Neutralisation: acid meets alkali
Add dilute sodium hydroxide drop by drop to dilute hydrochloric acid with an indicator, watch the pH change, and find the exact point of neutralisation.
Core concept
An acid and an alkali neutralise each other to form a salt and water; at the end point the pH is 7
Topic: Acids, bases and salts
acid: A substance that tastes sour, like lemon juice or vinegar. It turns blue litmus red. All key words 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: A substance that changes colour to show whether something is an acid or a base. All key words 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
- Record the pH of 25 mL of dilute hydrochloric acid with universal indicator.
- Add sodium hydroxide from the burette a few mL at a time, recording as you go.
- Find the volume that gives pH 7 and record it.
- Add more alkali past the end point and record.
- 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.