Question

why does the pH of the acid solution initially increase very slowly when metal is first added to the acid solution, but gradually increase at a faster rate as the reaction proceeds to completion?
I thought it may be because the metal is just then being completely dissolved but that didn't make sense because we were instructed to wait until the first little scoop of the metal powder was dissolved before adding the next bit.

Answer 1

When a metal reacts with an acid, say HCl, it reacts with the H+ atoms of the acid to produce H2 gas and chloride ions. Since pH is the negative log of H+ concentration, then it would sensibly decrease because it was used up. The metal oxide would form a coating over the metal. However, as the reaction proceeds, the acid corrodes the metal and exposes a purer surface removing the coating. Thus increasing the concentration of H+.