CBSE Class 7 Science Revision Notes Chapter 2: Exploring Substances – Acidic, Basic, and Neutral

Substances can be classified as acidic, basic or neutral by testing them with suitable indicators. Litmus, red rose, turmeric and onion help identify the nature of substances through colour or smell changes.

Many substances around us have different properties. Lemon juice tastes sour, soap feels slippery, and sugar solution may show neither of these properties. Scientists use indicators to test whether such substances are acidic, basic or neutral.

These notes follow the current 2026–27 chapter. Revise litmus, red rose extract, turmeric paper, olfactory indicators and neutralisation. The chapter also explains how acids and bases are used in treating ant bites, correcting soil conditions and reducing the harmful effects of industrial waste.

Key Takeaways

• Acidic substances: They turn blue litmus paper red.
• Basic substances: They turn red litmus paper blue.
• Neutral substances: They do not change either red or blue litmus.
• Neutralisation: An acid reacts with a base to form salt, water and heat.

Access Class 7 Science Chapter 2 Exploring Substances Notes in 30 Minutes

Revise the chapter in three parts:

First 10 minutes: Acids, bases, neutral substances and the litmus test

Next 10 minutes: Red rose, turmeric and olfactory indicators

Final 10 minutes: Neutralisation and its applications in daily life

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Acidic, Basic and Neutral Substances in Class 7 Science Chapter 2 Notes

Substances around us can be grouped according to their chemical nature.

The three main groups are acidic, basic and neutral substances.

Acidic Substances

Substances that turn blue litmus paper red are called acidic substances.

Many edible acidic substances taste sour.

Examples include:

• Lemon juice
• Amla juice
• Tamarind water
• Vinegar
• Curd
• Orange juice
• Grape juice

Acidic substances contain acids.

Basic Substances

Substances that turn red litmus paper blue are called basic substances.

Many bases feel soapy or slippery when touched.

Examples include:

• Soap solution
• Baking soda solution
• Lime water
• Washing powder solution

Bases may taste bitter, but students should never taste unknown substances to identify them.

Neutral Substances

Substances that do not change either red or blue litmus paper are called neutral substances.

They are neither acidic nor basic.

Examples include:

• Tap water
• Sugar solution
• Common salt solution

Acidic, Basic and Neutral Comparison

Litmus as an Acid-Base Indicator

Litmus is one of the most common indicators used to test substances.

It is available as red litmus paper, blue litmus paper and litmus solution.

What Is Litmus?

Litmus is a natural substance obtained from lichens.

Lichens are formed by an association between:

• A fungus
• An alga

They commonly grow on rocks and trees in places with clean air and sufficient rainfall.

Why Is Litmus Called an Indicator?

Litmus shows different colours in acidic and basic solutions.

A substance that shows a visible change in the presence of an acid or base is called an acid-base indicator.

Litmus Test

To test a substance:

1. Place small pieces of red and blue litmus paper on a clean tile.
2. Put one drop of the sample on each paper.
3. Observe the colour change.
4. Compare the result with the indicator rules.
5. Record whether the substance is acidic, basic or neutral.

Results of Common Substances

Preparing Lime Water

Lime water is a solution of calcium hydroxide in water.

It can be prepared by:

1. Mixing lime or chuna with water
2. Leaving the mixture undisturbed for about an hour
3. Filtering the liquid into another container

The filtered liquid is lime water.

Common Acids and Bases in Everyday Life

Many foods contain naturally occurring acids.

Their sour taste is often linked to the presence of acids.

Common Acids in Food

Sour Taste and Acids

Edible acidic substances generally taste sour.

However, taste should never be used to identify an unknown substance.

Some acids can be harmful or corrosive.

Slippery Nature of Bases

Baking soda solution and soap solution feel slippery.

This is a common property of many basic substances.

However, touch alone is not always enough to identify a base.

An indicator should be used for confirmation.

Are All Bitter Substances Basic?

No.

Bitter gourd tastes bitter, but it is not basic in nature.

Therefore, taste is not a reliable scientific test.

Red Rose Indicator in Exploring Substances Notes

Red rose petals can be used to prepare a natural acid-base indicator.

The extract changes colour in acidic and basic solutions.

Preparing Red Rose Extract

1. Collect fallen red rose petals.
2. Wash them with clean water.
3. Crush them using a mortar and pestle.
4. Place the crushed petals in a glass tumbler.
5. Add enough hot water to cover the petals.
6. Cover the tumbler.
7. Wait for 5 to 10 minutes.
8. Filter the coloured liquid.

The filtrate is red rose extract.

Hot-water steps should be carried out under adult supervision.

Testing with Red Rose Extract

Place the extract in separate test tubes.

Add different samples and observe the colour.

Red Rose Indicator Results

Examples

• Lemon juice makes the extract red.
• Soap solution makes it green.
• Sugar solution produces no major change.

Other Flower and Vegetable Indicators

Other natural substances that may act as indicators include:

• Red hibiscus
• Purple cabbage
• Beetroot
• Jamun
• Turmeric

Their colour changes may differ, so observations should be recorded carefully.

Hydrangea Flowers and Soil Nature

Hydrangea flowers can show different colours depending on the nature of the soil.

In acidic soil, the flowers may appear blue.

In basic soil, they may appear pink or red.

This shows how the chemical nature of soil can affect plants.

Gardeners can sometimes alter flower colour by changing soil conditions.

Turmeric as an Indicator

Turmeric is a common natural indicator found in Indian homes.

It is especially useful for testing basic substances.

Preparing Turmeric Paper

1. Take a spoonful of turmeric powder.
2. Add a small amount of water to make a paste.
3. Dip filter paper into the paste.
4. Allow the paper to dry.
5. Cut it into thin strips.

These strips are called turmeric paper.

Turmeric Indicator Results

Limitation of Turmeric

Turmeric can identify a base because it turns red in a basic solution.

However, it cannot distinguish between an acid and a neutral substance.

Both leave turmeric yellow.

Turmeric Stain and Soap

A turmeric stain on cloth may turn reddish when soap is applied.

Soap is basic, so it changes the yellow turmeric colour.

This is the same principle used in secret-message activities.

Secret Message Using Turmeric

Turmeric paper can be used to create hidden or coloured messages.

Writing the Message

A basic solution can be used to write on turmeric-coated paper.

Suitable basic solutions include:

• Soap solution
• Baking soda solution
• Lime water
• Washing powder solution

The writing turns red against the yellow background.

Chapter-Opening Mystery

At the science fair, a message appeared after a liquid was sprayed on white paper.

One possible explanation is:

• The message was written using soap solution.
• The paper was sprayed with turmeric solution.
• The soap reacted with turmeric and produced a red or reddish message.

The order may also be reversed by preparing turmeric paper first and writing with a base.

Other Natural and Olfactory Indicators

Indicators do not always work through colour.

Some indicators show changes through smell.

What Are Olfactory Indicators?

Substances whose odour changes in acidic or basic media are called olfactory indicators.

Examples include:

• Onion
• Vanilla
• Clove oil

The chapter mainly explores onion.

Onion Strip Activity

1. Place chopped onion and clean cloth or filter paper strips in a closed container.
2. Leave them overnight.
3. Remove the strips and check their smell.
4. Add tamarind water to one strip.
5. Add baking soda solution to another.
6. Compare the odour again.

The smell may behave differently in acidic and basic media.

Why Are Olfactory Indicators Useful?

They can help identify substances without relying on colour.

This makes them especially useful for visually impaired learners.

Colour and Smell Indicators

Acharya Prafulla Chandra Ray

Acharya Prafulla Chandra Ray is known as the Father of Modern Indian Chemistry.

He contributed to scientific research and chemistry education in India.

He also:

• Earned a doctorate in chemistry
• Established India’s first pharmaceutical company in 1901
• Wrote about the history of chemistry in India
• Highlighted the work of ancient Indian scientists
• Supported the use of the mother tongue in education
• Worked as a social reformer

His life shows how science, education, society and culture can remain connected.

Neutralisation in Class 7 Science Chapter 2 Revision Notes

When an acid and a base are mixed in suitable amounts, they react with each other.

The acidic and basic effects are reduced or cancelled.

What Is Neutralisation?

A reaction in which an acid reacts with a base to form salt, water and heat is called a neutralisation reaction.

The general equation is:

Acid + Base → Salt + Water + Heat

Lemon Juice and Lime Water Activity

Lemon juice is acidic.

Lime water is basic.

The activity follows these steps:

1. Add diluted lemon juice to a test tube.
2. Add blue litmus solution.
3. The solution turns red.
4. Add lime water drop by drop.
5. Swirl the test tube after each addition.
6. The red colour eventually changes.
7. With sufficient base, the acidic effect is neutralised.
8. Excess base may make the solution basic.

Why Does the Colour Change?

Initially, the lemon juice makes the solution acidic.

As lime water is added, the base reacts with the acid.

When sufficient lime water is present, the acid is neutralised.

Heat During Neutralisation

Neutralisation releases heat.

This is why the reaction is written with heat among the products.

The amount of heat may not always be easy to feel in a small classroom activity.

Neutralisation in Daily Life

Neutralisation is useful in health, farming and environmental protection.

The chapter gives several daily-life examples.

Ant Bite

A red ant injects an acidic liquid into the skin.

This acid contains formic acid and causes:

• Stinging
• Redness
• Pain
• Irritation

Moist baking soda can provide relief because it is basic.

The base neutralises some of the acidic effect.

Acidic Soil

Plants may not grow well if soil becomes too acidic.

Excessive use of some chemical fertilisers can increase soil acidity.

Acidic soil can be treated with:

• Lime
• Slaked lime
• Other suitable basic substances

The base reduces the acidic nature of the soil.

Basic Soil

If soil is too basic, organic matter may be added.

Examples include:

• Manure
• Compost
• Composted leaves

Organic matter releases acids as it decomposes.

These acids help reduce the basic nature of the soil.

Poor Plant Growth in Neutral Soil

Plants may still grow poorly even when the soil is neutral.

The reason may be a lack of nutrients.

Therefore, poor growth does not always prove that soil is acidic or basic.

Industrial Waste

Factories may release acidic or basic waste.

If such waste enters lakes or rivers without treatment, it can harm:

• Fish
• Aquatic plants
• Other organisms
• Water quality

Acidic factory waste can be treated with a suitable base before disposal.

Basic waste can be treated with a suitable acid under controlled industrial conditions.

Industrial treatment should be carried out by trained professionals.

Indicator Comparison Table

Best Indicator for Different Situations

• Use red and blue litmus together to classify acids, bases and neutral substances.
• Use red rose extract when visible colour changes are required.
• Use turmeric to identify bases.
• Use onion when smell-based testing is useful.

Can One Indicator Always Identify All Three Types?

No.

Turmeric identifies bases but cannot separate acids from neutral substances.

A single red litmus strip can identify a base, but it cannot tell whether a no-change sample is acidic or neutral.

Using more than one indicator provides a clearer result.

Testing Unknown Solutions

Suppose three unknown solutions must be identified.

A useful method is:

1. Test each with red litmus.
2. Test each with blue litmus.
3. Use red rose extract or turmeric for confirmation.
4. Compare all observations.
5. Classify each solution.

Example

• Red litmus turns blue: basic
• Blue litmus turns red: acidic
• Neither litmus changes: neutral

Class 7 Science Chapter 2 Summary Notes

Important Terms from Exploring Substances

Acid: A substance that shows acidic properties and turns blue litmus red.

Base: A substance that shows basic properties and turns red litmus blue.

Neutral substance: A substance that is neither acidic nor basic.

Indicator: A substance that helps identify whether a sample is acidic, basic or neutral.

Litmus: A natural indicator obtained from lichens.

Acid-base indicator: A substance that shows different changes in acidic and basic solutions.

Natural indicator: An indicator obtained from a natural source.

Synthetic indicator: An indicator prepared in a laboratory.

Olfactory indicator: An indicator whose smell changes in acidic or basic media.

Neutralisation: A reaction between an acid and a base.

Salt: One of the products formed during neutralisation.

Lime water: A solution of calcium hydroxide in water.

Formic acid: The acid injected during an ant bite.

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