Using a suitable solvent

1. A mixture can be separated because its parts have different physical properties, such as solubility, boiling point and state.
2. A mixture of two solids is often separated by adding a suitable solvent, one that dissolves one solid but not the other.
   1. Water dissolves sodium chloride but not sand, so adding water to a mixture of the two puts the salt into solution and leaves the sand as a solid.
   2. Water dissolves sugar but not chalk, so the same idea separates a mixture of sugar and powdered chalk.
3. Stirring and gentle warming help the soluble solid dissolve faster and more completely.
4. Once one substance has dissolved, the mixture is a solid in a liquid, which can be separated by filtration.

Filtration

1. Filtration separates an insoluble solid from a liquid or a solution.
2. A filter paper is folded into a cone and placed in a filter funnel above a conical flask or beaker.
3. The tiny holes in the filter paper let water and dissolved particles through but are too small for grains of solid to pass.
4. The solid trapped on the paper is the residue, and the liquid collected below is the filtrate.
5. The residue can be washed with a little distilled water to rinse off any solution clinging to it, and then dried.
6. Filtration cannot separate a dissolved solute from its solvent, because dissolved particles pass straight through the paper.

Crystallisation

1. Crystallisation obtains a pure, solid sample of a soluble substance from its solution.
2. The solution is heated in an evaporating basin to boil off some of the water and make the solution more concentrated.
3. A cold glass rod dipped into the solution shows when it is ready: if small crystals form on the rod as it cools, the solution is close to saturated.
4. Heating is then stopped and the hot solution is left to cool slowly.
   1. The solid is less soluble in cold water than in hot water, so as the solution cools, the solid that can no longer stay dissolved comes out as crystals.
   2. Slow cooling gives larger, better-shaped crystals.
5. The crystals are filtered off, washed with a little cold distilled water and dried between filter papers or in a warm oven.
   1. Cold water is used for washing because less of the crystals dissolves in it.
6. Any soluble impurities are present in only small amounts, so they stay dissolved in the remaining solution instead of forming crystals.
7. Crystallisation is better than evaporating to dryness, which leaves soluble impurities mixed with the product, can make the hot solid spit and drives off any water of crystallisation.

Simple distillation

1. Simple distillation separates a solvent from a solution, collecting the solvent and leaving the solute behind.
   1. It is used to obtain pure water from seawater or from a solution of ink.
2. The solution is heated in a flask until the solvent boils and turns to vapour.
3. The vapour passes into a condenser, a tube surrounded by a jacket of cold flowing water.
   1. The cold water enters at the bottom of the jacket and leaves at the top, so the jacket stays completely full and the condenser stays as cold as possible.
4. The vapour condenses back into a liquid inside the condenser and drips into a collecting vessel as the distillate.
5. The dissolved solid does not evaporate, so it stays in the flask as a residue.
6. A thermometer with its bulb level with the side arm measures the temperature of the vapour leaving the flask.
   1. A steady reading of $100{}^{\circ }C$ shows that the vapour, and so the distillate, is water.
7. Anti-bumping granules in the flask make the liquid boil smoothly instead of in sudden violent bursts.
8. Simple distillation can also separate two liquids whose boiling points are very far apart.

Fractional distillation

1. Fractional distillation separates miscible liquids whose boiling points are close together, such as ethanol, which boils at $78{}^{\circ }C$, and water, which boils at $100{}^{\circ }C$.
2. A fractionating column, a long glass tube packed with glass beads, is fitted between the flask and the condenser.
3. The column is hottest at the bottom and coolest at the top.
4. As the mixture boils, the vapour rises up the column, condenses on the cool beads and is boiled again by more hot vapour, many times over.
5. Each time this happens, the vapour becomes richer in the liquid with the lower boiling point.
6. The liquid with the lowest boiling point reaches the top of the column first and passes into the condenser.
   1. While ethanol distils over, the thermometer reading stays steady at about $78{}^{\circ }C$.
7. When almost all of the ethanol has distilled, the thermometer reading rises towards $100{}^{\circ }C$, and the collecting flask is changed so that the water is kept separate.

Choosing a technique

1. To choose a method, decide what state each substance is in and which property differs between them.
2. Insoluble solid and a liquid: use filtration, as for sand in water.
3. Soluble solid wanted from its solution: use crystallisation, as for copper(II) sulfate crystals from copper(II) sulfate solution.
4. Solvent wanted from a solution: use simple distillation, as for pure water from seawater.
5. Two solids, only one of them soluble: add a suitable solvent, filter, and then crystallise the soluble solid from the filtrate.
6. Miscible liquids with close boiling points: use fractional distillation, as for ethanol from a mixture of ethanol and water.

Melting points, boiling points and purity

1. Every pure substance has a fixed melting point and a fixed boiling point at a given pressure.
   1. Pure water melts at $0{}^{\circ }C$ and boils at $100{}^{\circ }C$ at normal atmospheric pressure.
2. You can identify an unknown substance by measuring its melting point or boiling point and comparing the value with data-book values.
3. A pure solid melts sharply, at a single temperature.
4. An impurity lowers the melting point of a solid and makes it melt over a range of temperatures.
5. An impurity raises the boiling point of a liquid and makes it boil over a range of temperatures.
   1. Seawater boils above $100{}^{\circ }C$, and its boiling temperature keeps rising as water boils away and the salt becomes more concentrated.
6. The closer a measured melting point is to the data-book value, and the narrower its range, the purer the sample.