Question

1. what is used to test for the presence of starch?
2. how can you tell by using this test that a substance contains starch?
3. what is used to test for the presence of simple reducing sugars such as monosaccharides?
4. how can you tell by using this test that a substance contains a simple reducing sugar?
5. why was water tested for each chemical?
6. what is used to test for the presence of protein?
7. how can you tell by using this test that a substance contains protein?
8. biuret reagent will turn the skin brownish - purple. explain why this occurs.
9. a. when greasy food is spilled on clothing, why is it difficult to clean with water alone?

b. what would be better than plain water for removing a greasy food stain? why?

Explanation:

1. Iodine solution (iodine - potassium iodide solution) is used to test for starch. This is a common chemical test in biology and chemistry.
2. When iodine solution is added to a substance containing starch, the solution changes color from brown (the color of iodine solution) to a deep blue - black. This color change is due to the formation of a starch - iodine complex.
3. Benedict's reagent is used to test for simple reducing sugars such as monosaccharides.
4. When Benedict's reagent is heated with a substance containing a simple reducing sugar, a color change occurs. The color change can range from green (low concentration of reducing sugar) to yellow, orange, and finally brick - red (high concentration of reducing sugar). This is because the reducing sugar donates electrons to the copper ions in Benedict's reagent, reducing them from \(Cu^{2 + }\) (blue) to \(Cu^{+}\) (which forms insoluble copper(I) oxide).
5. Water was tested for each chemical as a control. A control is a standard against which experimental results can be compared. In this case, testing water shows what the "negative" result (no reaction) looks like for each test. For example, if water (which should not contain starch, reducing sugar, or protein) is tested with iodine, Benedict's reagent, or biuret reagent, there should be no color change (or only the color of the reagent itself).
6. Biuret reagent is used to test for the presence of protein.
7. When biuret reagent is added to a substance containing protein, the solution changes color from blue (the color of biuret reagent) to a violet or purple color. This is because the peptide bonds in proteins react with the copper ions in biuret reagent (\(CuSO_4\) in a basic solution (\(NaOH\))).
8. Biuret reagent turns the skin brownish - purple because the skin contains proteins. The peptide bonds in the proteins in the skin react with the copper ions in the biuret reagent, similar to the reaction with proteins in a test tube.
9. a. Greasy food (which contains lipids) is difficult to clean with water alone because lipids are non - polar and water is polar. According to the principle of "like dissolves like", non - polar substances (lipids) do not dissolve well in polar solvents (water).

b. Soap or detergent would be better than plain water for removing a greasy food stain. Soap and detergent molecules have both a non - polar (hydrophobic) part and a polar (hydrophilic) part. The non - polar part can interact with the non - polar lipid (grease), and the polar part can interact with water. This forms micelles, which can remove the grease from the clothing and suspend it in water for rinsing away.

Answer:

1. Iodine solution (iodine - potassium iodide solution).
2. The solution changes from brown (iodine solution color) to deep blue - black.
3. Benedict's reagent.
4. When heated, a color change from green (low reducing sugar) to yellow, orange, or brick - red (high reducing sugar) occurs.
5. As a control to show the "negative" result (no reaction) for each test.
6. Biuret reagent.
7. The solution changes from blue (biuret reagent color) to violet or purple.
8. Because the skin contains proteins, and the peptide bonds in skin proteins react with the copper ions in biuret reagent.
9. a. Because lipids (in greasy food) are non - polar and water is polar (like dissolves like principle).

b. Soap or detergent. They have both non - polar (to interact with grease) and polar (to interact with water) parts, forming micelles to remove grease.