, leaving the salt completely behind; cooling the vapour on a cold surface held above turns it directly back into solid camphor, cleanly separated from the salt without either substance ever needing to melt.

Touch a black sketch-pen dot with a wet finger and you may notice it is not really one colour: it can spread into a smudge of several different colours. Paper chromatography makes this separation precise: a spot of the mixture is placed near the bottom of a strip of paper, and the paper's bottom edge is dipped in a solvent. As the solvent rises up the paper, it carries each dissolved substance along at a different speed, because each clings to the paper fibres, and dissolves in the solvent, to a different degree. Substances that cling less and dissolve more are carried further, so the original spot separates into distinct coloured bands.

Every technique in this chapter separated particles without asking what a particle itself is really made of. The next chapter goes there directly: if matter is built from particles, and those particles are atoms, what is actually inside an atom, and how did scientists ever manage to look inside something far too small to ever truly see?