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Explain, why (or give reasons for)
(i) In the laboratory preparation of hydrogen chloride from sodium chloride, the gas can be obtained below 200°C or above. But the lower temperature is preferred.
(ii) Hydrogen chloride is not collected over water.
(iii) Hydrochloric acid cannot be concentrated above 22.2% by boiling.
(iv) HCl gas does not conduct electricity, but hydrochloric acid conducts electricity.
(v) Only a very dilute solution of hydrogen chloride in water can be concentrated by boiling the solution while a very concentrated solution would become less concentrated when boiled.
(vi) When the stopper of a bottle full of HCl gas is opened there are fumes in the air.
(vii) A solution of HCl gas in water turns blue litmus red and conducts electricity, while HCl gas dissolves in toluene and it has no effect on litmus and does not conduct electricity.
(viii) An aqueous solution of chlorine is acidic in nature.

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Solution

(i) In the laboratory preparation of hydrogen chloride from sodium chloride, the gas can be obtained both below and above 200° C. However, the lower temperature is preferred because a higher temperature causes sodium sulphate to form a hard crust, which sticks to the bottom of the flask, and is difficult to remove.

NaCl(s) + H2SO4(aq) → NaHSO4(s) + HCl(g)
Sodium Sulphuric acid Sodium hydrogen Hydrogen chloride
chloride sulphate gas

NaCl(s) + NaHSO4(s) → Na2SO4(s) + HCl(g)
Sodium
Sulphate

(ii) Hydrogen chloride is not collected over water because it creates back suction of water in the tube. When the water in the tube comes in contact with concentrated sulphuric acid, a large amount of heat is produced, which may result in an explosion.

(iii) Hydrochloric acid cannot be concentrated above 22.2% by boiling it because the solution boils at a constant temperature of 110o C, without any change in its concentration. Such a mixture is known as a constant boiling mixture or an azeotrope.

(iv) Hydrochloric acid is a solution of hydrogen chloride gas in water. An aqueous solution of HCl has ions. These ions are attracted towards the corresponding electrodes during electrolysis, thereby, enabling it to conduct electricity. Ordinarily, hydrogen chloride gas is a covalent compound and does not consist of any ion. Hence, HCl gas does not conduct electricity.

(v) Dilute solution of hydrochloric acid cannot be concentrated beyond 22.2% by boiling as the solution boils at constant temperature 1100C and form azeotrope i.e., a constant boiling mixture without change in concentration.

(vi) HCl gas is highly soluble in water. When the stopper of a bottle containing HCl gas is opened, its vapours form tiny droplets of hydrochloric acid with water vapour and we observe fumes.

(vii) The acidic properties of a solution are due to the presence of ions.When hydrogen chloride dissolves in water, it ionises to produce H+ or H3O+ ions and Cl ions.
HCl(aq) → H3O+(aq) + Cl(aq)
The solution shows the acidic properties due to the presence of H3O+ ions.
When hydrogen chloride dissolves in toluene, it does not ionise. Hence, its solution in toluene does not behave as an acid and does not conduct electricity.

(viii) Chlorine, Cl2, reacts with water to produce hypochlorite, hydronium ions and a chlorine ion.
Thus, an aqueous solution of chlorine consists of hydrogen ions, which are responsible for the acidic behaviour of the solution.

Cl2(g) + H2O(l) → OCl(aq) + 2H+(aq) + Cl(aq)


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