Industrial Chemistry: Glass, Cement, Fertilisers and PYQs

Industrial chemistry is the part of the science syllabus that asks what everyday materials are made of and how they are made. Four groups come up again and again: glass, cement, fertilisers and explosives. The questions are rarely about mechanisms; they are about raw materials, the one substance added for a particular purpose, the percentage of a nutrient, and the man who invented a process. This chapter collects those facts in the order a paper setter uses them.

Glass and its varieties

Glass is not a crystalline solid. It is an amorphous solid, cooled from the melt so quickly that its particles never settle into a lattice, which is why it has no sharp melting point and is often loosely called a supercooled liquid. Ordinary window and bottle glass is soda-lime glass, made by melting silica in the form of sand with soda ash, which is sodium carbonate, and limestone, which is calcium carbonate. Changing one ingredient changes the glass completely.

Colour is given by metal oxides dissolved in the melt, and examiners love these pairs: cobalt oxide gives blue, chromium oxide gives green, cuprous oxide gives red, manganese dioxide gives violet, iron compounds give green or brown, cadmium sulphide gives yellow and colloidal gold gives a deep ruby red. After shaping, glass is annealed, that is cooled slowly, to relieve the internal stresses that would otherwise crack it.

Cement

Portland cement was patented by Joseph Aspdin in 1824 and named after the Portland stone it resembled when set. Its raw materials are a calcareous one, limestone, and an argillaceous one, clay, ground together and burnt in a rotary kiln at about 1450 degrees Celsius, where they fuse into hard nodules called clinker. The clinker is then ground with a small quantity, about two or three per cent, of gypsum. The approximate composition is lime about sixty to sixty-seven per cent, silica about seventeen to twenty-five per cent and alumina about three to eight per cent, with some iron oxide.

The four compounds that matter are tricalcium silicate, which gives early strength, dicalcium silicate, which gives strength over the following weeks, tricalcium aluminate, which sets almost instantly and must be controlled, and tetracalcium aluminoferrite. Gypsum is added precisely to slow the tricalcium aluminate and prevent a flash set, so that the mixed cement stays workable. Setting is not drying but hydration, a chemical reaction with water that gives out heat, which is why a large concrete pour has to be cooled. Cement with sand and water is mortar; with sand, stone aggregate and water it is concrete, and with steel bars inside it is reinforced concrete. Pozzolana cement contains fly ash and slag cement contains blast-furnace slag, both of which reduce the heat of hydration. Gypsum has a second use worth remembering: heated to about 120 degrees Celsius it loses part of its water and becomes plaster of Paris.

Fertilisers

Plants need three primary nutrients, nitrogen, phosphorus and potassium, and a fertiliser is classified by which of them it supplies. A straight fertiliser supplies one primary nutrient; a complex or mixed fertiliser supplies two or more, which is why diammonium phosphate, carrying both nitrogen and phosphorus, is a complex fertiliser.

Urea is manufactured from ammonia and carbon dioxide, and the ammonia itself comes from the Haber process, in which nitrogen and hydrogen combine over an iron catalyst at high pressure and a temperature of about 450 to 500 degrees Celsius. Superphosphate of lime is made by treating rock phosphate with sulphuric acid, and it is a phosphatic fertiliser, never a nitrogenous one, a distinction examiners test directly. Beside these manufactured fertilisers stand the organic manures. Farmyard manure, the decomposed mixture of cattle dung, urine and litter, is weak in nutrients, carrying only about half a per cent of nitrogen, but it improves the texture and water-holding capacity of the soil in a way no chemical fertiliser does. Excess fertiliser washed into lakes causes eutrophication, and nitrate in drinking water causes methaemoglobinaemia, the blue baby syndrome.

Explosives

An explosive stores chemical energy that is released as hot gas in a very short time. Low explosives burn rapidly, or deflagrate, and are used as propellants: gun powder, the oldest of them, is a mixture of about seventy-five parts potassium nitrate or saltpetre, fifteen parts charcoal and ten parts sulphur. High explosives detonate, with a shock wave travelling faster than sound through the material. Nitroglycerine, first made by Ascanio Sobrero, was too sensitive to handle until Alfred Nobel absorbed it in kieselguhr in 1867 to make dynamite, and later combined it with nitrocellulose to make blasting gelatin; the fortune he made from it endowed the Nobel Prizes. Trinitrotoluene, or TNT, is the standard of comparison for explosive power because it is stable enough to be melted and poured. RDX, also called cyclonite, is more powerful still and is the basis of plastic explosives such as C-4. Nitrocellulose, known as gun cotton, is made by nitrating cotton and is the basis of smokeless powder and cordite. Initiators such as mercury fulminate and lead azide are used in detonators, and in mining the cheapest charge is ANFO, a mixture of ammonium nitrate with fuel oil.