When acid and base are added dropwise separately to a solution, its pH changes each time. What does this show?

Correct answer: pH depends on the balance of hydrogen and hydroxide ions

The governing concept is that pH reflects the relative concentration of hydrogen ions and hydroxide ions in an aqueous solution. Adding an acid generally increases hydrogen-ion concentration, so pH decreases. Adding a base increases hydroxide-ion concentration and usually lowers the relative hydrogen-ion concentration, so pH increases. Colour and the container do not determine pH, and pH can change readily. Therefore option A is correct.