Question

what is the purpose of testing distilled water in tube 3?

to make sure there is a positive control.
to eliminate the possibility that residue in the graduated cylinder causes a positive result.
to eliminate the possibility that the water portion of the potato and onion solutions causes a positive result.

Explanation:

Identify the role of distilled water in biochemical assays

Distilled water contains no active organic solutes (such as reducing sugars, starch, or proteins) and is used as a negative control to show what a negative test result looks like.

Evaluate the purpose of the negative control

The potato and onion solutions are prepared using water as the solvent. Testing distilled water alone ensures that water itself does not react with the reagent to produce a false positive, confirming that any positive result is due to the solutes in the plant extracts rather than the water solvent.

Answer:

• To make sure there is a positive control.
• To eliminate the possibility that residue in the graduated cylinder causes a positive result.
• To eliminate the possibility that the water portion of the potato and onion solutions causes a positive result. (Correct answer)