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

Definition and Characteristics of Acids

Acids are substances that release hydrogen ions ($H^+$) when dissolved in water. They have a sour taste and can turn blue litmus paper red. Common examples include lemon juice (citric acid) and vinegar (acetic acid).

Key properties of acids:

• Sour taste
• Corrosive in concentrated form
• React with metals to produce hydrogen gas
• Conduct electricity in aqueous solution

Example: When hydrochloric acid (HCl) dissolves in water, it dissociates as:

$$\mathrm{HCl} \rightarrow \mathrm{H^+} + \mathrm{Cl^-}$$

This release of $H^+$ ions is what makes the solution acidic.

Definition and Characteristics of Bases

Bases are substances that release hydroxide ions ($OH^-$) in water. They usually have a bitter taste and a slippery feel. Bases turn red litmus paper blue.

Key properties of bases:

• Bitter taste
• Slippery texture
• Can be corrosive
• Conduct electricity in aqueous solution

Example: Sodium hydroxide (NaOH) dissociates in water as:

$$\mathrm{NaOH} \rightarrow \mathrm{Na^+} + \mathrm{OH^-}$$

This release of $OH^-$ ions makes the solution basic or alkaline.

Understanding Salts: Formation and Uses

Salts are compounds formed when acids react with bases in a neutralisation reaction. They consist of positive ions (from the base) and negative ions (from the acid).

Neutralisation reaction:

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

Example: Hydrochloric acid reacts with sodium hydroxide:

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

Here, sodium chloride (NaCl) is the salt formed.

Uses of salts:

• Table salt (NaCl) is used in cooking
• Baking soda (NaHCO₃) is used in baking
• Epsom salt (MgSO₄) is used in medicine

The pH Scale: Measuring Acidity and Alkalinity

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

Examples:

• Lemon juice has a pH around 2 (acidic)
• Pure water has a pH of 7 (neutral)
• Soap solution has a pH around 12 (basic)

A lower pH means higher acidity, and a higher pH means higher alkalinity.

Common Indicators for Acids and Bases

Indicators help us identify whether a substance is acidic or basic by changing colour.

Common indicators:

• Litmus paper:
• Blue litmus turns red in acids
• Red litmus turns blue in bases
• Phenolphthalein: Colourless in acids, pink in bases
• Methyl orange: Red in acids, yellow in bases

Using these indicators is a simple way to test substances at home or in the lab.

Everyday Examples and Importance of Acids, Bases and Salts

Acids, bases, and salts are part of our daily lives:

• Acids: Found in citrus fruits, sour foods, and stomach digestion
• Bases: Present in soaps, detergents, and antacids
• Salts: Used in food seasoning, medicines, and industrial processes

Understanding these helps in practical applications like cooking, cleaning, and health care.

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