What term refers to a chemical that, when added to water, protects the solution from large pH changes when acids or bases are added to them?

• A. Endpoint
• B. Overtitrations
• C. Organic
• D. Buffer

Answer: (D)

A buffer is a substance in water that keeps the solution’s pH from changing a lot when you add acids or bases. It does this by containing a weak acid and its conjugate base (or a weak base and its conjugate acid). When you add acid, the conjugate base part reacts with the added hydrogen ions to form more of the weak acid, which resists a big drop in pH. When you add base, the weak acid donates hydrogen ions to neutralize some of the hydroxide, forming more of the conjugate base and resisting a large rise in pH. The effectiveness depends on having appreciable amounts of both forms, and the pH shifts are governed by the ratio of the two forms (often summarized by pH ≈ pKa when the two forms are in similar concentrations). A common example is acetic acid with acetate. Other terms like endpoint refer to titration signals, not to a substance that stabilizes pH, and overtitration or “organic” describe different concepts entirely.