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The pH Scale
Acids, Bases, and Neutral Solutions
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The pH scale measures how acidic or basic (alkaline) a solution is. It runs from 0 (most acidic) to 14 (most basic), with 7 representing a neutral solution (pure water at 25°C). pH is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log[H⁺]. Because of the logarithm, each unit change represents a tenfold change in acidity - a pH of 3 is ten times more acidic than pH 4.
Acids donate hydrogen ions (H⁺) to solutions; bases accept them (Brønsted-Lowry definition) or donate hydroxide ions (OH⁾). In water at 25°C, pH + pOH = 14, which means knowing one immediately gives the other. pH affects biology, medicine, food science, and environmental chemistry - the ocean becoming more acidic by 0.1 pH units represents a 26% increase in acidity.
Key Facts
- pH = -log[H⁺]; pOH = -log[OH⁾]; pH + pOH = 14 (at 25°C)
- Acidic: pH < 7; Neutral: pH = 7; Basic: pH > 7
- Each unit = tenfold change in
- Strong acids (HCl, H₂SO₄, HNO₃) fully ionize in water.
- Strong bases (NaOH, KOH) fully dissociate in water.
- Kw = [H⁺][OH⁾] = 1 × 10⁻¹⁴ at 25°C
Vocabulary
- pH
- The negative base-10 logarithm of hydrogen ion concentration: pH = -log[H⁺].
- Acid (Brønsted-Lowry)
- A substance that donates a proton (H⁺) to another substance.
- Base (Brønsted-Lowry)
- A substance that accepts a proton (H⁺) from another substance.
- Buffer
- A solution that resists changes in pH when small amounts of acid or base are added; contains a weak acid and its conjugate base.
- Neutral solution
- A solution where [H⁺] = [OH⁾], giving pH = 7 at 25°C.
Common Mistakes to Avoid
- Thinking pH 0 means no acid. pH 0 means [H⁺] = 1 M - highly concentrated. There's no hard lower bound (battery acid can have negative pH).
- Assuming pH 7 is always neutral. pH 7 is neutral only at 25°C. At higher temperatures, Kw changes and neutral pH shifts below 7.
- Confusing a strong acid with a concentrated acid. 'Strong' refers to degree of ionization; 'concentrated' refers to molarity. Acetic acid is weak even when concentrated.
- Forgetting that pH + pOH = 14 only at 25°C. At other temperatures this sum changes because Kw is temperature-dependent.
Practice Questions
- 1 What is the pH of a solution with [H⁺] = 3.5 × 10⁻⁴ M? Is it acidic or basic?
- 2 A solution has pH = 11.2. Find [H⁺], [OH⁾], and pOH.
- 3 Lemon juice has pH ≈ 2.3 and black coffee has pH ≈ 5. How many times more acidic is lemon juice than coffee?