Black and White Reaction
Science Concept: Four solutions are mixed together so that colors are formed and then become colorless. Several chemical reactions take place.
Materials: Stock solution of diluted bleach (1 tbsp (15 mL) of household liquid bleach in 3 tbsp (45 mL) of water.
-1000 mg of Vitamin C supplement tablet
-2 tsp (10 mL) of tincture of iodine (However this solution of iodine did not work and I ended up using a solution of iodine from the lab (concentration unknown) which worked fine.)
-6 tbsp white vinegar (90 mL)
-1/2 tsp (2.5 mL) liquid laundry starch solution
-1/2 tsp Epsom salt
-7 tbsp (105 mL) of clear household ammonia
Crush 1000 mg of Vitamin C supplement tablets, stir in 2 tbsp (30 mL) of water, and stir until dissolved.
Add 2 tsp of tincture of iodine to 6 tbsp of white vinegar. Add a few small drops of the vitamin C solution to this. This will make Solution A colorless. Keep the remaining vitamin C solution because you will need it for Solution C. Then add 1/2 tsp, (2.5 mL) of starch solution to this and stir.
Add 1 tsp (5 mL) of the diluted bleach solution to 6 tbsp (90 mL) of water.
Dissolve 1/2 tsp of Epsom salt in 5 tbsp (75 mL) of water, and then add the entire remaining quantity of the vitamin C stock solution.
Use 7 tbsp (105 mL) of clear household ammonia.
Pour Solution B into Solution A while stirring. A black mixture is formed immediately. Then pour Solution D into Solution C. A white/beige mixture is formed in about 5 seconds. To complete the experiment, pour the black and white mixtures simultaneously into a larger container. The mixture will become clear and colorless, apparently identical to all the starting solutions.
Introduction: "What color are all these solutions that I have labeled A, B, C and D? Really boring because they are all colorless. I really want some color or something, I wonder what will happen when I mix some of these together?"
Explanation: When Solution A and B are combined, iodide ion is oxidized by hypochlorite ion in acid solution to make the elemental iodine. The iodine combines with the starch present to yield the deep blue or black starch-iodine complex, resulting in a black mixture. Starch is a long polymer molecule in one shape of a coil - like the spring on a spiral notebook. The iodine molecules goes inside of the coil and this is what makes it turn blue/black. Iodine is a diatomic element with the formula, I2.
When Solution C and D are combined, magnesium ion (Epsom salt) is precipitated as white magnesium hydroxide by aqueous ammonia.
When Solution (A+B) is combined with Solution (C+D) the iodine present in the black mixture is reduced to iodide ion by the excess of ascorbic acid that was originally present in Solution C. The vitamin C chemical gives an electron to each of the iodine atoms to form the iodide ion with a negative charge. This then shows that the iodide ion is colorless in water solution even in the presence of the starch. This reaction changes the form of the iodine element to the iodide ions.
Simultaneously, the magnesium hydroxide precipitate present in the white mixture is dissolved by the excess of acetic acid originally present in Solution A.
A + B:
I` + OCl` --> I2 + Starch (Blue/Black)
C + D:
MgSO4 + NH4OH --> Mg(OH)2¯ + (NH4)2SO4 (White Precipitate)
White + Black:
I2 + Vitamin. C --> I` Colorless
Mg(OH)2 + Acid (Vinegar) --> Mg+ 2 ions + H2O (Colorless)
Safety: Use normal precautions.
Disposal: Everything is fine to be poured down the drain.
Reference: Wright, Stephen W. Chemical Demonstrations with Consumer Chemicals: The Black and White Reaction J. Chem. Educ. 2002 79 44. (January 2002)