Explain why:
(a) Anhydrous HCl is a poor conductor while aq. HCl is an excellent conductor.
(b) When the stopper of a bottle full of hydrogen chloride gas is opened there are fumes in
the air.
(c) A solution of hydrogen chloride in water turns blue litmus red and conducts electricity ,
while a solution of the same gas in toluene:
(i) Has no effect on litmus ,and
(ii) Does not conduct electricity
(d) Thick white fumes are formed when glass rod dipped in NH4OH is brought near the
mouth of bottle full of HCl gas.
(e) Dry hydrogen chloride gas does not affect a dry strip of blue litmus paper but it turns red
in the presence of drop of water.
(f) Hydrogen chloride gas is not collected over water.
Answer
(a) Anhydrous HCl is poor conductor due to the absence of ions in it whereas aqueous HCl
is excellent conductor since it contains ions.
(b) When the stopper is opened HCl gas comes in contact with water vapors of air and gives
white fumes due to the formation of hydrochloric acid.
(c) A solution of HCl in water gives hydronium ions and conducts electricity, but HCl is also
soluble in dry toluene, but in that case it neither (i) turns blue litmus red (ii) nor does
conducts electricity. This indicates the absence of H+ ions in toluene showing thereby that
hydrogen chloride is a covalent compound.
(d) When ammonium hydroxide is brought near the mouth of HCl, dense white fumes are
formed due to the formation of ammonium chloride.
HCl + NH4OH —–→ NH4Cl + H2O
(e) Dry hydrogen chloride is not acidic whereas moist Hydrogen chloride is acidic. In
presence of a drop of water HCl gas dissolves in water and forms hydrochloric acid which
turns blue litmus paper red.
(f) Hydrogen chloride is not collected over water as it is highly soluble in water.
Question2: Mixture of sodium chloride and concentrated sulphuric acid does not heated
above the temperature of 170°C while preparing hydrogen chloride. Why ?
Answer: The mixture of sodium chloride and concentrated sulphuric acid is not heated above
170°C in preparing hydrogen chloride gas because at a higher temperature sodium sulphate
is formed which is a hard substance and difficult to remove from the reaction flask.
2NaCl + H2S04 → Na2S04 + 2HCl
Question3: Hydrogen chloride gas cannot be dried over quick time. Why ?
Answer: Because quick lime is basic in nature and combines with moist hydrogen chloride
gas forming calcium chloride.
Question4: Dilute hydrochloric acid cannot be concentrated by distilling (boiling) the dilute
acid. Why ?
Answer: When dilute hydrochloric acid is distilled, a constant boiling mixture containing 20 –
24% of hydrochloric acid distills over unchanged at 760 mm Hg pressure. This constant
boiling mixture cannot be separated into its constituents by simply distilling.
