Class 7 Science Notes on Chapter 2 Exploring Substances- Acidic, Basic and Neutral Notes: Comprehensive Study Guide

Everything around us, from lemon juice and soap to soil and water, can be acidic, basic or neutral. Studying these substances helps us explain many everyday observations, such as why turmeric changes colour with soap or why lime is added to acidic soil. 

Class 7 Science Notes on Chapter 2 Exploring Substances: Acidic, Basic and Neutral provides a simple and well-organised revision of all the important concepts covered in the latest curriculum.These notes explains acids, bases, natural and synthetic indicators, neutralisation reactions and more in easy-to-understand manner and revise quickly before exams. 

Topics Covered in Class 7 Science Notes on Chapter 2 Exploring Substances: Acidic, Basic and Neutral

What are Acids, Bases and Neutral Substances?	Indicators and their Types
Litmus Paper Test	Natural Indicators
Turmeric as an Indicator	Olfactory Indicators
Neutralisation Reaction	Neutralisation in Daily Life

Class 7 Science Notes on Chapter 2 Exploring Substances: Acidic, Basic and Neutral

What are Acids, Bases and Neutral Substances?

Substances around us can be grouped into three categories based on how they behave and how they react with different indicators. These categories are acids, bases, and neutral substances. Identifying them helps us understand many everyday materials used in homes, laboratories, agriculture, and industries.

Acids

Acids are substances that usually have a sour taste and change the colour of blue litmus paper to red. They are found naturally in many fruits and food items.

Common examples include: Lemon juice,vinegar,tamarind,orange juice and amla juice

Characteristics of Acids

• Turn blue litmus paper red.
• Generally taste sour (do not taste chemicals in laboratories).
• React with bases during neutralisation.
• Many natural fruit juices contain weak acids.

Bases

Bases are substances that generally feel soapy or slippery and turn red litmus paper blue.

Examples : Soap solution, baking soda solution, lime water and washing soda

Characteristics of Bases

• Turn red litmus paper blue.
• Feel slippery to touch.
• Neutralise acids.
• Many cleaning products are basic in nature.

Neutral Substances

Neutral substances are neither acidic nor basic. They do not change the colour of acid-base indicators.

Examples: Pure water, sugar solution and common salt solution

Characteristics of Neutral Substances

• Neither acidic nor basic.
• Do not change red or blue litmus paper.
• Have no acidic or basic properties.

Difference between Acids, Bases and Neutral Substances

Property	Acids	Bases	Neutral Substances
Taste	Sour	Bitter (never taste chemicals)	No characteristic taste
Litmus Test	Blue litmus turns red	Red litmus turns blue	No colour change
Nature	Acidic	Basic	Neither acidic nor basic
Reaction	Neutralises bases	Neutralises acids	No neutralisation behaviour
Examples	Lemon juice, Vinegar	Soap solution, Lime water	Water, Sugar solution

Know a Scientist: Acharya Prafulla Chandra Ray

Acharya Prafulla Chandra Ray (P. C. Ray) is known as the Father of Modern Indian Chemistry. He made significant contributions to chemical research in India and founded the country's first pharmaceutical company in 1901. He also encouraged scientific education through Indian languages and highlighted India's rich scientific heritage.

Indicators and Their Types

It is not always possible to identify whether a substance is acidic or basic simply by looking at it. Scientists use indicators to determine the nature of a substance.

An indicator is a substance that changes its colour in acidic or basic solutions, making it easy to identify the nature of the sample.

Indicators are mainly of three types:

• Natural indicators
• Synthetic indicators
• Olfactory indicators

Litmus Paper Test

Litmus paper is the most commonly used indicator in laboratories.

There are two types:

• Blue Litmus Paper
• Red Litmus Paper

Colour Changes

Indicator	Acid	Base	Neutral
Blue Litmus	Turns Red	No Change	No Change
Red Litmus	No Change	Turns Blue	No Change

Litmus paper provides a quick and reliable way to determine whether a substance is acidic or basic.

Natural Indicators

Natural indicators are obtained from plants, flowers, fruits, or other natural sources. They change colour depending on whether the solution is acidic or basic. Some common natural indicators are: Red rose extract, red hibiscus extract, purple cabbage extract, lichen,turmeric

Colour Changes of Natural Indicators

Natural Indicator	In Acid	In Base
Red Rose Extract	Red	Green
Turmeric	Yellow	Red-Brown
Purple Cabbage	Pink/Red	Greenish-Blue

Natural indicators are eco-friendly, inexpensive, and easy to prepare at home.

Turmeric as an Indicator

Turmeric is one of the most commonly used natural indicators. It helps identify basic substances but cannot distinguish between acidic and neutral solutions.

Observation

• Turmeric remains yellow in acidic solutions.
• Turmeric remains yellow in neutral solutions.
• Turmeric changes to reddish-brown in basic solutions.

This property explains why turmeric stains on clothes often turn reddish when soap is applied.

Olfactory Indicators

Some substances can be identified by the change in their smell when they come in contact with acidic or basic substances. These substances are called olfactory indicators. They are useful when colour changes cannot be observed clearly.

Common Olfactory Indicators: Onion, vanilla essence and clove oil

How They Work

• In an acidic medium, the characteristic smell generally remains unchanged.
• In a basic medium, the smell becomes faint or disappears.

Example:
The smell of onion becomes less noticeable when treated with a baking soda solution, while it remains unchanged with tamarind water.

What Happens when Acidic Substances Mix with Basic Substances?

When an acid and a base are mixed together in suitable amounts, they react with each other and reduce each other's effect. This process is called a neutralisation reaction.

During this reaction:

• The acidic nature decreases.
• The basic nature also decreases.
• A new substance called salt is formed.
• Water is produced.
• Heat is released.

General Equation

Acid + Base → Salt + Water + Heat

For example, when lemon juice is mixed with lime water, the solution gradually changes from acidic to basic as more lime water is added.

Neutralisation Reaction

A neutralisation reaction occurs when an acid reacts with a base to produce salt, water, and heat. It is one of the most important chemical reactions studied in Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic and Neutral Notes

Characteristics of Neutralisation

• Acid and base react together.
• Salt is formed as a product.
• Water is produced.
• Heat is released (exothermic reaction).
• The acidic and basic properties are neutralised.

Example

Hydrochloric Acid + Sodium Hydroxide

$\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}$

Here,

• HCl is the acid.
• NaOH is the base.
• NaCl is the salt formed.
• Water is also produced.

Neutralisation in Daily Life

Neutralisation reactions are not limited to laboratories. They are used in many everyday situations to solve practical problems.

1. Relief from Ant Bites:

Red ants inject formic acid when they bite, causing irritation and pain. Applying moist baking soda, which is a base, neutralises the acid and provides relief.

2. Treating Acidic Soil

Some crops do not grow well in acidic soil. Farmers add lime, a basic substance, to reduce the acidity and improve soil quality.

3. Treating Basic Soil

If soil becomes too basic, organic matter such as compost or manure is added. As it decomposes, it releases weak acids that help neutralise the soil.

4. Treating Industrial Waste

Factories sometimes release acidic waste into rivers and lakes. Before disposal, suitable basic substances are added to neutralise the waste and reduce environmental damage.

Everyday Examples of Acids, Bases and Neutral Substances

Substance	Nature
Lemon juice	Acidic
Vinegar	Acidic
Tamarind water	Acidic
Soap solution	Basic
Baking soda solution	Basic
Lime water	Basic
Sugar solution	Neutral
Distilled water	Neutral
Common salt solution	Neutral

Important Indicators and Their Colour Changes

Indicator	Acidic Solution	Basic Solution
Blue Litmus	Turns Red	No Change
Red Litmus	No Change	Turns Blue
Turmeric	Yellow	Reddish-Brown
Red Rose Extract	Red	Green