Test tube A and B are filled with water. C and D, with starch. A and C are used to test for iodine. What is the use of D and B?This is for my biology lab assignment.

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keithi eNotes educator| Certified Educator

I'm a little confused by your question but I do a diffusion demonstration using these materials in my class. Iodine is used to indicate whether or not a carbohydrate, such as starch, is present in a substance. First, I want to show my class what happens when you mix a cornstarch solution with an iodine solution. I will first mix up a solution of cornstarch, which is a carbohydrate, and water in a small beaker. I mix up a solution containing 3 parts water to one part iodine in another beaker. I pour about 10ml of the cornstarch solution into a medium sized test tube. Using a pipette (dropper), I'll put in 2 or 3 drops of the iodine solution into the test tube and the solution immediately turns to a darkĀ purple. You can do another demonstration by dropping several drops of the iodine solution onto a slice of potato and you will get the same reaction because of the starch found in the potato. After showing the reaction of what happens when you mix a cornstarch solution with an iodine solution, I set up my diffusion demonstration. I will pour about 20ml of the iodine solution into a 50ml beaker. I will pour about 20ml of the cornstarch solution into a cheap plastic bag (don't use Glad brand bags!) I will place the bag with the cornstarch solution into the beaker with the iodine solution. In about 10 minutes you will see the cornstarch solution in the bag start to turn purple because the iodine molecules are diffusing through the microscopic pores in the bag and combining with the cornstarch molecules. It's a pretty cool demonstration if you get the right plastic bags! I hope this helps!