The real reactivity of aluminium
In this demonstration the oxide coating on aluminium foil is removed. The aluminium is exposed and reacts rapidly with the oxygen in the air to reform the oxide. This allows the real reactivity of aluminium to be seen.
Read our standard health & safety guidance
Lesson Organisation
This demonstration takes only a few (less than 10) minutes.
A flexicam would work well with this demonstration and allow students a clearer view of what is going on.
Apparatus and Chemicals
For one demonstration:
Eye protection (goggles)
Disposable nitrile gloves
Beakers (250 cm3) 2
Watch glass
Forceps or tongs
Glass petri dish with lid
For method 2 only:
Cotton wool or cotton bud
Aluminium foil (Low hazard), 5 cm x 5 cm
Mercury(II) chloride, HgCl2, 200g dissolved in 200 cm3 water (Toxic at this concentration, Dangerous for the environment) (see note 1)
Sodium hydroxide solution, 1.0 mol dm-3 (Corrosive at this concentration), 200 cm3
Technical notes
Aluminium (Low hazard) Refer to CLEAPSS Hazcard 1
Mercury(II) chloride (Toxic at concentration used, Dangerous for the environment) Refer to CLEAPSS Hazcard 62
Sodium hydroxide (Corrosive at concentration used) Refer to CLEAPSS Hazcard 91 and Recipe Card 65
1 Wearing nitrile gloves, the 200 g of mercury(II) chloride should be dissolved in 200 cm3 water and the resulting solution given the appropriate hazard warnings.
2 Disposal of the waste. Mercury compounds should not normally be disposed of down the sink. The solution could be kept in a closed container for further demonstrations. Alternatively, treat the mercury chloride with excess concentrated sodium hydroxide to precipitate yellow mercury hydroxide. Filter this off, place it in a sealed plastic bag and store it for professional disposal.
3 Do not throw treated aluminium foil in the rubbish bin. It may continue to react exothermically and ignite other rubbish. One method of disposal is to return it to the sodium hydroxide solution until it has dissolved.
Procedure
Method 1
a Wear goggles and gloves. Cut off about a 5 cm x 5 cm piece of aluminium cooking foil. Using forceps or tongs, immerse the foil in the sodium hydroxide solution until it begins to effervesce which will take about a minute. Still using the forceps, rinse it with water.
b Again using the forceps, dip the foil into the mercury(II) chloride solution. Leave it for about a minute until the solution begins to turn slightly grey.
c Remove the foil using forceps, rinse it with water, shake it dry and leave it on the watch glass.
d After about a minute, the foil will start to tarnish rapidly and will become coated with a pale grey layer of oxide. It will get hot and steam will be produced as any remaining droplets of water evaporate.
Method 2
a Wear eye protection, gloves and use forceps. Soak the cotton wool or cotton bud in the mercury (II) chloride solution and wipe the surface of a little aluminium foil. The cleaned surface soon tarnishes with a layer of the oxide.
In both cases, the reacted foil can be placed in a glass petri dish with a lid and passed around the class. A piece of unreacted foil can be used for comparison.
Teaching notes
Electrode potentials place aluminium between zinc and magnesium in reactivity, but the presence of a firmly-attached oxide layer means that it is usually unreactive enough to be used unpainted. Sodium hydroxide dissolves away the oxide coating on aluminium as sodium aluminate.
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
Once the oxide has gone, effervescence starts due to:
2Al(s) + 2NaOH(aq) + 6H2O(l) → 2NaAl(OH)4(aq) + 3H2(g)
Mercury(II) chloride forms an amalgam with the freshly exposed surface.
The exposed aluminium reacts with oxygen from the air to form aluminium oxide.
4Al(s) + 3O2(g) → 2Al2O3(s)
Health & Safety checked, April 2008
Updated 10 Apr 2008
