Which of the following pH values indicates that a solution is a strong base?

Step 1: Analyze the question We need the pH value that shows a strong base.

Step 2: The concept The pH scale runs from 0 to 14 at 25°C:

• pH < 7 means acidic (closer to 0 means a stronger acid)
• pH = 7 means neutral
• pH > 7 means basic (closer to 14 means a stronger base)

Also, pH + pOH = 14, so a high pH means a high concentration of OH⁻ ions.

Step 3: Evaluate each option

• A. pH 1: strongly acidic (high H⁺ concentration).
• B. pH 5: weakly acidic.
• C. pH 9: weakly basic.
• D. pH 13: strongly basic. Here pOH = 14 − 13 = 1, so [OH⁻] = 10⁻¹ = 0.1 mol/dm³, which is typical of a fully dissociated strong base such as 0.1 M NaOH.

Step 4: Answer The pH closest to 14 is 13, so the answer is D.

Tip: Think of the pH scale as a line from 0 to 14. Near 0 is a strong acid, near 14 is a strong base, and 7 is in the middle. For the strength of an acid or base, check how far the pH is from 7 and in which direction.