|Aluminium foil + mercuric chloride
CAUTION: mercuric chloride is VERY poisonous. Wear chemical safe gloves. Be careful not to get any on your hands when doing this teacher only experiment.
Aluminium does not appear to react very well with the oxygen in the air. But aluminium is in fact quite a reactive metal. The reason it does not appear to react is that is has already reacted, but the aluminium oxide has formed in a thin, tightly held layer that protects the aluminium foil from further attack.
The aluminium oxide layer is so thin that is it not noticeable. It is possible to remove this layer with mercuric chloride and watch the aluminium reacting naturally with the oxygen in the air.
Place the aluminium foil onto a suitable tray or protected board.
2. Place one drop of mercuric chloride** solution onto the surface of aluminium foil and rub over the surface using a cotton wool bud.
Over the next few minutes as a white layer appears on the aluminium surface and the foil slowly disintegrates into a fine powder.
**Alternately put a small amount of solid HgCl2 on a watch glass. Take a wad of cotton wool, dampen it, then dab this onto the mercury chloride, and then rub this briskly over a reasonably large piece of Al foil (15 x 15cm)***. The reaction is so rapid that the water ‘boils’ off. Leave this sheet overnight and there will be holes through the foil.
***you might ask a willing pupil to hold their hand underneath the foil. They withdraw their hand very quickly.
Kathryn Coakley CGHS