ü Hydrochloric Acid (HCl):
Ø  Preparation: It is prepared in the laboratory, by heating sodium chloride with concentrated sulphuric acid.
NaCl + H2SO4 ⎯⎯420⎯K⎯→ NaHSO4 + HCl
NaHSO4 + NaCl ⎯⎯823⎯K⎯→ Na2SO4 + HCl 

Ø  Properties:
·       These are colourless, pungent smelling gas with acidic tastes.
·       It is heavier than air, can be liquified to colourless liquids.
·       These are neither combustible nor supporter of combustion.
·     When perfectly dry, they have no action on litmus, but in presence of moisture, they turn blue litumus red, showing acidic nature. Among HX, HI is the strongest and HF is the weakest acid.
·       These are quite soluble in water. HCl ionises as below.
HCl(g) + H2O (l) ® H3O+ (aq) + Cl– (aq) Ka = 107.
·   Its aqueous solution is called hydrochloric acid. High value of dissociation constant (Ka) indicates that it is a strong acid in water.
·       It reacts with NH3 and gives white fumes of NH4Cl.
NH3 + HCl → NH4Cl
·       When three parts of concentrated HCl and one part of concentrated HNO3 are mixed, aqua regia is formed which is used for dissolving noble metals, e.g., gold, platinum.
Au + 4 H+ + NO3- + 4Cl- →AuCl4- + NO + 2H2O
3Pt + 16H++ 4NO3- + 18Cl- → 3PtCl62- + 4NO + 8H2O
·       Hydrochloric acid decomposes salts of weaker acids like carbonates, hydrogen carbonates, sulphites, etc.
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
NaHCO3 + HCl → NaCl + H2O + CO2
Na2SO3 + 2HCl → 2NaCl + H2O + SO2
·       Reaction with metals oxides, hydroxides and bicarbonates
Zn + 2HCl ¾® ZnCl2 + H2 
MgO + 2HCl ¾® MgCl2 + H2O
NaOH + HCl ¾® NaCl + H2O
Detection of cation: HCl:
AgNO3 + HCl ¾® AgCl¯ (white) + HNO3
(CH3COO)2Pb + 2HCl ¾® PbCl2¯ (white) + 2CH3COOH Hg(NO3)2 + 2HCl ¾®¾ Hg2Cl2 ¯ (white) + 2HNO3
Ø  Uses: It is used
·  In the manufacture of chlorine, NH4Cl and glucose (from corn starch).
§  For extracting glue from bones and purifying bone black,
§  In medicine and as a laboratory reagent.
§  HI is used as reducing agent in organic chemistry.