April Altott

Disappearing Blue, Reappearing Blue

Science Concept: A chemical reaction causes color to disappear and then reappear again.

Materials:
two 600 mL beakers filled with 200 mL of water

20 mL of liquid starch solution

1 dropper full of iodine/KI solution

1 spatula scoop of sodium carbonate

stirring rod

Pre-Demo Preparation: Before the demonstration, fill both beakers with 200 mL of water and pre-measure the liquid starch solution. Also, the demonstration calls for the sodium carbonate to be dissolved well in the water before the demonstration can begin, so it would be beneficial to dissolve the sodium carbonate in the water before beginning the demo.

Directions: Add 200 mL of water to each beaker. In the first beaker, add 20 mL of starch solution and a dropper full of iodine/KI solution. A deep blue/black color should form. In the second beaker, add a spatula scoop of sodium carbonate, and use a stirring rod to stir the powder into the water until it is completely dissolved. Then pour the first beaker containing the starch and iodine solutions into the second beaker. Pour slowly and watch as the blue/black color turns colorless. Stop pouring to show the colorless solution, and then continue pouring until the color changes back to blue.

Introduction & Commentary: Does anyone like magic tricks? I'm going to show you a trick. Watch as I change the colors of this liquid right before your eyes.

Explanation: The iodine and the starch added to water form a deep blue color, which is the color of the iodine in its element form. 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 this solution is added to the water and sodium carbonate, the iodine solution is "neutralized" by the carbonate, which is a base. A chemical reaction takes place in which the iodine gains an electron and changes from I2 element to iodide ions, I- . Due to this change, the iodine changes to an iodide ion which is colorless. Because there is only a small amount of sodium carbonate in the water, however, it is quickly used up and the excess I2 overpowers the carbonate and returns the solution back to its blue color.

I2 --------> 2 I-

(blue) -------------(colorless)

 

Safety Precautions: Caution should be used when adding iodine to the water. If it contacts skin or clothing, it will stain.

Waste Disposal: All contents of the beakers can be emptied down the sink.

Reference: Science Demonstrations, January 1999