Equal masses of powdered calcium carbonate were added to separate solutions of hydrochloric acid. The calcium carbonate was in excess. The volume of carbon dioxide produced was measured at regular intervals. Which curves best represent the evolution of carbon dioxide against time for the acid solutions shown in the table below.

C

Opinion expressed through the G2 forms was divided with some commenting that this was a good question, whilst others felt it was too time consuming. Though a significant number of blank responses would seem to indicate that some candidates found the format of the question confusing, those that answered it performed quite well as indicated by a difficulty index of 44%. The question also proved to be quite a good discriminator, with a discrimination index of 0.33.