Method
Before the Demonstration
Line up three 250 cm
beakers on the bench. Place about 1 cm
of phenolphthalein solution in the first, place about 1 cm
of saturated lead nitrate in the second and place about 1 cm
of saturated copper sulphate solution in the third.
The volumes are not critical - a single squirt from a teat pipette
will be accurate enough. The audience should not know about these
additions. Only the most sharp-eyed observers will notice even the
copper sulphate.
Place 250 cm
of ammonia solution in one 500 cm
flask and about 125 cm
of the nitric acid in the other, which should be kept well out of
sight of the audience.
Mark the ammonia flask at approximately the 125 cm
level.
|
|
The Demonstration
Pour about 40 cm
of ammonia solution in turn into each of the three beakers on the
bench. Aim to leave the flask full to the mark at 125 cm.
The phenolphthalien will turn red, the lead nitrate will form a
milky white precipitate of lead (II) hydroxide and the copper sulphate
will form the deep blue tetraamminecopper (II) ion.
Now use some sleight of hand to switch over the ammonia-containing
flask with that containing the nitric acid. The levels of liquid
in both flasks will now be about the same.
Pour the contents of the three beakers in turn into the nitric acid
flask and the colours will disappear, leaving a clear, colourless
solution. (In fact it may be a very pale blue due to the copper
ions and there may be a few specks of undissolved lead hydroxide,
but the audience is unlikely to notice this.)
|