Chemical Demonstrations for Classes and Displays

This page describes some interesting demonstrations that can be used in classrooms or for Chemistry magic shows.

If you have any comments or suggestions regarding the demonstrations, please contact Jeff Hughes

Look at:

The Remarkable Properties of Mercuric Iodide
The Magic Jug
The Magic Solutions
A Spectacular Catalysis

The Remarkable Properties of Mercuric Iodide:

A. Thermochromic Solids [1]

A reversible colour change occurs on heating and cooling salts of the form M2HgI4 ,where M = Ag(I) or Cu(I).

Preparation, M = Ag(I):

  1. Dissolve 3.25 g of Hg(NO3)2 in boiling water.
  2. Add 10% KI solution until the initial precipitate of HgI2 dissolves to give a clear solution.
  3. Add 50 mL of a solution of 3.4 g of AgNO3 and boil for a few minutes.
  4. Filter off the yellow Ag2HgI4 and dry in a desiccator.

Preparation, M = Cu(I):

  1. Prepare a solution of Cu(I) by adding 25 mL of a solution of 2.5 g CuSO4 to a boiling solution of 6 g Na2SO3.7H2O + 5 g NaCl in 50 mL of water. A greenish precipitate forms initially, but redissolves to give a yellow solution.
  2. Dissolve 1.6 g of Hg(NO3)2 in 25 mL of boiling water.
  3. Form the HgI42-ion using 10% KI as in the Ag(I) preparation.
  4. To this solution, add the hot Cu(I) solution. A deep purple solid precipitates, which changes colour to red on filtering.

Demonstration:

The solids change colour (yellow to orange for Ag(I) and red to purple for Cu(I) ) when heated. Smearing some of the solution on a filter paper and heating with a hot plate or hair dryer is a convenient way to demonstrate the colour change. The change is reversible, and can be repeated many times. Both solids are quite stable on storage.

Chemical Principle:

The colour change is due to a subtle change in structure. The beta or low temperature form is tetragonal, with tetrahedral HgI42-units occupying the corners of the unit cell, but the M+ions occupy only 4 out of 6 face-centre positions. In the cubic alpha form (high temperature), all 6 faces, as well as the corners, are occupied at random by M or Hg in the ratio 2:1. In both alpha and beta forms, the positions of the iodides are fixed (cubic close-packed).

Another interesting thermochromic solid is (C2H5NH2)2CuCl4 [2]. This is a salt of CuCl42-, which is green at room temperature but becomes orange when heated. As for the reactions above, this change is also reversible. The green form is square planar, whilst in the orange form the CuCl42-ion forms a distorted tetrahedron.

B. Mercury Tornado [3]

A colourless solution is rapidly stirred on a mechanical stirrer. Addition of a drop of colourless liquid causes an orange 'tornado' to appear and gradually dissipate.

Chemicals Required:

  • Solution A : 2 g Hg(NO3)2and 100 mL of 0.1M HNO3
  • Solution B : 16 g KI and 100 mL of water

Procedure:

  1. Place a large beaker with about 700 mL of water on a mechanical stirrer.
  2. Add 7 mL of A and mix.
  3. Add B dropwise. An orange precipitate forms and, after a few mL, the precipitate persists.
  4. Continue adding B until the solution becomes clear again, then add an extra mL of B.
  5. If the solution is stirred rapidly to create a vortex, adding a drop or two of solution A to the vortex will cause an orange 'tornado' to appear and gradually dissipate. If, after several additions of A, the solution turns orange, adding 1 mL of B will restore the original clear solution.

Chemical Principle:

The initial orange precipitate is the kinetically favoured product, HgI2, which is orange and insoluble. This eventually gives way to the thermodynamically favoured HgI42-ion, which is colourless and soluble.

Hg2++ 2I-=> HgI2 + 2I-=> HgI42-

C. "Old Nassau" Reaction [4-6]

When three colourless solutions are mixed, the mixture suddenly turns orange after a few seconds. After a few more seconds, the solution suddenly turns black. By varying proportions of the solutions, other sequences can be produced.

Chemicals Required:

  • Solution A : 15 g/L KIO3
  • Solution B : 15 g/L NaHSO3 and a few mL of starch solution
  • Solution C : 3 g/L HgCl2

Procedure:

  1. Mix equal volumes of A and B. Solution turns black. Dilution delays colour development.
  2. Mix equal volumes of C + B + A (in that order ). Solution turns orange, then black.
  3. Mix C + A + 2B. Colour sequence is clear,orange, clear.

Chemical Principle:

The first reaction is the Iodine Clock or Landolt reaction (steps 1 to 3 in the scheme below). In the second reaction, the iodide being slowly formed in step 1 reacts quickly to form the insoluble orange HgI2 (step 4). Only when all the Hg2+is consumed does I2 form via step 2 and, hence, the deep blue -black iodine-starch colour forms and masks the orange colour. In the third reaction, enough HSO32-is added to reduce all the I2 formed (via step 3).

Mechanism:

1

IO3-+ 3HSO3-=> I-+ 3SO42-+ 3H+

2

5I-+ IO3-+ 6H+=> 3I2 + 3H2O

3

I2 + HSO3- => 2I-+ SO42-+ 3H+

4

Hg2++ 2I-=> HgI2 (orange)

5

I2 + starch => blue -black complex

References

  1. W.G. Palmer, "Experimental Inorganic Chemistry", p.192
  2. M.J.M. van Oort, J. Chem. Educ., 65 (1988) 84
  3. W.T. Lippincott, "Source Book for Chemistry Teachers", p.7
  4. H.N. Alyea, J. Chem. Educ., 54 (1977) 167
  5. A. Moss, J. Chem. Educ., 55 (1978) 244
  6. J.L. Lambert, G.T. Fine, J. Chem. Educ., 61 (1984) 1037

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The Magic Jug

Brief Description (What the Audience Sees):

A colourless solution is poured from a jug into five empty glasses, giving red, purple, brown, blue and colourless solutions. The glasses are tipped into an empty beaker and the whole mixture is colourless again.

Chemicals Required:

  • Stock solution (2.5 g NaOH, 0.5 g KI and 0.5 g KIO3 dissolved in 50 mL H2O)
  • Dropping bottles:
    1. 1% phenolphthalein
    2. 1% thymolphthalein
    3. 25% H2SO4
    4. 25% H2SO4 + starch indicator
    5. 20% Na2SO3

Equipment Required:

  • 5 x glasses or beakers (200 mL)
  • opaque jug
  • 600 mL beaker

Procedure:

  1. Take 5 mL of stock solution and mix with 600 mL of water in the jug.
  2. Place 8 drops of solution from bottle 1 into glass 1 and similarly for the other 4 bottles.
  3. Pour the stock solution from the jug into each glass to get the colours described above.
  4. Tip the glasses into the beaker in the order 5-4-3-2-1 and the solution turns colourless.

Chemical Principle:

On addition of alkaline solution from the jug into glasses 1 and 2, the acid/base indicators change colour from colourless in neutral solution to the colours observed in alkaline solution.

In glasses 3 and 4, the excess acid neutralises the alkalinity of the solution from the jug and, in acid solution, the following reaction occurs :

6H++ I-+ IO3- => I2 + 3H2O

The starch in glass 4 reacts with the iodine to give a deep blue colour.

The sodium sulfite in glass 5 gives no colour and, when the solutions are recombined, reduces the iodine to iodide :

H2O + SO32-+ I2 => 2I-+ SO42-+ 2H+

The acid in glasses 3 and 4 neutralises the alkaline solutions in glasses 1 and 2 as they are poured into the beaker. Hence, all solutions give rise to a colourless solution as they are poured into the large beaker.

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The Magic Solutions

Brief Description (What the Audience Sees):

A colourless solution is poured into 6 beakers in turn until each beaker is about half full. The solution in all six beakers remains colourless. However, when the pouring is resumed and the beakers are filled they turn the colours of the rainbow.

Chemicals Required:

Six indicator solutions:

  • Solution 1 : 150 mg phenolphthalein + 300 mg m-nitrophenol
  • Solution 2 : 45 mg phenolphthalein + 600 mg m-nitrophenol
  • Solution 3 : 600 mg m-nitrophenol
  • Solution 4 : 600 mg thymolphthalein + 600 mg m-nitrophenol
  • Solution 5 : 150 mg thymolphthalein
  • Solution 6 : 90 mg phenolphthalein + 40 mg thymolphthalein

Each mixture is dissolved in 5 mL of 95% ethanol.

  • 1 L of 0.012M NaOH
  • 25 mL 95% ethanol
  • 25 mL of 0.4M H2SO4

Equipment Required:

  • 1 x 600 mL beaker or jug
  • 6 x 200 mL beakers
  • 10 mL measuring cylinder

Procedure:

  1. Add 5 mL of the acid solution and a few drops of indicators 1-6 to beakers 1-6.
  2. Half fill each beaker with the NaOH solution. Each solution should remain colourless.
  3. The colours should appear when the beakers are filled. The colours, in order, are red, orange, yellow, green, blue and violet.

Chemical Principles:

This demonstration uses acid-base indicators which are colourless in their acid forms. The concentrations of acid and base are such that the base form of the indicator is not seen until nearly all of the base has been added.

Reference:

  • Journal of Chemical Education, 61 (1984) 172

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A Spectacular Catalysis

Brief Description (What the Audience Sees):

The oxidation of tartaric acid by hydrogen peroxide is slow in the absence of a catalyst. When the catalyst is added, the reaction becomes very vigorous and the green intermediate complex can be observed. When the reaction is complete, the original pink colour of the catalyst is restored.

Chemicals Required:

  • Solution A : 25g/300mL of potassium sodium tartrate
  • Solution B : 20 mL of 30% peroxide in 100mL water

(Caution! 30% peroxide is corrosive and causes painful burns!)

Equipment Required:

  • Hotplate-stirrer
  • 2 x 400 mL beakers

Procedure:

  1. Heat both solutions to 60-70ºC.
  2. Combine them and divide the solution into two beakers. The reaction (judged by the evolution of CO2) seems to be slow.
  3. Add about 1g of CoCl2 to one of the beakers. The solution turns green and evolution of CO2 is very rapid. When the reaction is complete, the original pink colour of CoCl2 returns.

Chemical Principles:

Tartrate is oxidised by H2O2 to give CO2. Co2+complexes with the tartrate ion giving a green complex. This complex activates the tartrate ion towards oxidation.

Reference:

  • Journal of Chemical Education, 55 (1978) 652

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