Are you a Class 7 CBSE student trying to make sense of acids, bases, and neutral substances? You’re not alone. This chapter, “Exploring Substances: Acidic, Basic, and Neutral,” is Chapter 2 of your Science (Curiosity) textbook . It introduces you to the chemical nature of everyday substances and how we can test them safely using indicators.

At Dhingra Classes, we simplify complex topics for our students. Here is a detailed summary and analysis tailored specifically for the CBSE Class 7 syllabus.

🧪 What Are Acids, Bases, and Neutral Substances?

Every substance around you can be classified into one of three groups based on its chemical nature .

Acids: The Sour Ones

Acidic substances have a pH less than 7. They taste sour and turn blue litmus paper red . Common examples include:

• Lemon juice (contains citric acid)

• Vinegar (contains acetic acid)

• Curd (contains lactic acid)

• Tamarind (contains tartaric acid)

Bases: The Bitter Ones

Basic substances (also called alkaline) have a pH greater than 7. They taste bitter, feel slippery (soapy), and turn red litmus paper blue . Common examples include:

• Baking soda (sodium bicarbonate)

• Soap solution

• Lime water (calcium hydroxide)

• Toothpaste

Neutral Substances: The Balanced Ones

Neutral substances have a pH of exactly 7. They are neither acidic nor basic and do not change the colour of any indicator . Common examples include:

• Water

• Sugar solution

• Salt solution

🔬 Indicators: Tools That Reveal the Truth

Indicators are special substances that change colour or smell when mixed with acidic or basic solutions . They help us test substances safely—without tasting or touching them directly.

Natural Indicators

These are obtained from plants and natural sources .

💡 Easy Memory Trick: A-B-R (Acid → Blue litmus → Red) and B-R-B (Base → Red litmus → Blue) .

Synthetic Indicators

These are man-made indicators used in laboratories :

• Phenolphthalein

• Methyl Orange

Olfactory Indicators

These indicators change their smell in acidic or basic solutions :

• Onion extract

• Vanilla extract

⚗️ The Magic of Neutralisation

When an acid and a base are mixed in the right proportions, they cancel each other’s effect. This reaction is called neutralisation .

The Word Equation

Acid + Base → Salt + Water + Heat

The products of this reaction are:

• Salt (any ionic compound, not just table salt)

• Water

• Heat (the reaction is exothermic)

Everyday Applications of Neutralisation

Neutralisation is not just a lab experiment—it solves real problems every day :

• 🐜 Ant Stings: Ants inject formic acid when they bite. Applying baking soda (a base) neutralises the acid and relieves pain .

• 🦷 Toothpaste: Toothpaste is basic. It neutralises the acids produced by bacteria in your mouth, protecting your teeth from decay .

• 🌱 Agriculture: Farmers add lime (a base) to acidic soil to balance the pH and improve crop growth .

• 🏭 Waste Treatment: Industrial acidic waste is neutralised with bases before being released into water bodies to protect aquatic life .

📝 Important Points for CBSE Class 7 Students

❓ Frequently Asked Questions (CBSE Pattern)

Q1: How can you test whether a substance is acidic, basic, or neutral?
Use litmus paper. If blue litmus turns red, the substance is acidic. If red litmus turns blue, it is basic. If there is no change, it is neutral .

Q2: Why should we never taste unknown substances in the laboratory?
Unknown substances may be harmful, poisonous, or corrosive. They can cause burns or irritation. Always use indicators instead .

Q3: What is a neutralisation reaction? Give one example.
A neutralisation reaction occurs when an acid reacts with a base to form salt and water. Example: Adding baking soda to an ant bite (formic acid) neutralises the acid and gives relief .

Q4: What happens to turmeric paper when it comes in contact with soap solution?
Turmeric paper turns reddish-brown in basic solutions. Soap solution is basic, so turmeric turns reddish-brown .

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🎯 Exam Tip for Class 7 CBSE Students

When answering questions on this chapter in your CBSE exam:

1. Memorise the litmus rule: Acid → Blue to Red; Base → Red to Blue.

2. Remember natural indicators like turmeric and red rose extract—these are frequently asked in application-based questions.

3. Use the neutralisation equation correctly: Acid + Base → Salt + Water + Heat.

4. Link concepts to real life: ant bites, toothpaste, soil treatment, and waste management.