One classic experiment that showcases two chemical reactions taking place is the iodine clock reaction, discovered by Hans Heinrich Landolt in 1886. He took two colorless solutions and mixed them together. After a little time had passed, the solution suddenly turned dark blue! When he repeated the experiment, he found that the solution turned blue after the same amount of time had passed. Clock reactions (or oscillating reactions) are labeled as such because there is a sudden property change after a predictable amount of time.
Recreate the Experiment at Home!
What You’ll Need:
- Distilled (or tap) water
- A few disposable cups
- One 1000 mg vitamin C tablet
- Tincture of Iodine (2%)
- Hydrogen peroxide (3%)
- Liquid laundry starch
First, prepare three solutions: a vitamin C stock, an iodine solution, and a hydrogen peroxide solution. For the vitamin C stock, crush the tablet and dissolve it in 2 oz (59 mL) of water.
The next solution will be one of the reactants. Mix 1 tsp (5 mL) of the vitamin C stock with 1 tsp of iodine and 2 oz of water. Label this as “Solution A”.
To prepare the second reactant, add 2 oz of water to 1 tbsp (15 mL) of hydrogen peroxide and 1/2 tsp (2.5 mL) of liquid starch solution. This will be “Solution B”.
Now you’re ready to begin the reaction! Carefully pour Solution A into Solution B. Then pour the entire solution into the now empty cup. Continue transferring the solution back and forth until you see the color change!
What is Happening?
Because the laundry starch contains sulfuric acid, we created the first reacting solution: hydrogen peroxide with sulfuric acid. Then we added it to a solution containing potassium iodide, sodium thiosulfate, and starch. When these are combined, it creates both the elemental and ion forms of iodine.
You can repeat this experiment as many times as you’d like! You could even try timing the reaction to see if the blue color appears at the exact same time in each trial.
“Iodine Clock Reaction”. Imagination Station, www.imaginationstationtoledo.org.
“Iodine Clock”. Released into the public domain under the Creative Commons Attribution-Share Alike 3.0 Unported license. Uploaded on 2/12/18 from commons.wikimedia.org
Hodan, George. “Laboratory Glassware”. Released into the public domain. Uploaded on 2/13/18 from publicdomainpictures.net