Question

Give reasons for the following:
(i) In laboratory preparation of nitric acid all the apparatus including cork should be made up of glass.
(ii) Commercial nitric acid is yellow in colour, but when it is diluted with water, it turns colourless.
(iii) Aluminium does not react with nitric acid of any concentration.
(iv) Conc. nitric acid renders iron passive.
(v) Nitric acid is used in purification of gold.
(vi) Aqua regia dissolves gold and platinum.
(vii) Nitric acid usually does not yield hydrogen from acids.
(viii) Lightning is a blessing in disguise.
(ix) Concentrated nitric acid is brown in colour.
(x) Concentrated nitric acid is a stronger oxidizing agent.

Answer 1

1. When we prepare nitric acid in the laboratory, we heat sodium nitrate or potassium nitrate with sulphuric acid. This combination is highly corrosive. Therefore, even the cork must be made of glass.
2. Pure nitric acid is colourless, while commercial nitric acid is yellowish brown in colour due to the presence of lower oxides of nitrogen, i.e., nitrogen dioxide. When it is diluted with water, it turns colourless because nitrogen dioxide is finally absorbed in the water to give nitric acid.
3. When aluminium metal reacts with nitric acid, a thin layer of oxide is formed on the surface, which is unreactive. This inhibits further reaction.
4. Concentrated nitric acid causes the formation of a metal oxide layer. The formation of this layer is known as passivation. It stops reaction.
5. Nitric acid can dissolve most metals, but not noble metals such as gold. Impurities of other metals are dissolved in it. Therefore, nitric acid treatment is used for the purification of gold.
6. Aqua regia is a 1:3 mixture of concentrated nitric acid and hydrochloric acid to form nascent chlorine. The nascent chlorine then reacts with the noble metal to form metal chloride.
7. Nitric acid is a strong oxidising agent. So, oxygen is liberated along with hydrogen to yield water.
8. During lightning, nitrogen in air combines with oxygen to form nitric oxide. This is further oxidised into nitrogen dioxide, which then reacts with atmospheric water to form nitric acid. This acid then falls to the ground and reacts with minerals to form nitrates that add fertility to the soil.
9. Concentrated nitric acid contains fumes of nitrogen dioxide dissolved in it. This makes the concentrated nitric acid brown.
10. Concentrated nitric acid gives nascent oxygen, which makes it a strong oxidising agent.