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

Acids, bases, and neutral substances are part of many familiar materials, but telling them apart requires more than simply looking at or touching them. Indicators such as litmus, turmeric, and China rose help identify their nature through visible colour changes. Concepts such as pH and neutralisation also explain how acidic and basic substances behave and interact.

PW Class 7 Science Chapter 2 Notes help you revise these concepts with important examples, reactions, and everyday applications. From acid rain and salt formation to neutralisation in daily life, the notes cover the key points you need to recall before textbook questions and school assessments.

What Are Acids and Bases?

Acids and bases have different properties and can be found in several natural and household substances.

Acids

• Acids are chemical substances that generally have a sour taste.

• Lemon, orange, curd, and vinegar contain acids.

• Acids that occur naturally are called natural acids.

• Acidic substances contain acids.

Bases

• Bases are chemical substances that generally have a bitter taste and a soapy texture.

• They are found in substances such as soap, washing powder, bleach, and ammonia.

• Bases are also called alkaline substances.

• Basic substances contain bases.

Neutral Substances

A neutral substance is neither acidic nor basic in nature.

What Are Indicators?

You cannot safely identify every substance by tasting or touching it. Indicators are used to determine whether a substance is acidic or basic.

• Indicators show the nature of a substance through a colour change.

• Litmus, turmeric, and China rose are examples of natural indicators.

• Natural indicators are obtained from sources found in nature.

How Do Litmus, Turmeric, and China Rose Work?

Litmus

Litmus is a natural indicator obtained from lichens. It is available as a solution and as red and blue litmus paper.

Turmeric

• Turmeric can be mixed with water to make a paste.

• The paste can be applied to blotting paper and dried to make turmeric paper.

• Turmeric solution can also be used as an indicator.

China Rose

China rose petals are kept in warm water to obtain a coloured solution. This solution can be used to test whether substances are acidic or basic.

What Is the pH Scale?

The pH scale indicates how acidic or basic a substance is.

• The pH scale ranges from 1 to 14.

• A pH value of 1 represents a highly acidic substance.

• A pH value of 14 represents a highly basic substance.

• A pH value of 7 represents a neutral substance.

• pH can generally be determined using pH strips or solutions.

What Is Acid Rain?

Rainwater containing increased amounts of acids is called acid rain.

• Air pollutants such as nitrogen dioxide, sulphur dioxide, and carbon dioxide contribute to acid formation.

• These pollutants can mix with rainwater to form acids.

• Acid rain can harm vegetation, animal life, and buildings.

Why Are Acids Stored in Glass Containers?

Acids are generally stored in glass containers rather than metal containers because they can react with metals and cause them to erode. Glass does not generally react with acids, making it suitable for their storage.

What Is Neutralisation?

Neutralisation is a chemical reaction in which an acid and a base react to reduce their acidic and basic effects.

• The reaction produces salt and water.

• Neutralisation also produces heat, which can raise the temperature of the mixture.

• Phenolphthalein is a synthetic indicator used in neutralisation reactions.

• Phenolphthalein is pink in a basic solution and becomes colourless in an acidic solution.

Formation of Salt

Acid + Base → Water + Salt

HCl + NaOH → H₂O + NaCl

How Does Neutralisation Help in Everyday Life?

Neutralisation has several practical applications.

Indigestion

The stomach produces hydrochloric acid to help digest food. Excess acid can cause indigestion or hyperacidity. Basic substances called antacids help neutralise the excess acid. Milk of magnesia is one example.

Ant Bite

An ant bite causes irritation because of formic acid. Basic substances such as baking soda, calamine solution, or zinc carbonate can help neutralise its effect.

Soil Treatment

• Chemical fertilisers can make soil acidic.

• Quicklime (calcium oxide) or slaked lime (calcium hydroxide) can be used to treat acidic soil.

• Basic soil can be treated by adding organic substances that release acids as they decompose.

Factory Waste

Factory waste may be acidic and should not be released directly into rivers or streams. Bases can be added to neutralise the waste before disposal and reduce its harmful effect on aquatic life.

Class 7 Science Chapter 2 Notes PDF

The Class 7 Science Chapter 2 Notes PDF covers acids, bases, indicators, pH, acid rain, neutralisation, salt formation, and their everyday applications for quick revision.

How Can Physics Wallah Class 7 Science Chapter 2 Notes Help You Revise?

Physics Wallah Class 7 Science Chapter 2 Notes make it easier to revisit the concepts covered in Exploring Substances: Acidic, Basic, and Neutral. They help you recall important properties, reactions, indicator tests, and real-life applications without going through lengthy explanations.

• Recall Acids and Bases: Revise their properties along with familiar examples.

• Learn Indicator Tests: Remember the colour changes shown by litmus, turmeric, and China rose.

• Review the pH Scale: Understand how pH values indicate acidic, neutral, or basic substances.

• Revise Neutralisation: Recall how acids and bases react to form salt and water.

• Explore Everyday Uses: Revisit neutralisation in indigestion, ant bites, soil treatment, and factory waste.

• Understand Acid Rain: Remember its formation, causes, and effects.

• Strengthen Exam Preparation: Go through key definitions, examples, reactions, and applications before assessments.

From identifying substances to understanding their reactions, the chapter connects basic chemistry with several everyday situations. Exploring Substances: Acidic, Basic, and Neutral introduces concepts such as indicators, pH, acid rain, and neutralisation. These notes help you revisit the important points and strengthen your understanding before school exams.

Related Resources

• CBSE Class 7 Science Syllabus

• CBSE Class 7 Science Notes

• CBSE Class 7 Science NCERT Solutions

• CBSE Class 7 Science Important Questions

• CBSE Class 7 Science Sample Papers