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Lesson 38: pH Scale and Indicators pH scale (0–14) Acids vs alkalis Universal indicator, litmus
Year 8 • Chemistry
pH Scale and Indicators
In this lesson, students learn about the pH scale, understand the difference between acids and alkalis, and use indicators such as universal indicator and litmus to identify substances.
Lesson Objectives
- Understand the pH scale (0–14).
- Identify acids, alkalis, and neutral substances.
- Use indicators to test pH.
- Interpret colour changes in indicators.
1. The pH Scale
The pH scale measures how acidic or alkaline a substance is.
It ranges from 0 to 14.
[ Image Placeholder – pH Scale Diagram ]
• pH 7 → Neutral (pure water)
• pH below 7 → Acidic
• pH above 7 → Alkaline
• pH below 7 → Acidic
• pH above 7 → Alkaline
2. Acids and Alkalis
Acids have low pH values and often taste sour.
Alkalis have high pH values and feel slippery.
[ Image Placeholder – Acids vs Alkalis Examples ]
Examples:
• Acid: lemon juice, vinegar
• Alkali: soap solution, bleach
• Acid: lemon juice, vinegar
• Alkali: soap solution, bleach
3. Universal Indicator
A universal indicator shows different colours depending on the pH of a solution.
[ Image Placeholder – Universal Indicator Colours ]
• Red → strong acid
• Yellow/green → weak acid/neutral
• Blue/purple → alkali
• Yellow/green → weak acid/neutral
• Blue/purple → alkali
4. Litmus Indicator
Litmus is a simple indicator used to test whether a substance is acidic or alkaline.
[ Image Placeholder – Red and Blue Litmus ]
• Acid → blue litmus turns red
• Alkali → red litmus turns blue
• Neutral → no change
• Alkali → red litmus turns blue
• Neutral → no change
Practice Questions
A. Fill in the Blanks
- The pH scale runs from __________ to __________.
- A neutral substance has pH __________.
- Acids have pH values __________ than 7.
- Alkalis have pH values __________ than 7.
- Universal indicator shows __________ changes.
B. True or False
- Pure water is neutral.
- Soap is acidic.
- Red litmus turns blue in alkali.
- Universal indicator gives one colour only.
- Strong acids have low pH values.
C. Short Answer
- What does the pH scale measure?
- Give one example of an acid.
- Give one example of an alkali.
- What colour does universal indicator show in neutral solution?
- How does blue litmus react with acid?
✅ Show Answer Key
A. Fill in the Blanks
- 0, 14
- 7
- less
- greater
- colour
B. True or False
- True
- False
- True
- False
- True
C. Short Answer
- How acidic or alkaline a substance is.
- Lemon juice / vinegar.
- Soap / bleach.
- Green.
- It turns red.
© Aviate Learning – pH Scale and Indicators (Year 8)