What is Acids, Bases and Salts Class 7: Complete Science Guide

Definition of Acids, Bases and Salts for Class 7

In Class 7 Science, acids, bases, and salts are fundamental chemical substances studied under the NCERT syllabus. Here's how each is defined:

• Acids: Substances that release hydrogen ions ($H^+$) when dissolved in water. They taste sour and can turn blue litmus paper red.
• Bases: Substances that release hydroxide ions ($OH^-$) in water. They taste bitter, feel slippery, and turn red litmus paper blue.
• Salts: Compounds formed when an acid reacts with a base, usually resulting in a neutral substance.

Understanding these definitions helps students grasp chemical reactions and properties essential for further studies.

Properties and Examples of Acids and Bases

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

Properties of Acids:

• Sour taste (e.g., lemon juice, vinegar)
• Corrosive in concentrated form
• React with metals to produce hydrogen gas
• Turn blue litmus paper red

Properties of Bases:

• Bitter taste (e.g., baking soda, soap)
• Slippery to touch
• Do not react with metals like acids
• Turn red litmus paper blue

Examples:

These properties help in identifying acids and bases in everyday life.

The pH Scale: Measuring Acidity and Alkalinity

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

• pH less than 7: Acidic solution
• pH equal to 7: Neutral solution (like pure water)
• pH greater than 7: Basic (alkaline) solution

For example, lemon juice has a pH around 2 (strong acid), while soap solution has a pH around 12 (strong base).

Why pH is important?

• It helps determine the strength of acids and bases.
• It is used in agriculture, medicine, and water quality testing.

Worked Example: If a solution has pH 3, it is acidic because the pH is less than 7.

Using pH indicators like litmus paper or universal indicator solution helps identify the pH value practically.

How Salts are Formed: Neutralisation Reaction

Salts are formed when acids react with bases in a process called neutralisation. This reaction produces salt and water.

General formula:

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

Example: Hydrochloric acid reacts with sodium hydroxide:

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

• Here, sodium chloride (NaCl) is the salt formed.
• Water ($H_2O$) is also produced.

This reaction is important in daily life, such as in antacid tablets that neutralise stomach acid.

Common Uses of Acids, Bases and Salts in Daily Life

Acids, bases, and salts are present in many household items and have practical uses:

• Acids:
• Vinegar (acetic acid) for cooking and cleaning
• Citric acid in fruits for flavor
• Stomach acids help digestion

• Bases:
• Baking soda for baking and cleaning
• Soap and detergents for washing
• Lime water in construction

• Salts:
• Table salt (sodium chloride) for food
• Epsom salt in bath salts
• Washing soda in cleaning agents

Knowing these uses helps students relate chemistry to real-world applications.

Identifying Acids and Bases Using Litmus Test

The litmus test is a simple method to identify whether a substance is an acid or a base:

• Blue litmus paper: Turns red in acidic solutions
• Red litmus paper: Turns blue in basic solutions

Steps to perform the test: 1. Dip blue litmus paper into the solution. If it turns red, the solution is acidic. 2. Dip red litmus paper into the solution. If it turns blue, the solution is basic. 3. If no change occurs, the solution is neutral.

This test is widely used in school laboratories and helps students quickly classify substances.