Turn ammonia solution red, white or blue by adding phenolphthalein, lead nitrate or copper(II) sulfate in this demonstration
In this experiment, students observe what happens when a solution of ammonia is poured into each of three beakers, containing (unknown to the audience) small amounts of phenolphthalein, lead nitrate and copper(II) 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.
The demonstration lasts about three minutes, or longer if explaining the chemistry to a post-16 audience. It takes about 30–45 minutes to set up.
- Eye protection (goggles)
- Disposable gloves (preferably nitrile)
- Beakers, 250 cm3, x3
- Conical flasks, 500 cm3, x2
- Beaker, 100 cm3
- Dropping pipettes, x3
- Ammonia solution, 1 M, 250 cm3
- Dilute nitric acid, 2 M (CORROSIVE), 250 cm3
- Copper(II) sulfate solution, 0.5 M, 1 cm3
- Lead(II) nitrate solution, saturated (TOXIC, DANGEROUS FOR THE ENVIRONMENT), 1 cm3
- Phenolphthalein solution (HIGHLY FLAMMABLE)
- Deionised or distilled water
Health, safety and technical notes
- Read our standard health and safety guidance.
- Wear eye protection throughout and use protective gloves.
- All solutions must be made up in deionised or distilled water.
- Ammonia solution, NH3(aq) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006.
- Dilute nitric acid, HNO3(aq), (CORROSIVE) – see CLEAPSS Hazcard HC067 and CLEAPSS Recipe Book RB061.
- Copper(II) sulfate solution, CuSO4(aq) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031.
- Lead(II) nitrate solution, Pb(NO3)2(aq), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC057a and CLEAPSS Recipe Book RB053. Add 14 g of lead nitrate to 10 cm3 of deionised water and stir to make a saturated solution. Do not use tap water to make up the solution since it will turn cloudy if there are any chloride ions present.
- Phenolphthalein solution (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000.
Before the demonstration
The following operations should be carried out of sight of the audience:
- Line up the three beakers on the bench.
- Place about 1 cm3 of phenolphthalein solution in the first, about 1 cm3 of saturated lead nitrate solution in the second and about 1 cm3 of saturated copper sulfate solution in the third. The volumes are not critical – a single ‘squirt’ from a teat pipette will be accurate enough.
- Place 250 cm3 of ammonia solution in one 500 cm3 flask and about 125 cm3 of the nitric acid in the other.
- Mark the ammonia flask at approximately the 125 cm3 level.
- Pour about 40 cm3 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 cm3. 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 [Cu(NH3)4(H2O)2]2+ 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 should by 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 the flask.
Only the most sharp-eyed observers will notice even the pale blue colour of the copper sulfate in the third beaker before the demonstration begins.
In the final part of the demonstration the solution in the third flask may be 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.
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.
The reactions taking place are:
Pb(NO3)2(aq) + 2NH3(aq) + 2H2O(l) → Pb(OH)2(s) + 2NH4NO3(aq)
[Cu(H2O)6]2+(aq) + 4NH3(aq) → [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)
These reactions are effectively reversed in acid:
Pb(OH)2(s) + 2HNO3(aq) → Pb(NO3)2(aq) + 2H2O(l)
[Cu(NH3)4(H2O)2]2+(aq) + 4H+(aq) + 4H2O(l) → [Cu(H2O)6]2+(aq) + 4NH4+(aq)
Go over the reactions with a suitable audience. Ask them to predict the contents of the second flask (after explaining the sleight of hand) and to perhaps to suggest ways of producing other colours.
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.
© Nuffield Foundation and the Royal Society of Chemistry
Health and safety checked, 2016
No comments yet