Classic Chemistry 1. Food Demonstrations SCN). 5 g of iron(iii) nitrate-9-water (ferric nitrate-9-water, Fe(NO 3

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1 Classic Chemistry 1. Food Demonstrations Magic writing Topic Timing Level Description Apparatus This demonstration will probably be done for entertainment/general interest. As such it can be an excellent introduction or finale to a lecture demonstration programme if a suitable message is written. However, there is a lot of interesting chemistry going on especially of transition metals. A couple of minutes, but more if discussion is to take place and repeats are to be done. Any for general interest. Post-16 students should be able to appreciate the chemistry behind the reactions. Messages are written on filter paper with a variety of colourless, dilute, aqueous solutions. Spraying with other solutions produces coloured products and the messages show up in a variety of colours. Large sheets of white filter paper, chromatography paper or blotting paper. Small paintbrushes, the size used for painting models (wooden spills will do if these are not available). Spray bottles such as those used to spray house plants. These are available from garden centres and DIY shops. One is needed for each solution to be sprayed so three are required for the basic method described. Hair drier (optional). Chemicals The quantities given are sufficient for several demonstrations. 5 g of potassium hexacyanoferrate(ii)-3-water (potassium ferrocyanide-3-water, K 4 Fe(CN) 6.3H 2 5 g of copper(ii) sulphate-5-water (hydrated copper sulphate, CuSO 4.5H 2 5 g of ammonium thiocyanate (NH 4 SCN). 5 g of iron(iii) nitrate-9-water (ferric nitrate-9-water, Fe(NO 3 ) 3.9H 2 5 g of lead(ii) nitrate (lead nitrate, Pb(NO 3 ) 2 ). 5 g of potassium iodide, KI. 100 cm 3 of approximately 2 mol dm 3 ammonia solution (bench ammonia, NH 3 (aq)). A few cm 3 of phenolphthalein solution. About 1 dm 3 of deionised water.

2 112 Classic Chemistry Demonstrations Method Before the demonstration Make up the solutions as follows. Phenolphthalein: dissolve 0.1 g in 60 cm 3 of ethanol and make up to 100 cm 3 with deionised water. This is the usual bench solution. Ammonia: approximately 10 cm 3 of 880 ammonia made up to 100 cm 3 with deionised water. All the other solids: dissolve 5 g of solid in 100 cm 3 of water. Put the solutions of iron(iii) nitrate, lead nitrate and ammonia into separate spray bottles. Adjust the nozzles of the bottles to give a fine mist and spray several times to ensure that the spray contains the solution and not water remaining from washing out the bottle. Use paintbrushes or wooden spills to write suitable messages on the filter paper using solutions of potassium iodide, potassium hexacyanoferrate(ii), ammonium thiocyanate, phenolphthalein and copper sulphate. Dry with 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 audience. Pin the paper to the wall where it can be easily seen. Visual tips Teaching tips The demonstration Spray the paper with the solution of lead nitrate. The message written in potassium iodide will show up as bright yellow lead iodide. Now spray with the iron(iii) nitrate. The message written with potassium hexacyanoferrate(ii) will turn dark blue (Prussian blue) and the one written with ammonium thiocyanate will turn red-brown (Fe(H 2 O) 5 SCN 2+ ). Now spray with the ammonia solution. The phenolphthalein will turn pink, the copper sulphate blue (Cu(NH 3 ) 6 2+ ) and the colour of the iron thiocyanate complex will disappear due to the formation of iron(iii) hydroxide which is much less strongly coloured. Do not overspray the paper with the reagents as the colours will run. A variety of suitable messages could be used such as Welcome or The end etc as appropriate. Alternatively, an equation could be written up so that the reactants are revealed by the first spray, the products with the second, and balancing coefficients with the third. Many of the reactions are worth discussing with a suitable audience. The reactions with ammonium thiocyanate and potassium hexacyanoferrate(ii) are used as tests for iron(iii). Even fairly young students will know the reaction of phenolphthalein with alkali. Students could be encouraged to suggest that the pink writing could be erased with a dilute acid such as hydrochloric acid. This could be tried later.

3 Classic Chemistry 1. Food Demonstrations Extensions One interesting 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 colour reactions can be used as the basis for magic writing demonstrations. Do try them out first, especially when mixtures are involved, as unexpected reactions might embarrass the unwary. Further details Instead of spraying the solutions onto the filter paper, they could be painted on with large paintbrushes. Safety Wear eye protection. Take care to avoid breathing in the fine sprays. Ensure that the room is well ventilated or do the demonstration outside or in a fume cupboard. Consider wearing a mask over the mouth. It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

4 Classic Chemical 1. Food Demonstrations The thermal decomposition of nitrates magic writing Topic Timing Level Description Apparatus Thermal decomposition of nitrates, general interest. About 5 min. Lower secondary. A message is written on filter paper with a solution of sodium nitrate and is then dried. Applying a glowing taper to the start of the message makes the treated paper smoulder and the message is revealed as the glow spreads its way through the treated paper only. Filter or blotting paper sheets as large as possible. Wooden taper. Bunsen burner or hair-drier. Small paint brush. Chemicals Method Teaching tips The quantities given are for one demonstration. About 10 g of sodium nitrate (sodium nitrate(v), NaNO 3 ). Before the demonstration Make a saturated solution of sodium nitrate by adding about 10 g of solid to 10 cm 3 of water and stirring. Using a small paintbrush (or a length of wooden taper), write a message on the filter paper. Use joined up writing! Dry the message using a hair-drier or by holding the paper well above a Bunsen flame. The message will be virtually invisible, so mark the start of it with a light pencil mark. The demonstration Pin up the filter paper in the sight of the audience. Apply a glowing taper to the start of the message until the treated paper starts to glow and char. Remove the taper and watch as the glow and charring works its way along the message, leaving the untreated paper untouched. This demonstration could be used to introduce the fire triangle: fuel, heat and oxygen. With older students it could be used to revise the equations for the decomposition of nitrates.

5 158 Classic Chemical Demonstrations Theory The reaction that occurs is: Extensions 2NaNO 3 (s) 2NaNO 2 (s) + O 2 (g) The oxygen produced is sufficient to keep the treated paper smouldering while the untreated paper does not burn. Try other metal nitrates (see below). What effect does the nitrate concentration have? Further details Potassium nitrate works in the same way as sodium nitrate. Lithium nitrate also works although it decomposes slightly differently due to the higher charge density on the lithium ion 4LiNO 3 (s) 2Li 2 O(s) + 4NO 2 (g) + O 2 (g) Lead nitrate will also work: 2Pb(NO 3 ) 2 (s) 2PbO(s) + 4NO 2 (g) + O 2 (g) Ammonium nitrate does not work because it does not give off oxygen as it decomposes: NH 4 NO 3 (s) N 2 O(g) + 2H 2 O(l) Safety Although nitrogen(i) oxide (N 2 O) will itself decompose to give oxygen, there is presumably either insufficient N 2 O to keep the paper smouldering or the temperature is too low to bring about decomposition. Wear eye protection. It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

CIE Chemistry A-Level Practicals for Papers 3 and 5

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