I did this science experiment in class where we tested substances for sugars and starch. We had twelve test tubes, divided into six and six. Both halves were labeled with numbers 1 through 6. All...

I did this science experiment in class where we tested substances for sugars and starch. We had twelve test tubes, divided into six and six. Both halves were labeled with numbers 1 through 6. All test tubes had 20 drops of distilled water. The corresponding numbers had the same substance. For example, #1 had 15 drops of sugar solution each, #2 had 1/2 scoop of potato buds each, #3 had 1/2 a ripped up marshmallow each, #4 had 1/2 a scoop of powdered eggs each, #5 had 1/2 a scoop of corn starch, and #6 only had the original 20 drops of distilled water. 

We put 3 drops of Lugol's solution each into the first 6 test tubes to test for starch. We put 5 drops of Benedict's solution each into the remaining 6 test tubes each to test for sugar. We put the test tubes with the Benedict's solution into a large beaker with 250 mL of water and we put it on a hot plate to heat it. We were watching for the colors of the test tubes. With the Lugol's, purple or black meant there was starch. With Benedict's, any color other than blue meant there was sugar.

I was wondering what the control in this experiment is? Is it the amount of distilled water, the solution that was put into the test tube(Lugol/Benedict), or something else?

Asked on by rg6150

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lynn30k | High School Teacher | (Level 1) Educator

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Your two test tubes that are labeled "6" are your controls. They are treated the same way as those labeled 1-5 as far as water and reagents (Benedict's or Lugol's); the only difference is the food you are testing for biomolecules. Ideally, you would add extra distilled water to your controls, to bring them up to the same volume as the tubes your are testing. You want to have everything be as much the same in all tubes in order to make valid comparisons. In order to see if the food makes a difference in the resulting color, you have to have something to compare it to. Another way of saying this is that you have to make sure the Benedict's or Lugol's doesn't simply color the water--you are looking for results of a chemical reaction. The color changes indicate the presence of sugars and starches, and the color of the water won't change, except perhaps for a bit of the tint of the diluted reagent.

There are other experiments commonly done in high school biology that test for proteins and lipids. One very easy test for lipids (fats) is to rub some food on a brown paper bag. If there is fat in the food, it will make the paper bag translucent (semi-transparent). 

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