HNO3 | Preparation, properties, acidic property |



HNO3, is a chemical of great commercial importance. There are three important methods for its manufacture.


Laboratory method:
From potassium nitrate (KNO3):
HNO3, can be prepared on large scale by beating Chile salt petre with conc. H2SO4


General principle:

The nitrogen and from the air can be made to combine by electric discharge. NO produced which is further oxidized to NO2, and is then dissolved in water.


N2 and O2 from atmosphere are allowed to combine with each other is the presence of electric are at 3000°C.

This reaction is endothermic and is favoured at high temperature.The gases which come out from this electric fieid contain 1. 25 % of NO. These gases are rapidly cooled down to 1000 °C.

At 600°C, NO combines with oxygen to form NO2.

NO2 is cooled down to 50°C by water pipes. NO2 is absorbed in water in a tower in which the water trickles down Fig (1.1)

HNO2 is oxidized to HNO3 and NO. NO is re-oxidized to NO2 and thus to HNO3.

making nitric acid from air

Preparation of conc. HNO3:

  • There are three stages to get 100 % HNO3. Dil HNO3, obtained by different methods can be concentrated to 68 % by the process of distillation. This percentage gives a constant boiling mixture. It is called commercial HNO3.
  • If concentrated H2SO4, is used and distilled with 68 % HNO3,, then we get 98% HNO3, H2SO4, absorbs the water.
  • If we want to get 100% HNO3, then 98% HNO3, is strongly cooled. Pure acid is deposited as colorless crystals at – 42°C. These crystals are separated and melted to get 100 % HNO3.

Physical properties:

  1. Pure HNO3, is a colourless liquid
  2. It has a characteristic choking smell
  3. its B is 86°C
  4. Its M.P. is – 42°C,
  5. The specific gravity of pure HNO3, at 15°C is 1.538.
  6. In the crystalline state, it is transparent
  7. It is soluble in water in all proportions.
  8. In moist air, it gives fumes.
  9. It is hygroscopic in nature.


HNO3, decomposes in the presence of light even at ordinary temperature

Acidic property:

HNO3 is a strong acid. It ionizes in water to give sufficient protons. it’s dissociation constant is in the range of 10+3. On the basis of acidic nature, it reacts with many oxides, hydroxides, carbonates bicarbonates, sulphites and bisulphites. In this way metal nitrates are produced.
  1. Reaction with calcium oxide (CaO):
  2. Reaction with NaOH and KOH:
  3. Reaction with Na2 CO3:

  4. Reaction with NaHCO3:

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