[bosonstein]

All volumes are in cm^3.

10 of solution A and 25 of HCl and 5 of deionised water are placed into beaker 'X'.
10 of Potassium iodide and 10 of sodium thiosulfate and 1 of starch solution into beaker 'Y'.
Beaker 'X' contents are then poured into Beaker 'Y'.
A dark blue colour appears after a short time.

I think its hydrogen peroxide but I just want to be sure.

[AWK]

May be. But concentrations are also important.