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Chemistry: Atoms to AP Chemistry · Module 7 of 15

Acids, Bases & pH

What makes something an acid or a base, how pH measures it, and what happens when they react.

In this module
  1. Properties of Acids & Bases
  2. The pH Scale
  3. Neutralization Reactions
  4. Titration Basics
  5. Check yourself

Properties of Acids & Bases

  • Acids taste sour, turn blue litmus paper red, and release H⁺ ions in water; bases taste bitter, feel slippery, turn red litmus paper blue, and release OH⁻ ions (or accept H⁺) in water.
  • Common examples: acids include vinegar (acetic acid) and stomach acid (HCl); bases include soap and ammonia.
Worked example

A solution turns red litmus paper blue. Is it likely an acid or a base?

Solution. A base — bases turn red litmus paper blue, while acids turn blue litmus paper red.

The pH Scale

  • pH ranges from 0–14: pH below 7 is acidic, pH = 7 is neutral, and pH above 7 is basic.
  • The pH scale is logarithmic — each whole-number change represents a 10-fold change in H⁺ concentration.
Worked example

How many times more acidic (higher H⁺ concentration) is a solution with pH 3 compared to one with pH 5?

Solution. The difference is 2 pH units, so it is 10² = 100 times more acidic (100 times higher H⁺ concentration).

Neutralization Reactions

  • An acid and a base react to form a salt and water: acid + base → salt + water.
  • At the exact point of neutralization, moles of H⁺ equal moles of OH⁻.
Worked example

Write the products of the neutralization reaction between HCl and NaOH.

Solution. HCl + NaOH → NaCl + H₂O — a salt and water.

Titration Basics

  • Titration slowly adds a solution of known concentration (the titrant) to a solution of unknown concentration until neutralization is reached — the equivalence point, often shown by an indicator color change.
  • At the equivalence point for a monoprotic acid and base: M(acid) × V(acid) = M(base) × V(base).
Worked example

25.0 mL of an HCl solution is exactly neutralized by 50.0 mL of 0.10 M NaOH. What is the concentration of the HCl?

Solution. M(acid)V(acid) = M(base)V(base) → M(acid)(25.0) = (0.10)(50.0) → M(acid) = 5.0 ÷ 25.0 = 0.20 M.

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. What ion do acids release in water?

2. What pH range is considered basic?

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