Category Science & Technology

               Plastics are synthetically produced materials that are capable of being moulded and made into useful shapes such as bottles and flat sheets. They usually soften under heat and pressure so they can be forced into a mould. Some types of plastic are set into a hard solid form by the application of heat, causing a chemical reaction between the component substances.

               Most plastics are light in weight, are good electrical insulators and are tough enough to have many domestic and industrial uses.

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               Some very light metals such as lithium and calcium can float in water. They also react with the oxygen in water, releasing hydrogen. The similar (but heavier) metals, such as sodium and potassium, generate so much heat when they are dropped into water that the hydrogen they release burns on the water’s surface.

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               When a substance burns, there is a very rapid chemical reaction with oxygen in the air. This reaction generates heat and light, producing a flame. There are other forms of burning too. Some metals burn in corrosive gases, for example. Explosions are a form of burning in which the reaction takes place extremely rapidly, releasing very large amounts of heat.

               When sugar is stirred into water, it dissolves and forms a solution. This means that the solid sugar molecules have split apart and become distributed equally throughout the water, so that no particles can be seen. The warmer the water, the more sugar can be dissolved into the solution. There are some molecules that do not behave in this way. Fats and oils, for example, will not dissolve in water — they can be seen floating on it —although they will dissolve in some other types of liquid.

               Solutions are not always liquids. Brass is a yellowish metal that is actually a solid solution containing copper and tin.

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            Crystals are solid substances that have their atoms arranged in regular patterns. Most naturally occurring substances form crystals under the right conditions, although they are not always apparent. Crystals form into regular geometric shapes, and crystals of the same substance or mixture of substances will always have the same regular appearance.

            Crystals are formed from solutions or melted materials. As the solutions evaporate or the melted materials cool, their atoms are forced closer together, producing a crystal. This crystal gradually grows as the process continues. Some crystals grow into complicated and beautiful shapes, which are often brightly coloured.

            Glass forms when melted solid materials are cooled quickly, so that they do not produce crystals. There are many types of glass, but the transparent glass that is most familiar to us is a very useful material. It is resistant to most corrosive substances, and it is a very good electrical insulator and heat insulator. Glass is stronger than most metals, but because it is brittle it shatters easily.

            The most common forms of glass are made from a melted mixture of silica (sand), sodium oxide and calcium oxide. Lead crystal is a form of glass in which the calcium oxide is replaced by lead oxide. The most heat-resistant glass can be made from pure silica.

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