📘 Chapter Overview
Everything around us is made up of substances. Some substances are acidic, some are basic, while others are neutral. In this chapter, we learn how scientists identify these properties using indicators and simple observations.
The chapter introduces important ideas such as acids, bases, neutral substances, indicators and neutralisation. These concepts are connected with many things we see in everyday life, such as lemon juice, soap, baking soda, vinegar and medicines.
📑 Table of Contents
1. Types of Substances
Substances can broadly be classified according to their chemical nature as:
- Acidic substances
- Basic substances
- Neutral substances
Indicators can be used to determine the nature of a substance safely instead of relying only on properties such as taste or touch.
2. Acidic Substances
Substances that contain acids and show acidic properties are called acidic substances.
🍋 Common Examples
- Lemon juice
- Vinegar
- Tamarind
- Amla
- Curd
Important Properties
- Many edible acidic substances have a sour taste.
- Acids turn blue litmus paper red.
- Different foods contain different naturally occurring acids.
3. Basic Substances
Substances that show basic properties are called basic substances or bases.
🧼 Common Examples
- Soap solution
- Baking soda solution
- Lime water
- Washing powder solution
Important Properties
- Many bases feel slippery or soapy.
- Bases generally turn red litmus paper blue.
- Many bases have a bitter taste, but tasting substances is unsafe.
4. Neutral Substances
Substances that are neither acidic nor basic are called neutral substances.
Examples
- Tap water
- Sugar solution
- Salt solution
A neutral substance generally does not change the colour of either red litmus or blue litmus paper.
5. What Are Indicators?
Indicators are substances that help us identify whether another substance is acidic or basic by showing a characteristic change.
The change may be in colour or, in some cases, smell.
Types of Indicators
- Litmus
- Turmeric
- China rose or other flower extracts
- Olfactory indicators such as onion
6. Litmus Test
Litmus is a commonly used indicator. It is available in the form of red and blue litmus paper.
| Substance | Effect on Blue Litmus | Effect on Red Litmus | Nature |
|---|---|---|---|
| Lemon juice | Turns red | No change | Acidic |
| Vinegar | Turns red | No change | Acidic |
| Soap solution | No change | Turns blue | Basic |
| Baking soda solution | No change | Turns blue | Basic |
| Sugar solution | No change | No change | Neutral |
| Salt solution | No change | No change | Neutral |
🔵 Blue litmus → Red = Acidic
🔴 Red litmus → Blue = Basic
No change in either = Neutral
7. Natural Indicators
Some plants and plant materials contain substances that can act as indicators. These are called natural indicators.
🌿 Turmeric
Turmeric can be used as a natural indicator. It shows a different colour in the presence of a basic substance.
🌹 Flower Extracts
Extracts obtained from certain flowers, such as red rose or china rose, can show characteristic colour changes when mixed with acidic or basic substances.
🧅 Olfactory Indicators
Some substances change their smell in acidic or basic conditions. Such substances are known as olfactory indicators.
8. Neutralisation
When an acid reacts with a base in suitable quantities, their acidic and basic effects can cancel each other. This process is called neutralisation.
During a neutralisation reaction, salt and water are formed and heat may be released.
Example
When an acidic solution is treated with a suitable basic solution, the acidic effect can be reduced. If sufficient base is added, the resulting solution may become neutral.
9. Neutralisation in Daily Life
Neutralisation is not limited to laboratories. It has many practical applications in everyday life.
🐜 Ant Bites
An ant bite can cause irritation because of acidic substances associated with the bite. A mild basic substance such as baking soda can help neutralise the acidic effect.
🌱 Soil Treatment
If soil is too acidic or too basic, suitable substances may be added to help correct its condition and make it more suitable for plant growth.
🏭 Industrial Waste
Some industrial wastes can be acidic or basic. Appropriate neutralisation treatment can help reduce their harmful effects before disposal.
💊 Acidity and Antacids
Excess acid in the stomach can cause discomfort. Antacids contain substances that can react with excess stomach acid and reduce its effect.
⚡ Quick Revision
📊 Acidic, Basic and Neutral – At a Glance
| Property | Acidic | Basic | Neutral |
|---|---|---|---|
| Blue litmus | Turns red | No change | No change |
| Red litmus | No change | Turns blue | No change |
| Examples | Lemon juice, vinegar | Soap solution, baking soda | Sugar solution, salt solution |
| General property | Often sour | Often slippery | Neither acidic nor basic |
📝 Important Questions
Substances that show acidic properties are called acidic substances. They generally turn blue litmus paper red.
Substances that show basic properties are called bases. They generally turn red litmus paper blue.
Substances that are neither acidic nor basic are called neutral substances.
An indicator is a substance that helps identify whether another substance is acidic or basic through a characteristic change such as colour or smell.
Blue litmus generally turns red in an acidic substance.
Red litmus generally turns blue in a basic substance.
Neutralisation is the reaction in which an acid and a base react so that their acidic and basic effects are reduced.
Turmeric and certain flower extracts are examples of natural indicators.
Unknown substances may be harmful or poisonous. Their nature should be tested using safe scientific methods such as indicators.
Acid + Base → Salt + Water + Heat.
🎯 Exam Tips
- Remember the litmus rule: Acid → Blue to Red.
- Remember: Base → Red to Blue.
- Learn at least two examples each of acidic, basic and neutral substances.
- Understand the meaning and examples of indicators.
- Learn the neutralisation equation.
- Remember practical applications of neutralisation in daily life.
❓ Frequently Asked Questions
What is Chapter 2 of Class 7 Science?
Chapter 2 is “Exploring Substances: Acidic, Basic, and Neutral” from the NCERT Curiosity textbook for the 2026–27 session.
What are the three main types of substances discussed?
The chapter discusses acidic, basic and neutral substances.
Which litmus paper turns red in an acid?
Blue litmus paper turns red in an acidic substance.
Which litmus paper turns blue in a base?
Red litmus paper turns blue in a basic substance.
What is neutralisation?
Neutralisation is the reaction between an acid and a base in which their acidic and basic effects are reduced.
What are natural indicators?
Natural indicators are substances obtained from natural sources that can help identify acidic or basic substances.