Acids Bases and Salts Class 7 PDF: Complete NCERT Guide

Introduction to Acids, Bases and Salts

In Class 7 Science, the chapter on acids, bases and salts introduces you to substances found in everyday life. Acids are compounds that taste sour and have a pH less than 7. Bases are substances that feel slippery and have a pH greater than 7. Salts are formed when acids react with bases in a neutralisation reaction.

Key definitions:

• Acids: Substances that release hydrogen ions ($H^+$) in water.
• Bases: Substances that release hydroxide ions ($OH^-$) in water.
• Salts: Ionic compounds formed from the neutralisation of acids and bases.

This chapter is essential for understanding chemical reactions and properties of common substances.

Properties and Examples of Acids and Bases

Acids and bases have distinct properties that help us identify them:

Common Examples:

• Acids: Vinegar (acetic acid), lemon juice (citric acid), hydrochloric acid (in stomach)
• Bases: Baking soda, soap, lime water

Understanding these properties helps you classify substances and predict their behaviour in reactions.

The pH Scale: Measuring Acidity and Alkalinity

The pH scale ranges from 0 to 14 and measures how acidic or basic a substance is:

• pH less than 7: Acidic
• pH equal to 7: Neutral
• pH greater than 7: Basic (alkaline)

For example, lemon juice has a pH around 2 (acidic), pure water is neutral with pH 7, and soap solution has a pH around 12 (basic).

Using pH indicators:

• Litmus paper changes colour based on pH
• Universal indicator shows a range of colours corresponding to pH values

Formula for pH:

$$pH = -\log[H^+]$$

where $[H^+]$ is the concentration of hydrogen ions in moles per litre.

Neutralisation Reaction and Formation of Salts

Neutralisation occurs when an acid reacts with a base to form salt and water:

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

For example:

$$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$$

Here, hydrochloric acid reacts with sodium hydroxide (a base) to form sodium chloride (common salt) and water.

Types of salts:

• Table salt (NaCl)
• Baking soda (NaHCO₃)
• Epsom salt (MgSO₄)

Salts have different uses in daily life and industry.

Common Uses of Acids, Bases and Salts

Acids, bases and salts are part of many household and industrial products:

• Acids: Used in cleaning agents, food preservation (vinegar), and digestion (stomach acid)
• Bases: Used in soaps, detergents, antacids to neutralise stomach acid
• Salts: Used in cooking (table salt), fertilizers, medicines

Understanding their properties helps you use these substances safely and effectively.

Solved Example: Identifying Acids and Bases Using Litmus Test

Question: A substance turns blue litmus red. Is it an acid or a base?

Solution:

• Blue litmus paper turns red in the presence of an acid.
• Therefore, the substance is an acid.

Another example:

If red litmus turns blue, the substance is a base.

This simple test helps you quickly identify acids and bases in the lab or at home.