A student added few pieces of aluminium metal to two test tubes A and B containing aqueous solutions of iron sulphate and copper sulphate. In the second part of her experiment, she added iron metal to another test tubes C and D containing aqueous solutions of aluminium sulphate and copper sulphate. In which test tube or test tubes will she observe a colour change?

In the activity series, the elements on the top can displace elements at the bottom, but not vice versa. From the given question, Aluminium can displace iron and copper from their sulphate solutions. Hence, a colour change will be observed in Test Tube A and Test Tube B. Iron metal cannot displace aluminium but can displace copper from its sulphate solution. Hence, Test Tube D's colour will also change. Thus, a colour change is observed in A, B, and D. Since aluminium displaces both iron and copper, it is the most reactive element among all. It is above both elements in the activity series.