What is the coefficient for I^– when the following equation is balanced using the smallest possible whole numbers?

\({\text{IO}}_{_3}^ - {\text{(aq)}} + \) ___ \({{\text{I}}^ - }{\text{(aq)}} + \) ___ \({{\text{H}}^ + }{\text{(aq)}} \to \) ___ \({{\text{I}}_{\text{2}}}{\text{(aq)}} + \) ___ \({{\text{H}}_{\text{2}}}{\text{O(l)}}\)

A.     1

B.     2

C.     3

D.     5

D

We accept that this is a difficult question but many candidates will have come across the reaction in their laboratory work – and they should know that an equation must be balanced for charge as well as for species. It was the most difficult question on the paper but there do need to be questions that will differentiate the grade 7 candidates from those who achieve grade 6.