This demonstration involves a bit of trickery, which some people do not like or approve of.
It involves some redox reactions involving chromium and iron that are visualised by the reduction of iodine to iodide prodicing (due to the presence of starch) a black solution.
The demonstration is set up such that the colour change only happens in a beaker but not in a conical flask (due to small amount of reagents secreted in them).
Beakers not Conicals
Beakers not Conicals – Risk Assessment