Investigating Acids, Bases and Neutralisation

Using Bromothymol Blue Indicator

Aim

To investigate acids, bases and neutralisation using bromothymol blue indicator.

Learning Outcome

• Identify acids and bases using an indicator.
• Explain what pH means.
• Use bromothymol blue to classify solutions.
• Observe a neutralisation reaction.
• Explain that an acid and a base react to form salt and water.
• Identify the neutralisation point using a colour change.

Time Allocation

Approximately 45 to 60 minutes.

How Does It Work?

The pH scale is used to show whether a substance is acidic, neutral or basic.

• Acids have a pH below 7.
• Neutral substances have a pH of 7.
• Bases have a pH above 7.

Indicators are substances that change colour depending on the pH of a solution. Bromothymol blue is useful because it shows clear colour changes in acidic, neutral and basic solutions.

Bromothymol Blue Colour Changes

Solution Type	Approximate pH	Bromothymol Blue Colour
Acidic	Below 7	Yellow
Neutral	Around 7	Green
Basic	Above 7	Blue

Scientific Background

A neutralisation reaction occurs when an acid reacts with a base.

For this experiment:

• Hydrochloric acid is the acid.
• Sodium hydroxide is the base.
• Sodium chloride is the salt formed.
• Water is also formed.

When bromothymol blue is added to sodium hydroxide, the solution is blue. As hydrochloric acid is added, the base is neutralised. The solution changes towards green when it becomes close to neutral. If too much acid is added, the solution turns yellow.

Hypothesis

If hydrochloric acid is added slowly to sodium hydroxide containing bromothymol blue, then the indicator colour will change from blue to green as the solution becomes neutral.

Variables

Independent Variable

Volume of hydrochloric acid added.

Dependent Variable

Colour of the bromothymol blue indicator.

Control Variables

• Same concentration of acid.
• Same concentration of base.
• Same volume of sodium hydroxide.
• Same number of indicator drops.
• Same apparatus.

Apparatus

• Dilute hydrochloric acid
• Dilute sodium hydroxide solution
• Bromothymol blue indicator
• Burette
• Funnel
• Retort stand and clamp
• Beaker or conical flask
• Measuring cylinder or pipette
• White tile
• Wash bottle with distilled water
• Test tubes or small beakers
• Bunsen burner, tripod and gauze, optional for evaporation
• Safety goggles
• Gloves

Safety Precautions

Part A: Testing Acids and Bases Using Bromothymol Blue

Aim

To observe the colour of bromothymol blue in acidic, neutral and basic solutions.

Method

1. Place small samples of dilute hydrochloric acid, distilled water and dilute sodium hydroxide into separate test tubes or beakers.
2. Add a few drops of bromothymol blue to each sample.
3. Observe and record the colour of each solution.
4. Use the colour table to classify each solution as acidic, neutral or basic.

Results Table: Indicator Colour

Substance	Expected Type	Indicator Colour	Classification
Hydrochloric acid	Acidic
Distilled water	Neutral
Sodium hydroxide	Basic

What Learners Should Observe

• Hydrochloric acid turns bromothymol blue yellow.
• Distilled water should appear green.
• Sodium hydroxide turns bromothymol blue blue.

Conclusion

Bromothymol blue can be used to identify acidic, neutral and basic solutions.

Part B: Neutralisation of Hydrochloric Acid and Sodium Hydroxide

Aim

To observe neutralisation between hydrochloric acid and sodium hydroxide using bromothymol blue indicator.

Method

1. Rinse the burette with a small amount of dilute hydrochloric acid.
2. Fill the burette with dilute hydrochloric acid.
3. Record the starting burette reading.
4. Measure sodium hydroxide solution into a clean beaker or conical flask.
5. Add a few drops of bromothymol blue indicator to the sodium hydroxide.
6. Place the beaker or flask on a white tile under the burette.
7. Slowly add hydrochloric acid from the burette into the sodium hydroxide solution.
8. Swirl the beaker gently after each small addition.
9. Continue adding acid until the solution changes from blue to green.
10. Stop adding acid as soon as the green colour is reached.
11. Record the final burette reading.
12. Calculate the volume of acid used.

Results Table: Neutralisation

Measurement	Reading
Starting burette reading (ml)
Final burette reading (ml)
Volume of acid used (ml)
Starting indicator colour
Final indicator colour

What Learners Should Observe

• The sodium hydroxide solution starts blue.
• The colour gradually changes as acid is added.
• The solution becomes green near neutralisation.
• If too much acid is added, the solution turns yellow.

Explanation

Sodium hydroxide is a base, so bromothymol blue appears blue. Hydrochloric acid neutralises the sodium hydroxide. At neutralisation, the acidic and basic properties are balanced. The solution changes to green, showing that it is close to neutral.

Conclusion

Hydrochloric acid reacts with sodium hydroxide in a neutralisation reaction. The products are sodium chloride and water. Bromothymol blue helps identify the neutralisation point through a colour change.

Part C: Identifying Unknown Solutions Using pH

Aim

To classify unknown solutions as acidic, neutral or basic using bromothymol blue.

Method

1. Place three unknown solutions into separate test tubes or small beakers.
2. Add a few drops of bromothymol blue to each sample.
3. Observe the colour of each solution.
4. Compare the colour with the bromothymol blue colour table.
5. Classify each solution as acidic, neutral or basic.

Results Table: Unknown Solutions

Unknown Sample	Indicator Colour	Classification
Sample A
Sample B
Sample C

Conclusion

Unknown solutions can be classified by comparing indicator colours with the pH colour range.

Optional Part D: Observing Salt Crystals

Aim

To observe the salt formed after neutralisation.

Method

1. Pour a small amount of the neutralised solution into an evaporating dish or beaker.
2. Heat gently using a Bunsen burner, tripod and gauze.
3. Allow the water to evaporate.
4. Observe the solid crystals left behind.

What Learners Should Observe

Learners should observe small salt crystals remaining after the water evaporates.

Explanation

Neutralisation produces a salt and water. In this reaction, the salt formed is sodium chloride. When the water evaporates, sodium chloride crystals remain.

Conclusion

The salt formed during neutralisation can be observed after evaporation.

Expected Results

• Acids turn bromothymol blue yellow.
• Neutral solutions appear green.
• Bases turn bromothymol blue blue.
• Sodium hydroxide changes from blue to green as hydrochloric acid is added.
• Excess acid causes the solution to turn yellow.
• Salt crystals may remain after evaporation.

Important Notes

Questions for Learners

1. What is an acid?
2. What is a base?
3. What is the pH scale used for?
4. What colour is bromothymol blue in an acid?
5. What colour is bromothymol blue in a neutral solution?
6. What colour is bromothymol blue in a base?
7. What is neutralisation?
8. What products form when hydrochloric acid reacts with sodium hydroxide?
9. Why must the acid be added slowly?
10. Why does salt remain after evaporation?

Common Mistakes

• Adding acid too quickly.
• Missing the green neutralisation point.
• Adding too much indicator.
• Forgetting to swirl the solution.
• Reading the burette incorrectly.
• Confusing endpoint colour with excess acid colour.
• Heating the solution too strongly during evaporation.

Teacher Notes

• Keep the main focus on pH, indicators and neutralisation.
• Avoid overloading Grade 10 learners with pKa or advanced acid-base theory.
• Bromothymol blue is useful because the colour change is easy to see.
• The unknown sample activity is useful for learner engagement.
• Evaporation of the neutralised solution helps learners see that a salt was formed.
• Small-scale practical work can reduce chemical use, improve safety and lower classroom costs.

Teacher Tip

Let learners first test known acids, bases and neutral substances before attempting the neutralisation reaction. This gives them confidence when interpreting the colour changes.

Extension Activity

Ask learners to test household substances such as:

• Lemon juice
• Vinegar
• Milk
• Baking soda solution
• Soap solution
• Salt water

Learners can classify each as acidic, neutral or basic.

Real-World Application

Acids, bases and neutralisation are important in agriculture, medicine, digestion, water treatment, cleaning products, food science, swimming pool testing and environmental monitoring.

Neutralisation is used to control pH in many real-world processes.