‘Magic’ writing with colour changing reactions

In this experiment, the teacher prepares messages or pictures drawn on absorbent paper using a variety of colourless, dilute, aqueous solutions. When these are allowed to dry, they are rendered invisible. Students then observe how spraying the paper with solutions that contain different reagents – each of which reacts with one or more of the chemicals on the paper – reveals the hidden messages or pictures.

Taken at an entertainment level there is plenty to interest an audience and there is wide variety of chemical reactions taking place. This demonstration would be a good introduction or finale to a general lecture demonstration on chemistry, or for an open day presentation.

A-level students should be able to appreciate the chemistry involved and could be encouraged to use this as useful extension work.

The demonstration lasts about three minutes, but more than this if discussion or repeats are involved.

Equipment

Apparatus

• Eye protection (goggles)
• Protective gloves (preferably nitrile)
• Large sheet of filter paper (see note 1 below)
• Beakers, 100 cm³, x6
• Small paintbrushes, several (see note 2)
• Spray bottles, x3 (see note 3)
• Hair drier (optional) (see note 4)

Apparatus notes

1. Chromatography paper or even white blotting paper can also be used.
2. Paint brushes used for painting models would be ideal. Alternatively, wooden splints could be used.
3. Ideal spray bottles are those used to spray house plants. However, instead of using spray bottles the solutions could be painted on with a large paint brush.
4. Make sure the hair drier has been electrically (PAT) tested.

Chemicals

• Potassium hexacyanoferrate(II)-3-water, 5 g
• Copper(II) sulfate-5-water (HARMFUL), 5 g
• Ammonium thiocyanate (HARMFUL), 5 g
• Iron(III) nitrate-9-water (OXISIDING, IRRITANT), 5 g
• Lead(II) nitrate (TOXIC, DANGEROUS TO THE ENVIRONMENT), 5 g
• Potassium iodide, 5 g
• Ammonia solution, 2 M, 100 cm³
• Phenolphthalein indicator solution (HIGHLY FLAMMABLE), a few cm³
• Deionised or distilled water, 1 dm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout and use protective gloves. To avoid the risk of breathing in the fine sprays of liquid, especially lead(II) nitrate which is harmful by inhalation and a category 1 teratogen, the room should be well ventilated or the experiment could be carried out using a fume cupboard. 
• Potassium hexacyanoferrate(II)-3-water (potassium ferrocyanide-3-water), K₄Fe(CN)₆.3H₂O(s) – see CLEAPSS Hazcard HC079.
• Copper(II) sulfate-5-water, CuSO₄.5H₂O(s), (HARMFUL, DANGEROUS TO THE ENVIRONMENT) – see CLEAPSS Hazcard HC027c. 
• Ammonium thiocyanate, NH₄SCN(s), (HARMFUL) – see CLEAPSS Hazcard HC009B. 
• Iron(III) nitrate-9-water, Fe(NO₃)₃.9H₂O(s), (OXIDISING, IRRITANT) – see CLEAPSS Hazcard HC055C. 
• Lead(II) nitrate, Pb(NO₃ )₂(s), (TOXIC, DANGEROUS TO THE ENVIRONMENT) – see CLEAPSS Hazcard HC057a. Wash hands after handling lead(II) nitrate solution.
• Potassium iodide, KI(s) – see CLEAPSS Hazcard HC047b. 
• Aqueous ammonia, NH₃(aq) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006. 
• Phenolphthalein indicator solution (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000.

Procedure

Before the demonstration

1. For each of the solids, dissolve 5 g in 100 cm³ of deionised/distilled water. The lead nitrate solution should be labelled TOXIC.
2. Transfer the solutions of iron(III) nitrate, lead(II) nitrate and aqueous ammonia into separate spray bottles. Adjust the nozzles of the bottles so that they deliver a fine mist, and spray several times to ensure that the spray contains the solution and not the water remaining from washing out the bottle.
3. Use the paintbrushes to write suitable messages or designs on the piece of filter paper using the solutions of potassium iodide, potassium hexacyanoferrate(II), ammonium thiocyanate, phenolphthalein and copper sulphate.
4. Leave the paper to dry or use a hair drier. Work on a piece of clean newspaper to avoid picking up chemicals from the surface of the bench. All of the solutions will dry colourless with the exception of copper sulphate, which will be a very pale blue – undetectable, except to the most sharp-eyed members of the audience.

For the demonstration

1. In a fume cupboard spray the paper with the solution of lead nitrate. The message written in potassium iodide will show up as bright yellow lead iodide.
2. Now spray with the iron(III) nitrate. The message written with potassium hexacyanoferrate(II) will turn dark blue. The one written with ammonium thiocyanate will turn red-brown.
3. Now spray with the ammonia solution. The phenolphthalein will turn pink, the copper sulphate blue and the red-brown colour of the iron thiocyanate complex will almost disappear.

Teaching notes

A variety of suitable messages could be used such as ‘Welcome’ or ‘The end’, as appropriate. Alternatively an equation could be written up so that the reactants are revealed by the first spray, the products with the second spray, and the balancing numbers for the third.

Some of the reactions are worth discussing with the audience. Even fairly young students will know that phenolphthalein turns red in the presence of alkali, and that the pink writing could be hidden again using a dilute acid as a spray.

The theory behind the appearance of the colours is as follows:

The spraying of potassium iodide with aqueous lead(II) nitrate involves the precipitation of bright yellow lead(II) iodide:

Pb²⁺(aq) + 2I^–(aq) → PbI₂(s)

The spraying with iron(III) nitrate causes a reaction with the hexacyanoferrate(II) ions, forming an intense blue colour known as ‘Prussian blue’. Recent research on the composition of the compound known as Prussian blue shows that it has a complex formula of the form Fe_x(CN)_y.zH₂O. Meanwhile, the intense red colour produced by spraying with ammonium thiocyanate writing involves a ligand exchange reaction:

[Fe(H₂O)₆]³⁺(aq) + CNS^–(aq) → [Fe(CNS)(H₂O)₅]²⁺ (aq)

Both of these reactions form the basis of the detection of iron(III) ions.

The spraying with the aqueous ammonia has a predictable effect on the phenolphthalein writing, and causes the pale blue hexaaquacopper(II) ions to form the intensely blue tetraamminediaquacopper(II) ions:

[Cu(H₂O)₆]²⁺(aq) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 4H₂O(l)

Meanwhile the intense red colour of the iron thiocyanate complex is replaced by a much less intense brown colour since hydroxide ions cause ligand exchange to form iron(III) hydroxide:

[Fe(CNS)(H₂O)₅]²⁺(aq) + 3OH^–(aq) → [Fe(OH)₃(H₂O)₃](s) + CNS^–(aq) + 2H₂O(l)

A possible variation is to spray with a mixture of lead nitrate and iron(III) nitrate. This brings out the yellow, blue and brown colours simultaneously.

A large number of alternative colour reactions can be used as the basis for ‘magic’ writing. It is suggested that these are tried out first, especially where mixtures are involved.

Try this similar experiment

There are other ways to reveal a hidden message using chemistry – try this demonstration using sodium nitrate and a lighted splint to make letters ‘glow’.