Purifying metals by electrolysis

Most metals need to be purified as the presence of impurities affects the properties of the metal, for example they can make impure iron brittle. The amount of different impurities which may be present in the final sample of the metal depends on what the metal is used for. Iron, for example, is rarely used in a pure form and impurities such as small amounts of carbon are important in steel, the main use of iron. Copper is a valuable metal which is widely used, particularly as an electrical conductor, unfortunately even small amounts of impurity can reduce its electrical conductivity considerably. To purify the metal an electrolytic process is used, with a sheet of pure copper as the negative cathode, and the impure copper as the positive anode. Copper dissolves from the anode to form copper ions, which are attracted to the cathode where they are deposited as the pure metal. Any impurities fall to the bottom of the cell forming a 'sludge', which often contains small amounts of silver and gold.

Other products from rocks and minerals

Although metals form an important group of materials extracted from the ground, they are not the only useful materials. Some rocks such as limestone, sandstone and granite are used for building, and minerals such as sand, shingle and clay are used in the construction industry and for road-building. In addition to these, materials such as limestone, and rock salt are important as raw materials for the chemicals industry.

Limestone - a useful rock

Limestone is a sedimentary rock (see also 'Sedimentary rocks') consisting of almost pure calcium carbonate. It is an important material, with about three-quarters of the amount quarried being used in construction. Much of this is as 'aggregate' - the crushed stone used in road-building. In addition it is used as a base to neutralise acid soils, acid gases produced by burning fossil fuels, and acidified lakes produced by acid rain. It is also used in iron and glass production. It can be converted into cement, by heating with clay, and quicklime by heating to a high temperature. Quicklime, and slaked lime (produced by adding water) are used to neutralise acid soils, in water treatment and in making mortar.

Chemicals from rock salt

Rock salt is chemically almost the same as the salt you may sprinkle on your food. It comes from the remains of ancient seas which have dried up and been buried under layers of rock. Most of this salt is used as a raw material for making other chemicals, particularly in the 'chlor-alkali' process. In this process, electricity is passed through a solution of sodium chloride. This converts the solution into sodium hydroxide, and releases chlorine and hydrogen as gases. These three products have important uses in their own right, as well as combining together to form bleach and hydrochloric acid.