Which of the following sentence best describes a buffer? a. a substance that resists small change in the acidity of a solution when an acid or base is added to the solution.
b. a chemical substance that prevents changes in hydrogen ion concentration.

In a solution, a buffer is a substance that releases and catches Hydrogen ions keeping the same pH. A buffer is resistible on pH change by taking Hydrogen ions whenever acids are being added to a solution. Whenever bases are added, it donates hydrogen ions. Therefore, the answer to this question is letter A.

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