Lesson Plan

• Describe the colour change of litmus in acidic solutions.
• Explain the response of thymolphthalein to acidic and basic environments.
• Identify the colour transition of methyl orange across pH ranges.
• Compare the behaviours of the three indicators to determine the acidity of unknown solutions.

• Projector and screen
• Whiteboard and markers
• Litmus paper, thymolphthalein solution, methyl orange solution
• Set of test tubes with prepared acid solutions
• Student worksheets
• pH strips (optional)

Begin by asking students what colour changes they have observed when testing household liquids with litmus. Review that they already know acids release H⁺ ions in water. Explain that today they will predict and observe how three common indicators respond to acids, and they will be able to state the expected colour for each indicator as a success criterion.

1. Do‑now (5'): Students write down the expected colour of litmus, thymolphthalein and methyl orange in an acidic solution.
2. Mini‑lecture (10'): Review acid definition, H⁺ release, and how indicators work; show pH ranges on slides.
3. Demonstration (15'): Teacher adds each indicator to separate acid test tubes; students record observed colours.
4. Guided practice (10'): In pairs, students complete a worksheet matching indicators to pH ranges and explain the results.
5. Quick check (5'): Whole‑class poll (clickers or show of hands) to confirm understanding of each indicator’s behaviour.
6. Extension (optional) (5'): Discuss how the same indicators could be used to identify an unknown solution.

Recap that acids turn blue litmus red, keep thymolphthalein colourless, and turn methyl orange red, shifting to orange‑yellow as the solution becomes less acidic. For the exit ticket, each student writes one indicator colour prediction for a given pH value. Assign the worksheet on indicator colour tables for homework to reinforce the concepts.