Separating and purifying mixtures

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.

In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state — pure milk, for example. Milk is a mixture of many compounds, so it is not pure in the chemical sense at all. Questions in this topic often turn on that difference.

Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.

You should be able to use melting point and boiling point data to distinguish pure from impure substances. Two things to check every time: whether the value is sharp or a range, and whether it matches the data book value for the pure substance.

A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged — which is exactly why a mixture can be separated physically and a compound cannot.

Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. These processes do not involve chemical reactions and no new substances are made.

You should be able to describe, explain and give examples of the specified processes of separation, and to suggest suitable separation and purification techniques for mixtures when given appropriate information.

That second one is where the marks are won or lost, because the mixture in the question is usually one you have never met. Work from what is being separated, not from what you did in class: an insoluble solid in a liquid means filtration; a solid dissolved in a liquid means crystallisation or distillation, depending on which part you want to keep; two liquids mean fractional distillation. Purification often needs two steps in order — filter off the insoluble impurity first, then crystallise what is left.

Water of appropriate quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes. Water that is safe to drink is called potable water.

Potable water is not pure water in the chemical sense, because it contains dissolved substances. You should be able to distinguish between potable water and pure water: pure water is H₂O and nothing else, while potable water is safe to drink but still a mixture — and the dissolved salts it contains are harmless at these levels.

The methods used to produce potable water depend on available supplies of water and local conditions.

In the United Kingdom, rain provides water with low levels of dissolved substances — fresh water — that collects in the ground and in lakes and rivers. Most potable water is produced by:

• choosing an appropriate source of fresh water • passing the water through filter beds • sterilising.

Sterilising agents used for potable water include chlorine, ozone or ultraviolet light.

You should be able to give reasons for the steps used: an appropriate source has low levels of dissolved substances to start with, so less treatment is needed; the filter beds remove insoluble solids such as grit and leaves; and sterilising kills microbes that would otherwise cause disease.

If supplies of fresh water are limited, desalination of salty water or sea water may be required. Desalination can be done by distillation, or by processes that use membranes such as reverse osmosis. These processes require large amounts of energy.

You should be able to describe the differences in treatment of ground water and salty water. The difference is what has to be removed. Ground water already has low levels of dissolved salts, so it only needs filtering and sterilising — both cheap. Salty water has high levels of dissolved salts, which filtering cannot remove, so the water must be separated from them by distillation or reverse osmosis — which is why desalination is used only where fresh water is scarce.

Urban lifestyles and industrial processes produce large amounts of waste water that require treatment before being released into the environment. Sewage and agricultural waste water require removal of organic matter and harmful microbes. Industrial waste water may require removal of organic matter and harmful chemicals.

Sewage treatment includes:

• screening and grit removal • sedimentation to produce sewage sludge and effluent • anaerobic digestion of sewage sludge • aerobic biological treatment of effluent.

Following the four steps through: screening removes large objects and grit that would damage the equipment. Sedimentation lets the mixture stand so that heavy solids sink as sewage sludge, while the liquid effluent floats above. The sludge is then digested anaerobically — by bacteria without oxygen — which produces methane that can be burned for energy, and a fertiliser. The effluent is treated aerobically, with air pumped through so that bacteria with oxygen break down the remaining organic matter.

You should be able to comment on the relative ease of obtaining potable water from waste, ground and salt water.