A technician at a metal-finishing factory receives an unlabelled pale-green solution. The factory uses iron compounds and sodium compounds in different trea...
Question 1 Report
A technician at a metal-finishing factory receives an unlabelled pale-green solution. The factory uses iron compounds and sodium compounds in different treatment tanks, so the technician carries out cation tests on separate 2 cm3 portions. With sodium hydroxide solution, a green precipitate forms. With aqueous ammonia, a green precipitate also forms and does not dissolve when excess ammonia is added. A fresh portion gives no flame colour apart from the usual blue Bunsen flame.
(a) Name the metal ions indicated by the green precipitate tests. [2]
(b) State the formula of the green precipitate. [2]
(c) Give the name of the reagent used in the first test. [2]
(d) Which observation shows that sodium ions are unlikely to be present? [2]
(e) Explain why separate portions of the original solution are used for the two tests. [4]
Answer Details
(a) The green precipitates indicate iron(II) ions, \(\mathrm{Fe^{2+}}\). [2]
(b) The precipitate is iron(II) hydroxide, \(\mathrm{Fe(OH)_2}\). [2]
(c) The reagent is sodium hydroxide solution, \(\mathrm{NaOH(aq)}\). [2]
(d) No yellow flame colour is seen. Sodium ions produce an intense yellow flame, so sodium ions are unlikely to be present. [2]
(e) If one sample were used, the first reagent would remain in it and could react with the next reagent or change the ions present. This could alter the observation. Separate portions give independent, reliable tests. [4]
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