This question was popular among the candidates and the performance was fair.

In (a)(i) and (ii), candidates correctly defined allotropy and isotopy and gave two elements as example of each of the two terms as follows:
           
(i)  I. Allotropy is the existence of an element in more than one form in  the same physical state.
II. Isotopy is the phenomenon in which atoms of the same element  have the same atomic number but different mass numbers.

(ii)   I.   Examples of allotropes:
             Carbon, sulphur, phosphorus, oxygen, tin etc.
II.         Examples of isotopes;
            Carbon, hydrogen, oxygen etc.

In (b)(i)-(iii), most candidates could not write balanced chemical equation as required.  The expected response from candidates were:

(i) CO2(g)  +  H2O(i)  ⇄  H2CO3       
(ii)  CO2(g)  +  2Mg(s)   →  2MgO(s)    + C(s) 
(iii)  CO2(g)  +  2NaOH(aq)   →   Na2CO3(aq)   +  H2O(l)  

                                                      OR
  CO2(g)  +  NaOH(aq)   →       NaHCO3(gaq) 

In (c)(i) and (ii), candidates showed good knowledge of pH scale and degree of acidity and alkalinity by correctly stating that:         

(i) I.  Solutions  A and D would liberate hydrogen from zinc;
II.  Solutions  C and E  change phenolphthalein;
III. Solution B  would not change the colour of litmus  

(ii) I strongly acidic – solution A
II strongly alkaline – solution C.

In (d), candidates could not correctly explain why aqueous solution of Na2CO3 is basic while aqueous solution of KCl is neutral.  They did not know that CO32- undergoes hydrolysis/ Na2CO3 is a salt of strong base and weak acid hence undergoes hydrolysis to produce excess OH- while KCl is a salt of strong acid and strong base and does not undergo hydrolysis.