Final answer:
The chemical substance and apparatus used to investigate the nature of pH of a sample of an onion solution is pH paper and pipette.
Step-by-step explanation:
The chemical substance and apparatus used to investigate the nature of pH of a sample of an onion solution is pH paper and pipette. pH paper is impregnated with acid-base indicators that change color at different pH values. By dipping the pH paper into the solution and comparing the color to the standards provided, the approximate pH of the solution can be determined.