Question

Practical Q2 C is a mixture of two inorganic compounds. Carry out the following exercises on C: record your observations and identify any gas(es) evolved. State the conclusions you draw from the results of each test.
(a) Test 10cm³ of distilled water with sample C, shake, filter; test the residue and filtrate with a further sample.
(b)(i) To about 2cm³ of the filtrate add a few drops of AgNO3, followed by dilute HNO3. (ii) Add excess NH3 solution to the resulting mixture.
(c)(i) Put the residue in a test tube, add about 2cm³ of dilute HCl and shake. (ii) Add NH3 solution in drops and then in excess.

Accepted Answer

(a) Part of the 10cm³ of distilled water sample dissolves the soluble part of C, leaving an insoluble residue; on shaking thoroughly and filtering, a colourless filtrate is obtained and an insoluble residue is retained — C is a mixture of a soluble and an insoluble part.
(b)(i) A white precipitate is formed with the filtrate and AgNO3, which is insoluble in dilute HNO3 — this indicates the presence of chloride (Cl⁻) ions.
(ii) On adding excess NH3, the white precipitate dissolves in excess NH3(aq), consistent with AgCl formation and its dissolution to form a soluble complex.
(c)(i) The residue partly dissolves in dilute HCl, giving a colourless (and odourless) gas with rapid effervescence, which turns lime water milky (identified as CO2, i.e. carbonate/CO3²⁻ present) — indicating the residue contains a carbonate.
(ii) A blue gelatinous precipitate is formed with NH3 in drops, which dissolves in excess NH3(aq) to form a deep blue solution — indicating the presence of Cu²⁺ ions.

WAEC Chemistry 2019 Theory — Question 27

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