ICSE Class 7 Chemistry Chapter 06 Useful Elements and Compounds

Useful Elements and Compounds

Chemistry plays an important role in our lives. The food we eat consists of complex chemical compounds. It is digested and burnt (on respiration) to give us energy. Digestion and respiration are long chains of chemical reactions. Chemical reactions are also involved in the extraction of metals and nonmetals, and the preparation of alloys, glass, fertilisers, soaps, paints, medicines, and so on. The list is inexhaustible.

Metals and Nonmetals

For a chemical study, elements are often divided into two classes-metals and nonmetals-on the basis of their general properties. Of the 114 elements known today, 90 are metals and the remaining 24 are nonmetals.

Metals

We generally look for a metal when we need a strong material. This is because metals are in general hard and strong. Some examples of metals are potassium (K), calcium (Ca), sodium (Na), magnesium (Mg), aluminium (Al), iron (Fe), zinc (Zn), tin (Sn), copper (Cu), mercury (Hg), silver (Ag), gold (Au) and platinum (Pt).

General Properties of Metals

Metals have the following general properties.

1. They are generally hard, strong solids. For example, aluminium, iron, zinc, tin and copper are hard, strong solids. They are strong enough to bear heavy loads.

Exceptions: Sodium and potassium are soft solids which can be cut with a knife, and mercury is a liquid in ordinary conditions.

2. They have a lustre, known as metallic lustre. For example, aluminium, iron, zinc, copper, silver and gold are all lustrous. Such metals can be polished also.

3. When struck, metals produce a sound called a metallic sound or metallic clink. Recall the ringing of a bell.

4. Metals are malleable. In other words, they can be beaten or rolled into sheets. A piece of aluminium, iron, copper or silver can be beaten or rolled into thin sheets. Common examples are, the silver foils used to decorate sweets and aluminium foils used to pack foodstuff.

5. Metals are ductile, i.e., they are flexible and can be drawn into wires. For example, iron, aluminium, copper, silver and gold can be easily drawn into wires.

6. Metals are good conductors of heat, i.e., they allow heat to pass through them easily. Hold one end of an iron nail with your fingers and place the other end in a flame. You will feel the heat. (Let go of the nail as soon as it warms up.) Due to this property, we use metal utensils in kitchens. Heat supplied at one part of the utensil is conducted all over it.

7. Metals are good conductors of electricity, i.e., they allow an electric current to pass through them easily. This is why electricity is transmitted from one point to another through metal wires, like those of copper or aluminium. Silver is the best conductor of electricity; the next best is copper and then aluminium.

Uses of Metals

Metals are of great use to us.

1. As metals are good conductors of electricity, they are used in making wires for electrical connection and transmission. Copper wires are used for domestic wiring and aluminium wires for long-distance transmission. Aluminium is lighter and cheaper than copper.

2. Iron is a very useful metal as it is tough, strong and cheap. It is used for making tools, machines and agricultural equipment, and also in house construction.

However, it rusts, which makes it lose its strength. To prevent rusting, iron is usually galvanised. Galvanisation is done by immersing an iron article in molten zinc, then taking it out and allowing it to cool. Zinc forms a firm coating over iron and prevents rusting.

Galvanised iron (GI) sheets are used in making trunks, GI pipes for supplying water, and GI wires for making barbed wire and wire nets.

Iron coated with tin is used to make cans.

3. Iron, aluminium and copper are used in making domestic utensils. Nonstick utensils are usually made of iron coated with such a polymer which is not affected by heat. You will learn about polymers in the next chapter.

4. When ignited, magnesium burns in air with a dazzling white light. Magnesium in the form of powder or turnings is, therefore, used to a great extent in fireworks.

5. Mercury is a liquid that does not stick to glass. At the same time, it is a good conductor of heat. So it is used for making thermometers. Mercury placed in a thermometer bulb, attached to a capillary, expands or contracts as it gets heated or cooled. The length of the mercury column in the thermometer gives a measure of the temperature.

6. Silver, gold and platinum are noble metals as they do not easily react with other substances. Once polished, they retain their sheen for a long time. They are, therefore, used for making jewellery. Small amounts of silver and copper are mixed with gold to make it tough.

7. Artificial teeth were earlier usually made of silver or gold. Tooth cavities are filled with what are known as dental amalgams-alloys of silver or tin with mercury.

8. Aluminium foil, which is very light, is used to pack food, medicines, etc. Aluminium is also used for making electric wires and utensils.

Nonmetals

In contrast to metals, nonmetals are not known for strength. They are

- usually lustreless (dull), except graphite and iodine;

- brittle, if solid;

- bad conductors of heat; and

- bad conductors of electricity (except graphite).

They do not produce a metallic clink.

Of the 24 nonmetals known today,

- 12 are solids (e.g., carbon, silicon, phosphorus, sulphur, iodine),

- 1 is a liquid (bromine), and

- 11 are gases (e.g., hydrogen, nitrogen, oxygen, fluorine, chlorine, helium, neon, argon).

Uses of Nonmetals

Let us study the uses of some common nonmetals.

Oxygen

You have already learnt that oxygen is a constituent of air. Let us briefly see how it is useful to us.

- All combustion processes need oxygen.

- It is required for respiration. It burns food in our body to give us energy.

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}\]

Glucose is shown with oxygen combining to produce carbon dioxide and water along with energy.

- Aquatic animals derive oxygen from dissolved air.

- Oxygen is given to patients who suffer from respiratory problems.

- Divers use a mixture of oxygen and helium for respiration.

- With acetylene, oxygen is used in oxyacetylene torches to cut and weld metals.

- It is required in large quantities in the iron and steel industry.

- It is used in the manufacture of sulphuric and nitric acids.

- Liquid oxygen is used to burn rocket fuel.

Nitrogen

Nitrogen is also a constituent of air.

- Plants use nitrogen to manufacture proteins.

- Liquid nitrogen is used to preserve blood, corneas or other donated organs.

- Due to its inertness, it is used for filling food packages. (Food does not go bad in an inert medium.)

- It is used in the manufacture of ammonia and urea.

Carbon

Charcoal and lampblack are non-crystalline forms of carbon. Carbon is found in two crystalline forms-diamond and graphite. Diamond and graphite have the following uses.

- Diamond is used as a gem as also for cutting rocks or glass.

- Graphite is a good conductor of electricity and is, therefore, used as an electrode.

- The crystal structure of graphite is such that one layer slides over the other, making it a good lubricant. Since its melting point is high, it can be used as a lubricant in machines that acquire high temperatures while being operated.

- Graphite is also used in the manufacture of pencils.

Chlorine

Chlorine is a greenish yellow gas and is highly reactive.

- It is used in the manufacture of the useful plastic polyvinyl chloride (PVC).

- It is used to disinfect water, as it kills germs. This is done either by passing chlorine gas through water or by treating water with bleaching powder.

- It is used as a bleach in paper and textile industries. Household bleaches also contain chlorine compounds.

- Chlorine is used in the manufacture of hydrochloric acid.

- It is also used in the manufacture of pesticides like Gammexene.

Iodine

Though a nonmetal, iodine is a lustrous black solid. It sublimes slowly to form a violet vapour over the solid inside a stoppered bottle.

- Iodine is an antiseptic. Its alcoholic solution (with some potassium iodide), which is reddish brown in colour and is known as tincture iodine, is applied on cuts.

- A paste of iodine in petroleum jelly gives relief from pain.

- Lack of iodine in the human body causes many diseases, mainly related to the thyroid gland. Physicians recommend that patients suffering from such diseases take what is known as iodised salt in their diet. Iodised salt is table salt mixed with sodium iodate.

Helium

Helium is a noble-gas element. It is the lightest gas next to hydrogen.

- It is used for filling balloons. It is safer than hydrogen as it does not catch fire.

- A mixture of helium and oxygen is used by divers for respiration.

Neon

Neon is also a noble-gas element.

- Neon is used mainly for illumination. It produces an orange-red light in a discharge tube and is used in advertisement signs.

Argon

Argon is the most abundant noble gas present in air.

- It is used to fill electric bulbs.

- Mixed with a little mercury vapour in a discharge tube, it gives a green light. So argon is used in green signs.

Alloys

A solid solution, i.e., a homogeneous mixture, of a metal with other metal(s) or nonmetal(s) is called an alloy.

For example,

(i) steel is an alloy of iron, manganese and carbon, and

(ii) brass is an alloy of copper and zinc.

An alloy is usually harder and stronger than the parent metal(s) and at the same time, it resists corrosion. This is why alloying is done. Some common alloys with their uses are mentioned in Table 6.1

Gold alloys: Pure gold is soft and so not suitable for making jewellery. It is, therefore, made harder and stronger by alloying it with silver, copper, or both. The alloy is still malleable.

The purity of gold is reckoned in carats. 22-carat gold is 24 carats. By 22-carat gold we mean that the sample contains 22 parts of pure gold and 2 parts of the alloying agents. Generally, 22-carat gold is suitable for making jewellery.

Amalgams

Alloys of mercury are called amalgams. Silver and tin amalgams are used in dentistry. Other amalgams have many laboratory and industrial uses.

Solutions We Use

Chemical reactions generally take place faster in solution. All chemical reactions in our body occur in solution.

We use several solutions in everyday life. Let us discuss some of them.

Soda water: Soda water is a solution of carbon dioxide (dissolved under pressure) in water. It is a highly refreshing drink when you are exhausted.

Glucose solution: A solution of glucose in water gives instant energy. Glucose burns during respiration to give carbon dioxide, water and energy.

Solution of sugar with lemon juice: In solution cane sugar is slowly converted into glucose and fructose in the presence of lemon juice. Glucose gives you energy.

Solution of sugar or glucose and salt: Blood cells require a minimum concentration of salt within them. If there is a loss of salt by way of excessive perspiration or vomiting, a solution of sugar or glucose with salt should be consumed.

A solution of glucose and salt, called glucose-saline solution, is intravenously injected to a patient suffering from severe dehydration or blood loss.

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