Observation

Majority of the candidates did not attempt this question. For those that did, their performance was average.

In part (a), majority of the candidates stated the reason why phenolphthalein is used in the titration of CH3COOH(aq) with NaOH(aq) but could not give a colour change that would be observed when ZnCO3 is heated strongly and left to stand for a few minutes.

The expected answers include:

(a)(i)    The anion (CH3COO-) formed from the salt during the titration hydrolyses to give OH- ions making the solution basic / titration is between a weak acid and a strong base.
(ii)   When heated turns yellow but upon cooling it turns white                                                                                                               
(b)        - Add distilled water to the mixture (in a beaker) and stir    
- filter to obtain starch as residue      
- concentrate the filtrate and cool to obtain crystals of glucose