Silver has a wide variety of everyday uses, including silverware and jewellery. Silver, when exposed to air for an extended period, turns black. This occurs when silver combines with sulphur in the air to form a silver sulphide coating.
Tarnishing of silver jewellery
Similarly, when copper reacts with moist carbon dioxide in the air, it loses its shiny brown surface and develops a green coating. This green substance is chemically known as basic copper carbonate.
Rusting
The usage of iron and steel articles has become an integral part of our daily lives. People use iron for a wide range of everyday items, including bridges, ships, cars, truck bodies, window grilles, and many other articles.
Chemical reactions on the surfaces of shiny metals and other items cause them to lose their shine. When iron is exposed to wet air for an extended period, it corrodes, forming a coating called rust, a brown, flaky substance.
Activity: Testing conditions in which iron objects develop rust
Take a few shiny iron nails and place them in three different clean test tubes. Label them as test tubes 'A', 'B', and 'C'.
- In test tube \('A'\), pour enough water to partially immerse the iron nail, leaving some air space at the top.
- In test tube \('B'\), pour enough clean water (without dissolved air) to completely immerse the iron nail. Then, add a layer of oil on top of the water to cover it. Boiling the water removes dissolved oxygen, and the oil layer prevents atmospheric oxygen from dissolving back into the water.
- In test tube \('C'\), add some calcium chloride (a drying agent) and seal it tightly with a lid.
Place all three test tubes in a stable environment at room temperature, undisturbed.
Testing of iron nails in different conditions
Observation:
Iron nails rust in test tube \('A'\), but not in test tubes 'B' or 'C', as you will observe. The nails in test tube 'A' are exposed to both air and water. The nails in test tube 'B' contains water without dissolved air, so ovygen is absent. While those in test tube 'C' are only exposed to dry air. Hence, rusting of iron requires both air and moisture.
Iron material loses its shine and turns into a red-brown, flaky substance when exposed to air and moisture for an extended period, a process known as rusting.
The chemical process of rusting is as follows,
\(Iron\) \(+\) \(Oxygen\) \(+\) \(Water\) \(\to\) \(Iron\) \(oxide\) (\(rust\))
\(4Fe + 3O_2 + xH_2O \to 2Fe_2O_3.xH_2O\)
Rusting occurs more rapidly in environments with high humidity (i.e., a high moisture content in the atmosphere) and a high salt content in the water.
Prevention of corrosion:
- Painting, oiling, greasing, and chrome plating, are all effective methods for preventing iron from rusting.
- Galvanisation is a process that coats steel and iron with a thin layer of zinc to prevent rusting. Even if the zinc coating on the galvanised item is damaged, it remains protected against rust.
Galvanised iron sheets
- Alloying is an excellent way to improve a metal's properties. This method can be used to obtain the desired properties. When iron is combined with nickel and chromium, stainless steel is formed, a hard, rust-resistant metal.
Some other metals also get corroded in this way. Have you ever observed the colour of the coating formed on copper and silver metals?
i. When exposed to oxygen, carbon dioxide, and water vapour in moist air, copper undergoes corrosion and forms a greenish layer called patina (basic copper carbonate). The green coating is primarily due to the reaction of copper with carbon dioxide (\(CO_2\)) and moisture (\(H_2O\)) in the air, forming basic copper carbonate.
ii. Silver metal reacts with hydrogen sulphide gas present in the air to form silver sulphide. As a result, silver objects appear dull and black due to the layer of silver sulphide that is created on their surface.
\(Silver + Hydrogen\ sulphide \to Silver\ sulphide\)
Reaction of materials with water
i. Reaction of metals with water:
The reactivity of metals towards water differs. It is based on the reactivity of the metals,
- Sodium and potassium react vigorously with cold water, which liberates a lot of heat during the reaction. The reaction is exothermic.
Sodium + Cold water \(\to\) Sodium hydroxide + Hydrogen gas + Heat
- Calcium reacts with normal water to form calcium hydroxide and hydrogen gas. Magnesium reacts with hot water to form magnesium hydroxide and hydrogen gas.
Calcium + Normal water \(\to\) Calcium hydroxide + Hydrogen gas
Reactivity series
- Iron, aluminium, and zinc do not react with normal, hot or cold water, but when they come into contact with steam, they produce metal oxide and hydrogen.
Iron + Steam (water vapour) \(\to\) Iron oxide + Hydrogen gas
Aluminium + Steam (water vapour) \(\to\) Aluminium oxide + Hydrogen gas
- Lead, copper, silver and gold do not react with water at all.
ii. Reaction of non-metals with water:
Most of the non-metals do not react with water, but most of the non-metals are reactive in air.
Non-metal + Water \(\to\) No reaction
Reference:
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