Question

what is the purpose of testing distilled water in tube 3?
to eliminate the possibility that the water portion of the potato and onion solutions causes a positive result.
to make sure there is a positive control.
to eliminate the possibility that residue in the graduated cylinder causes a positive result.

Explanation:

Distilled water is used as a negative - control in experiments to rule out the influence of the solvent (water in this case) on the experimental results. In the context of testing potato and onion solutions, testing distilled water can eliminate the possibility that the water part of these solutions causes a positive result. Positive controls are substances known to give a positive result, and this is not about positive control. Also, it has nothing to do with graduated - cylinder residue.

Answer:

To eliminate the possibility that the water portion of the potato and onion solutions causes a positive result.