TEST OF PRACTICAL KNOWLEDGE QUESTION
(a) Suggest how the following liquid reagents can be suitably stored in the laboratory
(i) X which fumes in moist air;
(ii) Y which is slowly decomposed by sunlight in ordinary reagent bottles.
(b) State what is observed when aqueous ammonia is added to:
(i) litmus paper;
(ii) Pb(NO\(_3\))\(_2\) solution in drops until in excess
(iii) freshly precipitated AgCI in excess.
(c) A salt sample was suspected to be either Na\(_2\)CO\(_2\) or NaHCO\(_3\). A student who was required to identify it tested a portion for solubility in water and then for effect on litmus paper.
(i) What was observed in each case?
(ii) Give the reason why the student's procedure was unsuitable
(iii) Describe briefly how you would have identified the salt.
(a) Storage of reagents
- (i) X, which fumes in moist air (e.g. concentrated hydrochloric or trioxonitrate(V) acid): store in a tightly stoppered (airtight) bottle so that it does not react with atmospheric moisture.
- (ii) Y, which is slowly decomposed by sunlight (e.g. silver trioxonitrate(V), hydrogen peroxide): store in a dark (amber/brown) coloured bottle kept away from light.
(b) Adding aqueous ammonia to
- (i) Litmus paper: red litmus turns blue (ammonia solution is alkaline).
- (ii) \(Pb(NO_3)_2\) solution, drops then excess: a white precipitate of lead(II) hydroxide forms, which is insoluble in excess ammonia.
- (iii) Freshly precipitated \(AgCl\) in excess: the white \(AgCl\) dissolves (it forms the soluble diamminesilver(I) complex \([Ag(NH_3)_2]^+\)).
(c) Distinguishing \(Na_2CO_3\) from \(NaHCO_3\)
- (i) In each case the salt dissolves in water and the solution turns red litmus blue (both are soluble and alkaline).
- (ii) The procedure is unsuitable because both salts give identical results (both are soluble and both are alkaline to litmus), so the tests cannot tell them apart.
- (iii) A better method is to heat a dry sample: sodium hydrogentrioxocarbonate(IV) decomposes readily, giving off \(CO_2\) that turns limewater milky \((2NaHCO_3 \to Na_2CO_3 + H_2O + CO_2)\), whereas sodium trioxocarbonate(IV) is stable to heat and gives no gas. (Alternatively, warm each with dilute acid; \(NaHCO_3\) effervesces more readily.)
