Majority of the responding candidates carried out the required tests, made their observations and drew inferences in the prescribed format. However, some candidates lost marks for not reporting that no visible reaction was observed on adding HNO3 to the filtrate and for including pb2+ as ions present when the filtrate was treated with NH3(aq)
Observation of jelly-like paste was also not recorded by many candidates when water was added to residyue and then heated.
On addition of iodine to the cooled mixture, most candidates recorded black ppt instead of blue-black colouration. General performance was fair.
The expected answers are:
|
TEST |
OBSERVATION |
INFERENCE |
a) |
F + water, stirred mixture
and filtered |
Partly dissoves/soluble Colourless filtrate
White residue |
F contains soluble and insoluble salts |
b) (i) I
II |
Filtrate + dil HNO3
+ Ag NO3(aq)
Mixture in (b)(i) +
excess NH3 solution |
No visible reaction
White precipitate formed
Precipitate dissovles |
Cl- present
Cl- confirmed |
b)(ii) |
Filtrate + HN3(aq) in drops
then in excess |
White gelatinous precipitate
Precipitate dissovles |
Zn+2 or A13+
may be present
Zn2+ confirmed |
c) |
Residue + water + heat
allow to cool + iodine solution |
Jelly-like paste is formed
Dark/deep blue/blue black colouration is obtained |
Starch is confirmed |