This question was attempted by most candidates and the performance was fair.
In part (a)(i), candidates correctly defined deliquescence and efflorescence. However, some of them lost marks because they confused process with substance or compound.
The expected responses were as follows:
I. Deliquescence is a process by which a compound absorbs moisture/water from the atmosphere and dissolves in it to form a solution.
II. Efflorescence is a process whereby a hydrated salt loses all or part of its water of crystallization when exposed to the air.
In part (a)(ii), candidates correctly gave one example of a substance that undergoes each of the processes from among the list thus:
Deliquescence: NaOH/FeCl3/CaCl2/PCl5 etc. .
Efflorescence: Na2CO3.10H2O/ Na2SO4 .10H2O/MgSO4 .7H2O etc.
In part (b), candidates correctly gave an outline of how the three colourless solutions suspected to be ethane, ethene and ethyne could be distinguished in the laboratory thus:
- To a sample of each of the gases, add acidified KMnO4 /Br2 water (vapour)
- No visible reaction shows ethane OR decolourization of KMnO4 /Br2 water (vapour) shows ethene and ethyne.
- To each of the two remaining samples, add ammoniacal copper chloride/ammoniacal AgNO3 /Ag2O
- No visible reaction shows ethene/formation of reddish-brown precipitate/white precipitate indicate the presence of ethyne.
In part (c)(i), most candidates did not know that water of crystallization was the definite amount of water some substances chemically combine with when they form salt from their aqueous solution. However, they correctly calculated the value of x in the hydrated calcium chloride as follows:
% of anhydrous CaCl2 is 100 – 49.32 = 50.68%
Number of moles of CaCl2 H2O
50.68 49.32
111 18
0.457 2.74
0.457 0.457
1: 6
x = 6
In part (d), most candidates used formula instead of the name as required by the question and hence, lost the marks. The expected answers from candidates were:
- Calcium trioxocarbonate (IV)
- Calcium chloride
- Calcium tetraoxosulphate (VI)
- Calcium oxide/calcium hydroxide
In part (e), most candidates stated the ions present in the solution as Na+ & Cl- and H+ & OH-, named the products at the anode and cathode as chloride and hydrogen respectively and gave the by – product of the electrolysis as NaOH(aq).
