# Exploring Substances: Acidic, Basic, and Neutral

## Overview

This chapter introduces students to the nature of substances found in everyday life. It explains how substances can be classified as acidic, basic, or neutral based on their reaction with indicators. It also explores natural and synthetic indicators, neutralisation reactions, and real-life applications of these concepts.

## Key Topics Covered

### 1. Introduction at the Science Fair

* Students were greeted with white paper that revealed hidden messages when sprayed with a liquid.
* This sparked curiosity about how substances interact and change properties.

### 2. Nature – Our Science Laboratory

#### 2.1.1 Litmus as an Indicator

* **Activity**: Various household substances tested with blue and red litmus paper.
* **Observation**:

* Acids: Turn blue litmus red (e.g., lemon juice, amla juice).
* Bases: Turn red litmus blue (e.g., soap solution, baking soda).
* Neutrals: No colour change (e.g., sugar solution, tap water).

#### 2.1.2 Red Rose as an Indicator

* **Process**: Red rose petals crushed and soaked in hot water to prepare indicator.
* **Findings**:

* Turns red in acidic medium.
* Turns green in basic medium.
* No change in neutral substances.

#### 2.1.3 Turmeric as an Indicator

* **Method**: Filter paper dipped in turmeric paste and dried to make ‘turmeric paper’.
* **Findings**:

* Turns red in basic substances.
* No colour change in acidic or neutral substances.
* **Limitation**: Cannot distinguish between acids and neutrals.

#### 2.1.4 Olfactory Indicators

* Certain substances change odour in acidic/basic media (e.g., onion extract).
* **Activity**: Onion-soaked cloth strips used to detect acid/base through smell.

### 3. Neutralisation Reactions

#### 2.2 What Happens When Acids and Bases Mix?

* **Experiment**: Mixing lemon juice and lime water shows colour change from red to blue.
* **Concept**: Neutralisation reaction

* **Equation**: Acid + Base → Salt + Water + Heat
* The solution becomes neutral (neither acidic nor basic).

### 4. Neutralisation in Daily Life

#### Situations and Applications

* **Ant Bite**: Formic acid is neutralised using baking soda (a base).
* **Acidic Soil**: Treated with lime to neutralise and support plant growth.
* **Industrial Waste**: Acidic waste neutralised with base before release.
* **Basic Soil**: Treated with organic matter (releases acids).

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## New Terms and Definitions

| Term | Simple Definition |
| ------------------- | ------------------------------------------------------------------------------- |
| Acidic | A substance that tastes sour and turns blue litmus paper red |
| Basic | A substance that tastes bitter, feels slippery, and turns red litmus paper blue |
| Neutral | A substance that is neither acidic nor basic |
| Litmus | A natural indicator made from lichens, available as red or blue paper |
| Indicator | A substance that shows a colour change when added to acidic or basic substances |
| Red Rose Extract | A flower extract that turns red in acid and green in base |
| Turmeric Paper | Yellow paper from turmeric that turns red in base but not in acid or neutral |
| Olfactory Indicator | A substance that changes its smell in acidic or basic medium |
| Neutralisation | Reaction where acid and base cancel each other, forming salt, water, and heat |
| Salt | A product formed when an acid reacts with a base |

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## Practice Questions

### Easy (3)

1. **What happens to blue litmus paper when dipped in lemon juice?**
**Answer**: It turns red.
**Explanation**: Lemon juice is acidic and acids turn blue litmus red.

2. **Which natural indicator is obtained from a flower?**
**Answer**: Red rose extract.
**Explanation**: It shows red in acids and green in bases.

3. **Name one neutral substance.**
**Answer**: Tap water.
**Explanation**: It does not change the colour of litmus paper.

### Medium (2)

4. **Why does turmeric paper not change colour in acidic solution?**
**Answer**: Turmeric only reacts to basic substances.
**Explanation**: It is not sensitive to acids or neutrals.

5. **What kind of reaction is observed when lemon juice is mixed with lime water?**
**Answer**: Neutralisation reaction.
**Explanation**: Acid (lemon juice) reacts with base (lime water) to form salt and water.

### Difficult (3)

6. **You are given three solutions. How can you identify acid, base, and neutral using turmeric paper?**
**Answer**: Only base will turn turmeric paper red.
**Explanation**: Acid and neutral show no colour change.

7. **Why do rose petals turn green when soap solution is added?**
**Answer**: Soap is basic in nature.
**Explanation**: Red rose extract turns green in base.

8. **Give two examples of neutralisation in daily life.**
**Answer**: Treating ant bites with baking soda, and treating acidic soil with lime.
**Explanation**: Both cases involve acid-base reactions.

### Very Difficult (2)

9. **A student uses turmeric to write on paper and sprays a solution to reveal it. Which solution was used and why?**
**Answer**: Soap solution.
**Explanation**: The base reacts with turmeric to turn it red and reveal the message.

10. **Explain how to test the nature of a liquid if you have red rose extract and turmeric paper.**
**Answer**: Use red rose extract to check for red (acid) or green (base) colour. Use turmeric paper—red means base, no change means acid or neutral.
**Explanation**: Combining both gives more reliable results.

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