Question

in each trial, you will add an indicator to the mystery food sample. but you will also add the indicator to a known substance and water. why is that important? check all reasons.
to practice measuring volume using the graduated pipette correctly
to match the mystery food sample to a definite positive result (gelatin, oil, etc.)
to match the mystery food sample to a definite negative result (water)
to increase the amount of data.

Explanation:

• For option B: Adding the indicator to a known positive substance (e.g., gelatin) helps establish what a positive result looks like, so we can compare the mystery sample’s result to this positive control.
• For option C: Adding the indicator to water (a negative control, as it lacks the target substance) shows what a negative result looks like, allowing comparison with the mystery sample.
• Option A is incorrect because the goal isn’t practicing pipette use (the focus is on identifying the mystery sample, not pipette technique).
• Option D is incorrect because the purpose isn’t just to “increase data” — the controls (known substance, water) are used for comparison, not just to add more trials.

Answer:

B. to match the mystery food sample to a definite positive result (gelatin, oil, etc.)
C. to match the mystery food sample to a definite negative result (water)