Separation using a suitable solvent

A mixture can be separated by adding a liquid that dissolves one component but not another. The liquid is the solvent. The component that dissolves enters the solution, while the insoluble component remains as an undissolved solid.

A suitable solvent is chosen because the components have different solubilities in it. For example, water can be used to separate a mixture of salt and sand: salt dissolves in water, but sand does not.

After the water is added and stirred, the mixture can be filtered. The sand remains on the filter, while the salt solution passes through. If the salt is needed, water can be evaporated from the solution, leaving the salt behind.

Filtration

Filtration separates an insoluble solid from a liquid. The mixture is passed through a filter, which allows the liquid through but retains the solid particles.

The liquid that passes through the filter is called the filtrate. The insoluble solid left on the filter is called the residue.

Substances dissolved in the liquid pass through the filter with it and remain in the filtrate. Filtration therefore does not separate a dissolved substance from the liquid.

Crystallisation

Crystallisation recovers a dissolved solid from a solution by forming solid crystals of the solute. The solute is dissolved in a liquid called the solvent.

Some of the solvent is removed by evaporation, making the solution more concentrated. When the solution is saturated, it contains as much dissolved solute as it can hold at that temperature. Further evaporation can cause excess solute to form crystals.

A concentrated solution can also form crystals when it cools. For many solids, solubility decreases as the temperature falls, so the cooler solution cannot keep as much solute dissolved. The excess solute forms solid crystals.

The crystals are separated from the remaining solution, for example by filtration, and then dried. Some solute may remain dissolved in the solution.

Simple distillation

Simple distillation separates a liquid from a mixture by heating the mixture to produce vapour, then cooling the vapour so that it condenses back into a liquid. The condensed liquid is collected separately and is called the distillate.

Separating a solvent from a dissolved solid

When a solution is heated, the solvent vaporises while the dissolved solid remains in the container. The solvent vapour passes into a condenser, where it cools and becomes liquid again. The liquid solvent is then collected as the distillate.

Separating liquids with different boiling points

Simple distillation can also separate liquids whose boiling points are sufficiently different. On heating the mixture, the liquid with the lower boiling point vaporises preferentially, so the vapour contains a greater proportion of that liquid than the original mixture. Cooling and condensing the vapour produces a distillate enriched in the lower-boiling liquid.

Fractional distillation

Fractional distillation separates a mixture of miscible liquids: liquids that mix together to form one uniform mixture. It uses differences in their boiling points. The liquid with the lower boiling point is more volatile, so it vaporises more readily.

When the mixture is heated, vapour rises into a fractionating column. The vapour contains both liquids, but is richer in the more volatile liquid than the original mixture.

Separation in the fractionating column

The column is hotter at the bottom and cooler towards the top. As vapour rises, some of it cools and condenses. The condensed liquid can be heated and vaporised again as it meets rising vapour. These repeated cycles of condensation and vaporisation make the vapour progressively richer in the more volatile liquid.

The vapour that reaches the top passes into a condenser, where it cools and becomes liquid. This liquid is collected first. More of the less volatile liquid remains in the flask or column, or is collected later as the temperature rises.

The repeated separation in the column is useful when the liquids have boiling points close enough that simple distillation would not separate them effectively.

Selecting separation and purification techniques

To choose a separation technique, identify which substance you want to obtain and how its physical properties differ from those of the other substances in the mixture. In particular, consider whether each substance dissolves in a chosen solvent and whether liquids have different boiling points.

Choosing a technique

• Filtration separates an insoluble solid from a liquid. The solid remains on the filter, while the liquid and any substances dissolved in it pass through. Choose filtration when an insoluble solid needs to be removed or collected.
• Crystallisation recovers a dissolved solid from a solution. Some solvent is evaporated, and the solution is allowed to cool so that crystals form. The crystals can then be separated from the remaining liquid and dried.
• Distillation can recover a liquid from a solution. The liquid is boiled to form a vapour, which is then cooled and condensed for collection. Distillation can also separate liquids with different boiling points.
• Fractional distillation is used to separate liquids whose boiling points are closer together. A fractionating column provides repeated evaporation and condensation, improving the separation before the vapour is collected as a liquid.

The desired product affects the choice. For a solution of a solid in a liquid, crystallisation can recover the solid, whereas distillation can recover the liquid.

Combining techniques

A mixture may need more than one technique if the desired substance first passes into an intermediate solution or mixture. Solvent choice can make this possible: select a solvent in which the desired substance dissolves but another component does not.

For example, to obtain salt from a mixture of salt and sand, add water. The salt dissolves, but the sand does not. Filter the mixture to remove the sand; the salt passes through the filter dissolved in the water. Then use crystallisation to obtain salt from the solution.

Identification and purity from melting and boiling points

A pure substance melts and boils at characteristic temperatures under specified conditions. Measuring one of these temperatures and comparing it with a reference value can therefore help identify the substance.

If the measured value agrees with the reference value, it supports the identification, but it does not prove it: different substances can have similar melting or boiling points. If the values disagree, the proposed identification is called into question. The comparison should be made under the same conditions, particularly the same pressure when comparing boiling points.

Assessing purity from temperature ranges

A pure substance melts or boils at a relatively definite temperature. In a measurement, the temperature stays close to that value while the change of state occurs.

Impurities commonly cause the change of state to occur over a range of temperatures instead. For a solid, impurities typically make it start melting at a lower temperature and melt over a wider range than the pure substance. A lower, broader melting range therefore suggests that the sample is impure.