State one appropriate observation when :

(a) Copper sulphide is treated with dilute hydrochloric acid.

(b) A few drops of dil. HCl are added to AgNO₃ solution. followed by addition of NH₄OH solution.

(c) Lead nitrate solution is mixed with dilute hydrochloric acid and heated.

(d) A small piece of zinc is added to dilute hydrochloric acid.

(e) Dilute HCl is added to sodium carbonate crystals.

(f) Concentrated HCl is added to manganese dioxide.

(a) A colourless gas having smell of rotten eggs is given off. The gas evolved is H₂S
CuS + 2HCl ⟶ CuCl₂ + H₂S ↑

(b) Curdy white precipitate of silver chloride [AgCl] is obtained, which is soluble in excess of NH₄OH.
AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃
AgCl + 2NH₄OH ⟶ [Ag(NH₃)₂Cl)] + 2H₂O

(c) White precipitate of PbCl₂ is formed which is soluble in hot water.
Pb(NO₃)₂ + 2HCl ⟶ PbCl₂ ↓ + 2HNO₃

(d) Hydrogen gas is evolved with bubbles and it burns with a pop sound
Zn + 2HCl ⟶ ZnCl₂ + H₂ ↑

(e) Effervescence of CO₂ seen which turns lime water milky.
Na₂CO₃ + 2HCl ⟶ 2NaCl + H₂O + CO₂ ↑

(f) When concentrated hydrochloric acid is added to manganese dioxide (MnO₂), a greenish-yellow gas (chlorine gas) is evolved with effervescence.
MnO₂ + 4HCl (conc.) $\xrightarrow{\Delta}$ MnCl₂ + 2H₂O + Cl₂↑