In the lab, you are testing an unknown solution using Benedict’s test. After you add Benedict’s reagent, the unknown solution appears yellow. As you write up your lab report, how should you explain this result?
This is a negative result, so the unknown must have a free aldehyde or ketone reactive group, and reducing sugar is not present.
This is a positive result, so the unknown must have a free aldehyde or ketone reactive group, and reducing sugar is present.
This is a positive result, so the unknown must have a free aldehyde or ketone reactive group, so a polysaccharide is present.
This is a negative result, so the unknown must have a free aldehyde or ketone reactive group, so a polysaccharide is not present.

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Question restatement: In the lab, you are testing an unknown solution using Benedict’s test. After you add Benedict’s reagent, the unknown solution appears yellow. As you write up your lab report, how should you explain this result?

Option 1: This is a negative result, so the unknown must have a free aldehyde or ketone reactive group, and reducing sugar is not present.
- This is misleading because Benedict’s test is used to detect reducing sugars, which react to form a brick-red precipitate if positive. A yellow color typically indicates a trace or inconclusive result, not a clear negative, and it does not support the claim that there is a free aldehyde or ketone reactive group while reducing sugar is not present. The wording also contradicts the idea that reducing sugar is present in a positive test, so this option confuses interpretation of the test outcome.

Option 2: This is a positive result, so the unknown must have a free aldehyde or ketone reactive group, and reducing sugar is present.
- Benedict’s test changes color in the presence of reducing sugars due to the reduction of copper(II) ions to copper(I) oxide, with color changes ranging from blue to green, yellow, orange, or red as the concentration of reducing sugar increases. A yellow result suggests a low to moderate amount of reducing sugar, indicating a positive test, which aligns with having a free aldehyde or ketone reactive group in the sample capable of reducing the reagent. Therefore, the description that it’s a positive result and that reducing sugar is present is consistent with Benedict’s test chemistry.

Option 3: This is a positive result, so the unknown must have a free aldehyde or ketone reactive group, so a polysaccharide is present.
- The presence of a reducing sugar detected by Benedict’s test does not necessarily imply a polysaccharide is present; polysaccharides are polymers of sugars and may or may not be reducing, depending on their terminal units and hydrolysis state. Benedict’s test detects reducing sugars, not polymers directly, and a positive result indicates free aldehyde or ketone groups capable of reducing the reagent, which can come from simple reducing sugars rather than specifically a polysaccharide. This makes the assertion about polysaccharide presence too strong and not strictly required by the test outcome.

Option 4: This is a negative result, so the unknown must have a free aldehyde or ketone reactive group, so a polysaccharide is not present.
- If the result were truly negative, we would expect no reducing sugars and no reactive aldehyde/ketone groups available to reduce Benedict’s reagent. However, Benedict’s reagent turning yellow indicates some degree of reduction, i.e., a positive result. Claiming a negative result while noting a free aldehyde or ketone reactive group is contradictory, and tying this to the absence of polysaccharide adds unnecessary complexity without matching the observed yellow coloration.

Summary of reasoning: The color change to yellow in Benedict’s test points to a positive result with some amount of reducing sugar present, due to the reagent’s reduction. This aligns with Option 2’s framing, while Options 1 and 4 misinterpret the color outcome as negative, and Option 3 overgeneralizes the presence to polysaccharides rather than reducing sugars and their terminal reactive groups. The correct interpretation centers on a positive Benedict’s test indicating reducing sugar activity, which Option 2 communicates.

BIOL109LV02 HW #4

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