3. METALS AND NON-METALS
- Silver articles become black after some time when exposed to air. This is because it reacts with sulphur in the air to form a coating of silver sulphide.
- Copper reacts with moist CO2 in the air and slowly loses its shiny brown surface and gains a green coat. This green substance is basic copper carbonate.
- Iron when exposed to moist air for a long time acquires a coating of a brown flaky substance called rust.
Conditions under which Iron rusts
- Take three test tubes A, B and C and place clean iron nails in each of them.
- Pour some water in test tube A and cork it.
- Pour boiled (to remove dissolved air) distilled water in test tube B, add 1 mL oil and cork it. The oil floats on water and prevent air from dissolving in the water.
- Put some anhydrous calcium chloride (drying agent to absorb moisture from the air) in test tube C and cork it.
- After few days, the iron nails rust in test tube A, but they do not rust in test tubes B and C.
- In test tube A, the nails are exposed to both air and water.
- In test tube B, the nails are exposed to only water.
- The nails in test tube C are exposed to dry air.
- This shows that both air and moisture are needed for rusting of iron.
Prevention of Corrosion
- Prevention of Rusting of iron: Painting, oiling, greasing, galvanising, chrome plating, anodising or alloying.
- Galvanisation: A method of protecting steel and iron from rusting by coating them with a thin layer of zinc. The galvanised article is protected against rusting even if the zinc coating is broken because zinc is more reactive than iron and hence can be easily oxidised.
- Alloying: A method of addition of other substances (metal or non-metal) to a metal to get new desired properties. Such a homogeneous mixture of two or more metals, or a metal and a nonmetal is called Alloy.
It is prepared by melting the primary metal, and then, dissolving the other elements in definite proportions. It is then cooled to room temperature. E.g.
· Iron is never used in its pure state because it is very soft and stretches easily when hot. If it is mixed with about 0.05 % of carbon, it becomes hard and strong.
· Mixing iron with nickel & chromium forms stainless steel. It is hard and does not rust.
Pure gold (24 carat gold) is very soft. So it is not suitable to make jewellery. It is alloyed with silver or copper to make it hard. In India, 22 carat gold is used to make ornaments (22 parts pure gold + 2 parts Cu or Ag).
- The alloy containing mercury is called an amalgam.
- Electrical conductivity & melting point of an alloy is less than that of pure metals. E.g., brass (alloy of Cu & Zn) and bronze (alloy of Cu & Sn) are not good conductors of electricity whereas copper is used for making electrical circuits. Solder (alloy of Pb & Sn) has a low melting point, it used for welding electrical wires.
The wonder of ancient Indian metallurgy
The iron pillar (built 1600 years ago) near the Qutub Minar in Delhi has high quality of rust resistance. It is 8 m high and weighs 6 tonnes.