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Grades 9 and 10 Intermediate About 20 min Version 1 Awaiting review Premium

Neutralisation: acid meets alkali

Acids contain hydrogen ions (H⁺) and alkalis contain hydroxide ions (OH⁻). When they are mixed, the ions join to make water: H⁺ + OH⁻ → H₂O. The other ions form a salt. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water. This is neutralisation. Adding alkali to acid a little at a time raises the pH slowly at first, then very suddenly near the end point, where exactly enough alkali has been added and the pH is 7. An indicator shows when this happens.

What you will learn

  • Describe neutralisation as acid + alkali → salt + water.
  • Follow the pH change during a titration.
  • Use an indicator to find the end point.

Before you start

  • Know the pH scale and that indicators change colour.

Materials

  • 25 mL of dilute hydrochloric acid (0.1 mol/L)
  • Dilute sodium hydroxide solution (0.1 mol/L)
  • Universal indicator and phenolphthalein

Equipment

  • Burette, conical flask, white tile and clamp stand

Procedure

  1. Record the pH of 25 mL of dilute hydrochloric acid with universal indicator.
  2. Add sodium hydroxide from the burette a few mL at a time, recording as you go.
  3. Find the volume that gives pH 7 and record it.
  4. Add more alkali past the end point and record.
  5. Repeat near the end point with phenolphthalein and record when it first turns pink.

How this simulation is simplified

  • 25 mL of 0.1 mol/L hydrochloric acid is titrated with 0.1 mol/L sodium hydroxide at room temperature.