Question 5
What happens if a drop of sulphuric acid is added to a hydrogen peroxide solution?
The right answer is the one marked above.
It lowers the rate of reaction — it is an inhibitor, or retarder. That is, the hydrogen peroxide decomposes more slowly.
The rate of the same substance can be steered either way: manganese dioxide speeds it up, a drop of sulphuric acid slows it down. The reaction does not stop altogether.