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Introduction
Even in the sixteenth century chemists knew that by heating common salt and concentrated sulphuric acid together, hydrogen chloride could be prepared. The sodium sulphate salt obtained during the reactions studied intensively for its properties is called Glauber's salt after the discoverer Johann Glauber.
Occurrence of Hydrochloric Acid
Every one of us carries a hydrochloric acid manufacturing unit in our body! It is present in the gastric juice of all mammals. In the stomach hydrochloric acid helps to produce pepsin, which is used for the digestion of proteins. It also helps to destroy harmful microorganisms that enter the stomach along with the food.
Preparation of Hydrogen Chloride Gas
By Synthesis
Hydrogen chloride gas can be prepared by the direct combination of hydrogen gas and chlorine gas in the presence of sunlight. The reaction is explosive in direct sunlight. However, in diffused sunlight the reaction is slow and controllable. In the dark, the reaction hardly takes place.
Hydrogen chloride gas can also be obtained by burning hydrogen in chlorine.
Physical Properties of Hydrogen Chloride Gas
Odour : It has a pungent and choking smell. If inhaled, it corrodes the delicate membrane of the respiratory passages.
Chemical Properties of Hydrogen Chloride Gas
Combustibility : Hydrogen chloride gas is neither combustible, nor does it support combustion.
Tests for Hydrogen Chloride Gas
Following tests could be conducted for checking the presence of hydrogen chloride gas:
1) The presence of a pungent, suffocating odour.
2) It produces dense white fumes with ammonia.
Hydrochloric Acid and its Preparation
Hydrochloric acid is prepared by dissolving hydrogen chloride gas in water. Hydrogen chloride is a covalent compound, but when dissolved in water it ionizes to form hydrogen ions and chloride ions as follows:
Industrial Preparation of Hydrochloric Acid
Hydrochloric acid is produced along with the industrial preparation of caustic soda (sodium hydroxide). During the electrolysis of sodium chloride, large quantities of hydrogen and chlorine gas are obtained as by-products. These two gases are burnt to form hydrogen chloride gas.
The hydrogen chloride gas so formed is dissolved in water to form hydrochloric acid. A saturated solution of the acid has a density of 1.2 g cm-3. It contains about 40% by mass of hydrogen chloride.
Properties of Hydrochloric Acid
Action on Indicators (Acidic Properties)
a) It turns litmus paper from blue to red.
b) It turns methyl orange from yellow to pink.
c) It turns phenolpthalein (alkaline) from deep pink to colorless.
Tests for Hydrochloric Acid
For checking the presence of the hydrochloric acid, the following tests could be conducted:
1) It turns blue litmus paper red.
2) When silver nitrate solution is added to hydrochloric acid, it gives white precipitate of silver chloride.
Uses of Hydrochloric Acid
The main uses of the hydrochloric acid are mentioned below:
1) Hydrochloric acid is used in the production of dyes, drugs, paints, photographic chemicals etc.
2) It is used for the production of glucose from starch.
3) It is used for the extraction of glue from bones.
Summary
Hydrochloric acid is used in the production of dyes, drugs, paints, photographic chemicals and in the preparation of aqua-regia for dissolving metals like gold and platinum.
