Acids and bases are everywhere: lemon juice, stomach acid, soap and baking soda. Chemistry classes test them in three ways: definitions, the pH scale and calculations. This guide covers all three with worked examples.
What makes an acid or a base
The most common definition in high school is the Brønsted-Lowry model: an acid donates a proton (an H⁺ ion) and a base accepts a proton. When hydrochloric acid dissolves in water, it gives H⁺ to water, forming hydronium (H₃O⁺) and chloride: HCl + H₂O → H₃O⁺ + Cl⁻.
The older Arrhenius model says acids produce H⁺ in water and bases produce OH⁻. It works for common cases like HCl and NaOH but cannot explain bases like ammonia, which has no OH⁻ of its own. Ammonia accepts a proton from water: NH₃ + H₂O → NH₄⁺ + OH⁻.
When an acid donates a proton, what remains is its conjugate base; when a base accepts one, it becomes its conjugate acid. In the ammonia example, NH₄⁺ is the conjugate acid of NH₃ and OH⁻ is the conjugate base of water.
Strong vs. weak
A strong acid ionizes completely in water; almost every molecule gives up its proton. The common strong acids are HCl, HBr, HI, HNO₃, H₂SO₄ and HClO₄. A weak acid, such as acetic acid in vinegar, ionizes only partly, so most molecules stay intact. "Strong" describes how completely it ionizes, not how concentrated it is: a dilute strong acid can be less harmful than a concentrated weak one.
Strong bases include the group 1 hydroxides (NaOH, KOH) and some group 2 hydroxides like Ca(OH)₂. Ammonia is a common weak base.
The pH scale
pH measures the concentration of hydrogen ions: pH = −log[H⁺], where [H⁺] is in moles per liter. At 25 °C:
- pH below 7: acidic
- pH 7: neutral (pure water)
- pH above 7: basic (alkaline)
Because the scale is logarithmic, each step of 1 is a factor of 10. A solution at pH 3 has ten times more H⁺ than one at pH 4 and a hundred times more than one at pH 5.
The related quantity pOH measures hydroxide: pOH = −log[OH⁻]. In water at 25 °C, pH + pOH = 14, because [H⁺][OH⁻] = 1.0 × 10⁻¹⁴.
Worked example 1: pH of a strong acid
What is the pH of 0.010 M HCl?
- HCl is strong, so [H⁺] = 0.010 M = 1.0 × 10⁻² M.
- pH = −log(1.0 × 10⁻²) = 2.0.
Worked example 2: pH of a strong base
What is the pH of 0.0010 M NaOH?
- NaOH is strong, so [OH⁻] = 1.0 × 10⁻³ M and pOH = 3.0.
- pH = 14 − 3.0 = 11.0.
Neutralization
When an acid and a base react, they form water and a salt: HCl + NaOH → NaCl + H₂O. The net reaction is simply H⁺ + OH⁻ → H₂O. In a titration, you add a base of known concentration to an acid (or the reverse) until it is exactly neutralized, often shown by an indicator changing color. Using M₁V₁ = M₂V₂ for a one-to-one reaction, you can find the unknown concentration. For example, if 25.0 mL of HCl is neutralized by 20.0 mL of 0.100 M NaOH, the acid's concentration is (0.100 × 20.0) / 25.0 = 0.080 M.
Common mistakes
- Thinking pH 4 is "a little" more acidic than pH 5. It is ten times more acidic.
- Confusing strong with concentrated.
- Forgetting to convert pOH to pH for bases.
- Using M₁V₁ = M₂V₂ for acids that release two protons, like H₂SO₄, without accounting for the 2:1 ratio.
Practice questions
- What is the pH of 1.0 × 10⁻⁴ M HNO₃?
- A solution has pH 9. Is it acidic or basic, and what is its pOH?
- How many times more acidic is pH 2 than pH 5?
- Identify the conjugate base of HF.
Answers: (1) 4.0. (2) Basic; pOH = 5. (3) 1,000 times (three steps of 10). (4) F⁻.
Last reviewed 2026-10-23 by the BrainBoost team.