3. METALS AND NON-METALS
CORROSION
- 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.
Select Your Next Topic 👇
👉 Part 1: Physical Properties
👉 Part 2: Chemical Properties
👉 Part 3: How do Metals and Non-metals React?
👉 Part 4: Occurrence of Metals
👉 Part 5: Corrosion
