Lesson Plan

• Describe the colour changes of litmus, thymolphthalein and methyl orange when an alkali is added.
• Explain how hydroxide ions cause these colour changes.
• Interpret indicator results to identify an alkaline solution.
• Apply the knowledge to predict indicator responses in simple experiments.

• Whiteboard and markers
• Projector or interactive whiteboard
• Sodium hydroxide solution, litmus solution, thymolphthalein solution, methyl orange solution
• Test tubes, beakers, and pipettes
• Litmus paper strips
• Student worksheets

Begin with a quick question: “What colour changes have you noticed when adding soap or cleaning agents to water?” Connect this to prior learning that acids turn indicators red while bases turn them blue. State the success criteria: by the end of the lesson students will be able to describe and predict how litmus, thymolphthalein and methyl orange respond to alkalis.

1. Do‑Now (5'): Students write predictions for each indicator’s colour when an alkali is added.
2. Mini‑lecture (10'): Define alkali, show the colour‑change table, and explain the role of OH⁻ ions.
3. Demonstration (10'): Teacher adds NaOH to three test tubes containing litmus, thymolphthalein and methyl orange; students record observations.
4. Guided practice (10'): Pairs complete a worksheet interpreting indicator results for given pH values.
5. Concept check (5'): Quick quiz (Kahoot or show‑of‑hands) to confirm understanding.
6. Summary discussion (5'): Review key points and clarify any misconceptions.

We revisited how litmus turns blue, thymolphthalein turns blue above pH 9.3, and methyl orange turns yellow above pH 4.4 in the presence of an alkali. For the exit ticket, each student writes the expected colour of each indicator in a strong base. Homework: complete the worksheet that matches pH ranges with the correct indicator colour.