Why does the pH change stop after diluting repeatedly diluting hydrochloric acid with water?

Question

Hydrochloric acid was repeatedly diluted with water until the pH stopped changing.

Answer 1

pH is the negative log of [H+] and [H+] for pure water is 10-7 M so pH is 7. When you dilute HCl, the contribution of H+ from the HCl becomes insignificant compared to that from the water, and so the pH approaches 7. It won’t actually reach 7, but the change in pH upon continual dilution is too small to detect.