How will a solution change if a base is added? The hydroxide ion concentration will decrease and lower the pH.
The hydroxide ion concentration will decrease and raise the pH.
The hydrogen ion concentration will increase and lower the pH.
The hydroxide ion concentration will increase and raise the pH.

The hydroxide ion concentration will decrease and lower the pH

Explanation:

If you add a base to a buffer solution, the hydrogen ion concentration decreases by less than the amount expected for the quantity of base added. The acid and its conjugate base consume the hydroxide ions. The pH of the solution does not rise significantly, which it would do if the buffer system was not in use.

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