3 minutes reading time (552 words)

    Red, White and Blue

    ​Description

    A solution of ammonia is poured into each of three beakers which contain (unknown to the audience) a little phenolphthalein, a little lead nitrate solution and a little copper sulfate solution. The beakers' contents turn red, milky white and deep blue respectively. Pouring the contents of the beakers into acid reverses the changes to give a colourless solution.

    Apparatus

    • Beaker, 250mL, 3x
    • Flask, 500mL, 2x
    • Teat pipettes, 3x

    Chemicals
    • Ammonia, 1M, 250mL
    • Nitric acid, 2M, 250mL
    • Copper sulfate, 0.5M, 1mL
    • Lead (II) nitrate, saturated, 1mL
    • Phenolphthalein solution, 1mL

    Ensure that all of the solutions are made up using deionised water otherwise the lead-containing solutions will be cloudy due to the formation of lead chloride from chloride ions in tap water.

    Method
    Before the demonstration
    Line up the three 250mL beakers on the bench. Place about 1mL of phenolphthalein solution in the first, place about 1mL of saturated lead nitrate solution in the second and place about 1 mL of saturated copper sulfate 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 sulfate. Place 250mL of ammonia solution in one 500mL flask and about 125mL of the nitric acid in the other, which should be kept out of sight of the audience. Mark the ammonia flask at approximately the 125mL level.

    The demonstration

    Pour about 40mL of ammonia solution in turn into each of the three beakers on the bench. Aim to leave the flask full to the mark at 125mL. The phenolphthalein will turn red, the lead nitrate will form a milky white precipitate of lead(II) hydroxide and the copper sulfate will form the deep blue tetraamminecopper(II) ion. Now use some sleight of hand to switch 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.)

    Visual tips

    Scale the volumes up if the audience is some way away. Stand the phenolphthalein and copper sulfate flasks on white filter paper and the lead nitrate one on black paper for maximum impact.

    Teaching tips

    Go over the reactions with a suitable audience. Ask them to predict the contents of the second flask (after explaining the sleight of hand). Ask them to suggest ways of producing other colours.

    Theory

    The reactions are:
    Pb(NO3)2(aq) + 2NH3(aq) + 2H2O(l) → Pb(OH)2(s) + 2NH4NO3(aq)
    Cu2+(aq) + 4NH3(aq) → Cu(NH3)42+(aq)


    These are reversed in acid:
    Pb(OH)2(s) + 2HNO3(aq) → Pb(NO3)2(aq) + 2H2O(l)
    Cu(NH3)42+(aq) + 4H+(aq) → Cu2+(aq) + 4NH4+(aq)

    Further details
    A version of this demonstration in which the evolution of nitrogen dioxide generated by the reaction of copper and concentrated nitric acid drives the solutions from one flask to another has been reported by TC Swinfen and DJ Hearn, Sch. Sci. Rev., 1989, 71 (255), 94.

    Safety
    Wear eye protection. It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

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    Tuesday, 21 August 2018

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