Notes Chapter 2 Class 7 Science Curiosity NCERT

Exploring Substances: Acidic, Basic, and Neutral

Introduction

• This chapter explains acidic, basic, and neutral substances.
• It begins with a science fair where a white paper revealed a message when sprayed with a liquid, sparking curiosity about substances that change color when mixed.

1. Nature – Our Science Laboratory

Litmus as an Indicator

1. What is Litmus?

• Litmus is a natural substance from lichens (fungus and alga combination).
• Available as blue and red paper strips or solution.
• It’s an acid-base indicator that changes color in acidic or basic solutions.

2. How it Works:

• Acidic substances (e.g., lemon juice, vinegar) turn blue litmus red.
• Basic substances (e.g., soap solution, lime water) turn red litmus blue.
• Neutral substances (e.g., tap water, sugar solution) don’t change litmus color.

3. Key Substances:

• Acidic: Lemon juice, amla juice, tamarind water, vinegar.
• Basic: Soap solution, baking soda solution, lime water, washing powder solution.
• Neutral: Tap water, sugar solution, salt solution.

4. Lime Water: Made by mixing calcium oxide (lime) with water, letting it sit, and filtering.

2. Red Rose as an Indicator

• Red Rose Extract:
• Acts as an acid-base indicator.
• Turns red in acidic solutions (e.g., lemon juice).
• Turns green in basic solutions (e.g., soap solution).
• No color change in neutral solutions.

Other Natural Indicators: Beetroot, purple cabbage, turmeric, Indian blackberry (jamun), red hibiscus.

Fascinating Fact:

• Hydrangea flowers change color based on soil:
• Acidic soil: Blue flowers.
• Basic soil: Pink or red flowers.

 3. Turmeric as an Indicator

• Turmeric:
• Acts as an indicator for basic substances.
• Turns red in basic solutions (e.g., soap solution).
• Stays yellow in acidic or neutral solutions.

Fascinating Fact:

• Turmeric is a “Golden” spice used in food, Ayurvedic medicine, and known for health benefits.

Olfactory Indicators

• Substances like onions whose smell changes in acidic or basic conditions.
• Example: Onion smell changes differently with tamarind water (acidic) or baking soda (basic).

4.What Happens When Acidic and Basic Substances Mix?

• Neutralization Reaction:
• When an acid (e.g., lemon juice) and a base (e.g., lime water) mix in the right amounts, they form a neutral solution.
• Reaction: Acid + Base → Salt + Water + Heat.
• Example: Lemon juice (acidic) turns blue litmus red; adding lime water (basic) turns it blue again, showing neutralization.

5. Neutralization in Daily Life

Ant Bite:

• Ants inject formic acid, causing pain.
• Moist baking soda (base) neutralizes the acid, relieving pain.

Acidic Soil:

• Chemical fertilizers can make soil acidic, harming plants.
• Lime (base) neutralizes acidic soil.
• Basic soil is treated with organic matter (e.g., manure) to release acids.

Industrial Waste:

• Acidic factory waste harms aquatic life.
• Adding basic substances neutralizes waste before release.

6.Science Fair Mystery

• A paper showed the message “Welcome to the Wonderful World of Science” when sprayed.
• Explanation: Likely written with a basic solution (e.g., soap solution) on turmeric-coated paper. Spraying turmeric solution turned basic areas red, revealing the message.

7. Key Points

• Substances are acidic, basic, or neutral.
• Indicators identify substance nature:
• Litmus: Blue to red (acids), red to blue (bases).
• Red rose extract: Red (acids), green (bases).
• Turmeric: Red (bases), yellow (acids/neutral).
• Olfactory indicators (e.g., onion): Smell changes in acidic/basic conditions.
• Acids: Taste sour (e.g., lemon, vinegar), contain acids like citric, tartaric.
• Bases: Feel slippery, taste bitter (e.g., baking soda, soap).
• Neutral substances: No effect on indicators (e.g., sugar, salt).
• Neutralization: Acid + base forms salt, water, and heat.
• Neutralization is used for ant bites, soil treatment, and waste management.

8.Common Acids in Edible Substances

• Lemon: Citric acid
• Curd: Lactic acid
• Tamarind: Tartaric acid
• Vinegar: Acetic acid

9. Know a Scientist

Acharya Prafulla Chandra Ray:

• “Father of Modern Indian Chemistry”.
• Earned a UK doctorate, advanced Indian scientific research.
• Founded India’s first pharmaceutical company (1901).
• Promoted Indian culture and mother tongue in education.