Question

Dr. Sanchez tests the pH of the frog pond in her backyard. The pond has a pH of 9.5. The frogs are healthiest at pH of 7. What type of substance should Dr. Sanchez add to the pond to decrease the pH? Why?

Answer 1

pH of 9,5 is really appropriate to alkali solution.

As you can see pH of 7 is neutral, but Dr.Sanchez have alkali water in her frog pond. Alkali means that there are some extra OH- ions in water, and only one way to neutralize these ions is adding some $\mathrm{H}^+$ ions and it is adding of acid. Why?

When solution is neutral it means that concentration of $\mathrm{H^{+}}$ and OH- is in equilibrium, as we said alkali means that there are some extra OH- ions. The reaction of neutralization is:

$$\mathrm {H} ^ {+} + \mathrm {O H} ^ {-} \rightleftharpoons \mathrm {H} _ {2} \mathrm {O}$$

So any acidic compound is acceptable for decreasing pH if only it is not poison for frogs of course.