ICSE Class 7 Chemistry Chapter 07 Man made Substances

7 - Man-made Substances

Man has made many substances that help him lead a comfortable, healthy life. These are generally prepared in factories from naturally occurring substances. Man uses cement in the construction of buildings and bridges. He makes fertilisers to increase crop yield. He manufactures glass, which is used to make a range of products, from spectacles to microscopes to bulletproof windows. Man also prepares medicines to fight diseases. Let us now study these and some other man-made substances.

Cement

Cement is a grey, powdery substance manufactured from limestone, clay (aluminium silicate) and gypsum. The product is a complex material containing the silicates of calcium and aluminium. A paste of it in water sets into a hard rocky mass - a phenomenon called the setting of cement.

A paste of sand, cement and water, called mortar, is very conveniently used for joining bricks and plastering walls.

A mixture of stone chips (gravel), sand, cement and water, known as concrete, sets harder than ordinary mortar. It is used for flooring and making roads.

Concrete with steel bars and wires embedded in it, called reinforced concrete (RC), forms a very strong material. It is used for constructing roofs, bridges and pillars.

Glass

You are quite familiar with glass. It has been known for centuries for its varied uses. Glass does not melt at one temperature, rather it softens over a range of temperatures. On being softened, it can be bent and blown to different shapes. This property of glass greatly increases its utility.

There are several types of glass, depending on composition, though silica (SiO2) is a common constituent. The different types suit different purposes. Let us acquaint ourselves with the important kinds.

Soda glass or soda-lime glass

When a mixture of soda ash (Na2CO3), limestone (CaCO3) and sand (SiO2) is fused, a transparent liquid is formed. On being cooled, the liquid turns into a transparent, hard material called soda glass or soda-lime glass. It is sodium calcium silicate, and its approximate composition is represented by Na2O-CaO-5SiO2. The Na2O and CaO constitute the basic part (as these are the oxides of metals) and SiO2 constitutes the acidic part (as it is the oxide of a nonmetal).

Soda glass is the cheapest of all glasses and is used for making window panes and bottles. Soda-glass bottles are easily attacked by chemicals.

Potash glass

Potash glass contains potassium in place of sodium. Compared to soda glass, it has a higher softening temperature as also a greater resistance to chemicals. So, potash glass is more suitable for making chemical apparatus - beakers, flasks, funnels, etc.

Optical glass

Optical glass is used to make lenses, prisms and components of optical instruments, like telescopes and microscopes. It contains boric oxide (B2O3) and silica (SiO2) as the acidic component. Optical glass can be of two types, depending on the basic component.

Crookes glass for spectacles

Glass used to make spectacles must absorb ultraviolet rays, which are harmful for the eyes. Crookes glass is useful for this purpose as it contains some compounds, which help it absorb these rays.

Lead crystal and crystal glass

Lead glass sparkles. It has a soft surface and so it can be ground and cut relatively easily. Engravings can also be made on such glass. It is, therefore, used for making decorative items. If the glass contains 24% or more of lead oxide, it is called lead crystal. If it contains less than 24% lead oxide, it is called crystal glass.

Borosilicate glass

Borosilicate glass contains less alkali (K2O or CaO) and more silica than potash glass, and some boron trioxide. It is not easily attacked by chemicals. It can stand sudden cooling or heating, and is, in fact, better than potash glass. It finds extensive use in the manufacture of laboratory apparatus.

Coloured glass

Coloured glass is manufactured by adding metals or metal oxides to the glass melt, as mentioned in Table 7.1.

Coloured glass is used to make artificial jewellery. It is also used to make crockery and stained-glass windows.

Milky glass

Milky glass is prepared by adding tin oxide (SnO2), calcium phosphate (Ca3(PO4)2) or cryolite (AlF3-3NaF) to the glass melt. All these substances are white, and so they make the glass look milky.

Glass laminates

Glass laminates are made by fixing polymer (about which you will soon learn) sheets between layers of glass. They do not shatter and are, therefore, used to make the windows and screens of cars, trains and aircraft. Specially manufactured glass laminates are used as bulletproof material.

Plastics

Plastics are highly useful materials that have been developed during the last century. Polythene (or polyethylene) carrybags, plastic bottles, soda-water crates, the body of your TV, the body of your washing machine, electrical insulations and furniture foam are all plastic materials.

Plastics are cheap, light and strong. They can be moulded into different shapes - a property known as plasticity. They are not generally attacked by chemicals. For these reasons, they are ideal substitutes for wood, metals, glass and fabrics.

But, on the other hand, plastics are a great source of pollution. This is because they are not easily decomposed or degraded. So, with the increasing use of plastics, the volume of undegradable garbage is also increasing every day. When burnt, they produce poisonous gases. The use-and-throw plastic materials like carrybags and bottles block drains and sewers.

On the basis of how they behave towards heat, plastics are classified into two types - thermoplastics and thermosetting plastics. Thermoplastics retain their plasticity even after repeated heating and cooling. So they can be moulded over and over again. But thermosetting plastics, once set after being melted, cannot be moulded again.

Chemically, plastics are polymers.

A single unit, called a monomer, is repeated many times over to form what is known as a polymer.

For example, the monomer ethene is repeated thousands of times to form the polymer polythene. Teflon, which is used for coating nonstick kitchenware, is another polymer.

A class of polymers called silicones are used as lubricants and as sealing material.

Apart from the man-made polymers (also called synthetic polymers), there are natural polymers also, e.g., starch, cellulose, proteins and rubber.

Some man-made polymers are mentioned in Table 7.2.

Paints

Paints can be applied on a surface to protect it from corrosion and weathering, or to give it an attractive look. Refrigerators, cars, ships, railway coaches and houses are some things that are painted. Paints are also used in works of art and signboards.

A paint contains a pigment, a vehicle and a thinner. A pigment is a white or coloured matter usually obtained from mineral sources. Zinc oxide, white lead and titanium dioxide are the commonly used white pigments. Some coloured minerals are used as coloured pigments. The pigment is mixed with a vehicle, which is an oil like linseed or soya bean oil, or a polymer. A thinner is a solvent such as turpentine or kerosene. It makes the paint more fluid so that it may be applied easily.

Luminous paints glow when exposed to light. So they are used in marking signs on roads and vehicles.

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