17. Red, White and Blue   Previous PageNext Page


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.)

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