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Classic Chemical Demonstrationsk 253

90. Turning red wine into waterTopic

This is a fun demonstration suitable for an open day.

TimingAbout 5 min.

LevelAny, for general interest, but a post-16 group could be asked to work out thereactions involved.

DescriptionA solution of acidified potassium permanganate (red wine) is poured into a set ofglasses previously laced with small amounts of colourless solutions. The wineturnsto water, milk, raspberry milk shakeand fizzy lemonade.

Apparatus One wine (or Ribena) bottle, about 750 cm3or larger. A laboratory flask will

do but the effect is partly lost.

Five wine glasses (or other drinking glasses). Laboratory beakers will do but theeffect is partly lost.

ChemicalsThe quantities given are for one demonstration.

About 0.4 g of potassium permanganate(potassium manganate(VII), KMnO4).

About 24 g of barium chloride-2-water(BaCl2.2H2O).

About 25 g of sodium thiosulphate-5-water (Na2S2O3.5H2O).

About 22 g of sodium carbonate (Na2CO3).

About 100 cm3of 2 mol dm3sulphuric acid.

About 1.5 dm3of deionised water.

MethodBefore the demonstrationMake up the following solutions:

24 g of barium chloride in 100 cm3of deionised water (approximately1 mol dm3);

25 g of sodium thiosulphate in 100 cm3of deionised water (approximately1 mol dm3);

21 g of sodium carbonate in 100 cm3of deionised water (approximately2 mol dm3);

0.4 g of potassium permanganate dissolved in 100 cm3

of 2 mol dm3

sulphuricacid and made up to 1 dm3with deionised water (approximately 0.0025 moldm3with respect to potassium permanganate). Put this solution in a suitablebottle red wine or Ribena, for example.

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254 Classic Chemical Demonstrations

Line up the five glasses on the bench. Leave the first (glass 1) empty. Then out of sightof the audience, lace the remaining glasses as follows:

glass 2: 1 cm3of the sodium thiosulphate solution;

glass 3: 1 cm3of the sodium thiosulphate solution and 3 cm3of the bariumchloride solution;

glass 4: 3 cm3of the barium chloride solution; and

glass 5: 1 cm3of the sodium thiosulphate solution and 3 cm3of the sodiumcarbonate solution.

The small volumes of liquid will almost certainly not be spotted by the audience.None of the concentrations or volumes is critical.

The demonstrationWith a suitable patter, pour a glassful of the permanganate solution into each glass.Do not exceed 200 cm3for any glass to ensure that the reactants already in theglasses remain in excess.

The following changes will be seen.

Glass 1: no change.

Glass 2: the permanganate will be decolorised as it is reduced to virtuallycolourless Mn2+ions by the thiosulphate ions. The red winehas turned towateror white wine.

Glass 3: the permanganate will be decolorised as above and a whiteprecipitate of barium sulphate will form as the barium ions react with sulphateions from the acidified permanganate solution. The red winehas turned tomilk.

Glass 4: a white precipitate forms as above, but the colour of thepermanganate remains. The red winehas turned to raspberry milk shake.

Glass 5: the permanganate will be decolorised as above and the acidicsolution will react with the sodium carbonate to cause bubbles. The red winehas turned into fizzy lemonade.

Visual tips It is best to remove the drinksfairly quickly as glasses 2 and 5 may gradually gocloudy due to the formation of colloidal sulphur from excess thiosulphate and acidand the white precipitates in glasses 3 and 4 will settle out, spoiling the illusion.

To avoid this, a saturated solution of sodium metabisulphite (Na2S2O5) can beused instead of the sodium thiosulphate. However, this has the disadvantage that theinitial mixture with barium chloride in glass 3 is cloudy. The cloudiness can beminimised by adding a little dilute hydrochloric acid but this does not remove itentirely and it could be spotted by the audience. This also produces some sulphurdioxide gas which is smelly and may affect asthmatics.

Teaching tips

A good post-16 group could be asked to predict, and write balanced equations for,the reactions used.

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TheoryThe reactions are:

2MnO42(aq) + 16H+(aq) + 10S2O32(aq)2Mn2+(aq) + 5S4O62-(aq) + 8H2O(l)

Ba2+(aq) + SO42-(aq)BaSO4(s)

CO32(aq) + 2H+(aq)H2O(l) + CO2(g)

ExtensionsOther drinkscould be devised based on this, or other, systems. For examplemanipulating the amount of thiosulphate in glass 5 so that the permanganate ends up

in slight excess could givepink champagne

.

SafetyWear eye protection.

A warning that chemicals should only be stored in correctly labelled bottles mightbe appropriate. Do not leave the solutions in the glasses or bottle in case they aremistaken for real drinks.

It is the responsibility of teachers doing this demonstration to carry out anappropriate risk assessment.