Question 3(2012)
State one chemical test between each of the following pairs :
(i) Sodium carbonate and Sodium sulphite
(ii) Ferrous nitrate and Lead nitrate
(iii) Manganese dioxide and Copper (II) oxide
(i) 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.
(ii) 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.
(iii) 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