Question 2

Give a chemical test to distinguish between :

(a) Sodium chloride soln. and sodium nitrate soln.

(b) Hydrogen chloride gas and hydrogen sulphide gas.

(c) Calcium nitrate soln. and zinc nitrate soln.

(d) Carbon dioxide gas and sulphur dioxide gas.

(e) Sodium carbonate and sodium sulphite

(f) Ferrous nitrate and lead nitrate.

(g) Manganese dioxide and copper (Il) oxide.

(h) Sodium nitrate and sodium sulphite [using dilute sulphuric acid].

(i) Sodium chloride and sodium sulphide

(j) Sodium hydroxide solution and ammonium hydroxide solution.

(k) Ammonium sulphate and sodium sulphate.

(l) Sulphuric acid from nitric acid and hydrochloric acid.

(m) Magnesium chloride and magnesium nitrate solution

(n) Zinc sulphate solution and zinc chloride solution

(o) Iron (II) chloride soln. and iron (III) chloride soln.

(p) Calcium nitrate soln. and calcium chloride soln.

(a) Add silver nitrate soln. to the given solns., sodium chloride reacts to form a white ppt. which is soluble in NH₄OH and insoluble in dil. HNO₃. The other soln. is sodium nitrate.

NaCl + AgNO₃ ⟶ AgCl ↓ [white ppt.] + NaNO₃

NaNO₃ + AgNO₃ ⟶ no white ppt.

(b) Hydrogen sulphide gas turns moist lead acetate paper silvery black or black whereas, no change is observed in case of HCl gas.

Pb(CH₃COO)₂ [colourless] + H₂S ⟶ PbS [black] + 2CH₃COOH

(c) When NaOH is added to the given soln., Zn(NO₃)₂ reacts to form a gelatinous white ppt. which dissolves in excess of NaOH whereas, Ca(NO₃)₂ forms a milky white ppt. which is insoluble in excess of NaOH. Hence, the two can be distinguished.

(d) Carbon dioxide gas has no effect on acidified potassium permanganate (KMnO₄) and acidified potassium dichromate (K₂Cr₂O₇) solution whereas sulphur dioxide gas turns acidified potassium permanganate from pink to clear colourless and acidified potassium dichromate from orange to clear green.

2KMnO₄ + 2H₂O + 5SO₂ ⟶ K₂SO₄ + 2MnSO₄ + 2H₂SO₄

K₂Cr₂O₇ + H₂SO₄ + 3SO₂ ⟶ K₂SO₄ + Cr₂(SO₄)₃ + H₂O

(e) When dil. sulphuric acid is added to sodium carbonate and heated, colourless, odourless gas is evolved which turns lime water milky and has no effect on KMnO₄ or K₂Cr₂O₇ solutions.

When dil. sulphuric acid is added to sodium sulphite and heated, colourless gas with suffocating odour is evolved which turns lime water milky. It turns acidified K₂Cr₂O₇ from orange to clear green and pink coloured KMnO₄ to clear colourless.

Hence, the two compounds can be distinguished.

(f) When NaOH is added to each of the compounds, a dirty green precipitate of Iron [II] hydroxide [Fe(OH)₂] is formed in case of Ferrous nitrate whereas a chalky white precipitate of lead hydroxide [Pb(OH)₂] is formed in case of lead nitrate. Iron [II] Hydroxide [Fe(OH)₂] is insoluble in excess of NaOH, whereas Lead Hydroxide [Pb(OH)₂] is soluble in excess of NaOH. Hence, the two compounds can be distinguished.

(g) When each of the compound is heated with conc. hydrochloric acid, greenish yellow (chlorine) gas is evolved in case of manganese dioxide and filtrate is brownish in colour whereas, no chlorine gas is evolved in case of copper (II) oxide and filtrate is bluish in colour.

MnO₂ + 4HCl ⟶ MnCl₂ + 2H₂O + Cl₂

CuO + 2HCl ⟶ CuCl₂ + H₂O

(h) Sodium nitrate will not react with dilute sulphuric acid.

Sodium sulphite reacts with dil. sulphuric acid and colourless gas with suffocating odour is evolved which turns lime water milky.

(i) When dil. sulphuric acid is added to sodium sulphide and heated, hydrogen sulphide gas is evolved which has the smell of rotten eggs and turns lead acetate paper black.
Whereas, when conc. sulphuric acid is added to sodium chloride and heated, HCl gas is evolved which gives dense white fumes when a glass rod dipped in ammonia is brought near it.

(j) When ammonium hydroxide is reacted with a solution of copper sulphate, a pale blue ppt. of copper hydroxide is formed which dissolves in excess of ammonium hydroxide forming a deep blue solution of a soluble complex salt [Cu(NH₃)₄]SO₄ [tetramine copper [II] sulphate] .

CuSO₄ + 2NH₄OH ⟶ (NH₄)₂SO₄ + Cu(OH)₂ ↓

Cu(OH)₂ + (NH₄)₂SO₄ + 2NH₄OH ⟶ [Cu(NH₃)₄]SO₄ + 4H₂O

Whereas, when copper sulphate is added to sodium hydroxide solution, pale blue ppt. of copper hydroxide is obtained which is insoluble in excess of sodium hydroxide.

CuSO₄ + 2NaOH ⟶ Na₂SO₄ + Cu(OH)₂ ↓

(k) When ammonium sulphate is heated with NaOH, ammonia gas is produced which turns red litmus blue whereas, sodium sulphate does not react with sodium hydroxide.

(NH₄)₂SO₄ + 2NaOH ⟶ Na₂SO₄ + 2H₂O + 2NH₃

Na₂SO₄ + NaOH ⟶ no reaction.

(l) Barium chloride reacts with dil H₂SO₄ to form a white ppt. of barium sulphate.

BaCl₂ + H₂SO₄ [dil.] ⟶ 2HCl + BaSO₄ ↓ [white ppt.]

Whereas, HCl and nitric acid does not form a white ppt.

BaCl₂ + HCl ⟶ No white ppt.

BaCl₂ + HNO₃ ⟶ No white ppt

(m) Add silver nitrate solution to the given solns., magnesium chloride reacts to form a white ppt of silver chloride (AgCl). Whereas, magnesium nitrate show no reaction with silver nitrate. Hence, they can be distinguished using silver nitrate.

MgCl₂ + 2AgNO₃ ⟶ 2AgCl↓ + Mg(NO₃)₂

(n) When BaCl₂ soln. is added to ZnSO₄, a white ppt. of BaSO₄ is formed, whereas, no ppt. is formed in case of ZnCl₂. Hence, the two solns. can be distinguished.

BaCl₂ + ZnSO₄ ⟶ BaSO₄ ↓ [white ppt.] + ZnCl₂

BaCl₂ + ZnCl₂ ⟶ No white ppt.

(o) When NaOH soln. is added to the given solns., FeCl₂ reacts to form a dirty green ppt. of Fe(OH)₂ whereas, FeCl₃ reacts to form a reddish brown ppt. of Fe(OH)₃.

(p) Add silver nitrate soln. to the given solns., calcium chloride reacts to form a white ppt. which is soluble in NH₄OH and insoluble in dil. HNO₃. The other soln. is calcium nitrate.

CaCl₂ + 2AgNO₃ ⟶ 2AgCl ↓ [white ppt.] + Ca(NO₃)₂

Ca(NO₃)₂ + AgNO₃ ⟶ no white ppt.