What this chapter is about
This chapter teaches you how to group substances based on their chemical nature. You will learn that the things around us can be acidic, basic, or neutral. Acids taste sour, bases taste bitter and feel slippery, and neutral substances are neither acidic nor basic.
You will discover how to test whether a substance is acidic or basic using special materials called indicators. Indicators change colour when they touch acids or bases. Litmus paper, turmeric, and China rose are common indicators you can try at home or in school.
After studying this chapter, you should be able to identify acidic and basic substances in your kitchen, understand why certain foods taste sour, and explain how indicators help us know the nature of a substance without tasting it.
Key ideas
- Acids are substances that taste sour and turn blue litmus paper red. Examples: lemon juice, vinegar, curd, orange juice.
- Bases are substances that taste bitter, feel soapy or slippery, and turn red litmus paper blue. Examples: baking soda solution, soap water, lime water.
- Neutral substances are neither acidic nor basic. They do not change the colour of litmus paper. Example: pure water, common salt solution.
- Indicators are substances that show different colours in acidic and basic solutions. They help us test without tasting.
- Litmus comes from a plant called lichen. Blue litmus turns red in acid; red litmus turns blue in base.
- Turmeric is a natural indicator. It stays yellow in acids but turns red or brown in bases.
- China rose (hibiscus) solution turns dark pink or magenta in acids and green in bases.
- Mixing an acid and a base can produce a neutral substance; this reaction is called neutralisation.
Formulas and facts to remember
1. Blue litmus + Acid → turns red. 2. Red litmus + Base → turns blue. 3. Turmeric + Base → turns reddish-brown. 4. China rose solution + Acid → dark pink; + Base → green. 5. Acid + Base → Salt + Water (neutralisation). 6. Rainwater is slightly acidic because it dissolves gases like carbon dioxide from the air. 7. Our stomach produces hydrochloric acid to help digest food. 8. Ant sting and bee sting contain acids; applying a mild base like baking soda paste can give relief.
Worked examples
Example 1: Testing lemon juice with litmus
A student dips blue litmus paper into fresh lemon juice.
Step 1: Lemon juice contains citric acid, so it is acidic. Step 2: Blue litmus paper changes colour in acids. Result: The blue litmus paper turns red.
---
Example 2: Checking soap solution with turmeric
Riya adds a few drops of soap solution to turmeric paste.
Step 1: Soap solution is basic in nature. Step 2: Turmeric stays yellow in acids but changes colour in bases. Result: The turmeric paste turns reddish-brown.
---
Example 3: Identifying a neutral liquid
Arun tests distilled water with both red and blue litmus paper.
Step 1: He dips blue litmus paper in the water. It stays blue. Step 2: He dips red litmus paper in the same water. It stays red. Conclusion: Since neither litmus paper changes colour, distilled water is neutral.
Common mistakes
Thinking all sour things are dangerous acids → Many safe foods like lemon and tamarind are acidic; the word acid does not always mean harmful.
Believing bases are the opposite of acids in taste (so they must be sweet) → Bases taste bitter, not sweet.
Assuming neutral means the substance has no chemicals → Neutral only means it is neither acidic nor basic; salt solution is neutral but still contains dissolved salt.
Using the same strip of litmus for several tests → Always use a fresh strip for each test; leftover colour can give wrong results.
Forgetting that turmeric does not change colour in acids → Turmeric stays yellow in acids; it changes only in bases.
Quick revision
- Acids turn blue litmus red; bases turn red litmus blue.
- Neutral substances do not change either litmus.
- Indicators like litmus, turmeric, and China rose help test without tasting.
- Neutralisation happens when an acid and a base react to form salt and water.
- Turmeric turns reddish-brown only in bases.
- Many kitchen items (lemon, curd, soap, baking soda) can be grouped as acidic or basic.