Question 3A
State the observations that would be made when each of the following reactions is carried out in the laboratory:
- addition of 2 cm3 of bench H2SO4 to 2 cm3 of barium chloride solution;
- addition of 2 cm3 of dilute hydrochloric acid to 1 g of powdered iron(II)sulphide (FeS);
- addition of 2 cm3 of dilute hydrochloric acid to 1 g of iron filings and allowed to stand for some time.
[7 marks]
Observation
In part (a), some candidates carried out the tests on solid sample.
In part (b), majority of the candidates did not write their observations when they felt the test tube after adding distilled water to D.
In part (c), the knowledge of practical from theoretical perspective was tested. From the addition of 2 cm3 of dilute hydrochloric acid to 1 g of powdered iron(II) sulphide (FeS), majority of the candidates wrote formation of hydrogen sulphide and iron(II)chloride instead of writing the observable properties such as the odour and colour of the gas evolved.
The expected answers include:
(a) White precipitate of BaSO4 barium tetraoxosulphate(VI) is formed.
(b) Effervescence / Evolution of gas producing a gas with a smell of rotten eggs and a green solution is formed.
(c) Effervescence / Evolution of colourless, odourless gas (Light green) solution turned brown after sometime.