What is the pH of distilled water?

• A. 0
• B. 7.0
• C. 14
• D. 6.0

Answer: (B)

pH tells us how acidic or basic a solution is by looking at the hydrogen ion concentration. In pure water at room temperature, water molecules self-ionize into equal amounts of H+ and OH-. When these are equal, the solution is neutral, and the pH is 7 because pH = -log[H+] and [H+] = 1×10^-7 M gives pH = 7. Distilled water is essentially pure water, so its pH is 7 under standard conditions. In practice, factors like CO2 from the air dissolving in the water can push the pH slightly below 7, and temperature changes can shift it a bit, but the typical classroom answer for distilled water is 7.0. The other values imply far more H+ (pH 0) or far less H+ (pH 14) than neutral water, or a slight acidity (pH 6.0) that doesn’t match pure distilled water at standard conditions.