Question 3
(a) (i) State two solid substances that give effervescence with dilute hydrochloric acid.
(ii) Write a balanced chemical equation for one of the reactions stated in 3(a)(i).
[4 marks]
(b) (i) State two trioxocarbonate (IV) salts that are not decomposed by heat.
(ii) Name two salts that form acidic solutions when dissolved in water.
(iii) Name the process involved in 3(b)(ii).
[5 marks]
(c) State four methods of purification used in the laboratory. [4 marks]
(d) Give a reason why aqueous solution of FeSO4 are freshly prepared when required for
use. [1 mark]
Observation
Majority of the candidates responded to this question, and their performance was above average.
In part (a), majority of the candidates stated two solid substances that give effervescence with dilute hydrochloric acid.
In part (b), majority of the candidates stated two trioxocarbonate (IV) salts that are not decomposed by heat.
In part (c), majority of the candidates stated four methods of purification used in the laboratory.
In part (d), majority of the candidates gave a reason why aqueous solutions of FeSO4 are freshly prepared when required for use.
The expected answers include:
(a) (i) - CaCO3
- Zn
- Mg
- Na2S
- FeS
(ii) CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
(b) (i) - Na2CO3 / Sodium trioxocarbonate (IV)
- K2CO3 / Potassium trioxocarbonate (IV)
- BaCO3 / Barium trioxocarbonate (IV)
(ii)
- aluminium chloride
- ammonium trioxonitrate (V)
- zinc tetraoxosulphate (VI)
- iron(III)chloride
(iii) The process is hydrolysis
(c)
- recrystallization
- chromatography
- distillation
- sublimation
- fractional distillation
- solvent extraction
- filtration
- electrolysis
- precipitation
(d)
- because it is easily oxidized /
- It is easily converted to Fe3+