Question

There are two test, one where we indicate if the solutions have starch and another where we indicate it it has sugar. For the sugar and starch experiments, what were the positive and negative controls in each of the two tests. Please explain your answers.

Here is some background:

First test with sugar: We had 4 test tubes- milk, distilled water, albumin and glucose. We add 40 drops of Benedict’s reagent to each of the 4 test tubes and place in boiling water for 10 minutes. The water and glucose was clear, albumin was a white hazey color and milk was white. The final colors were glucose was Dark red/orange, water was light blue, albumin was dark blue and milk was burnt orange. Reduced sugar was present in milk and glucose.

Second test with starch: We had 4 test tubes- milk, distilled water, albumin, starch and a potatoe. At the beginning, H2O was clear, albumin clear with white haze, milk was white, starch was clear with haze, and a raw potatoe skin was brown white tanish inside color. We put 1 drop of IKI on all samples. H2O turned light brown/orange, albumin was same, milk was a dirty white color, starch and raw potatoe was black. The starch and raw potatoe had starch present.

Answer 1

A positive control is the the sample which acts as the standard that is we are definitely sure that the testing compound is present in it. A negative control is the sample in which we are sure that the testing compound is not present.

So for the first experiment.

Positive control is glucose because it is pretty sure the sugar is present in it. Glucose is the sugar.

Negative control is clear distilled water. Distilled water as such does not contain any sugars.

For the second experiment

Positive control is starch

Negative control is distilled water and explanation is same as said earlier.