Chapter-Chapter 2. Exploring Substances – Acidic, Basic and Neutral Science Curiosity class 7 in english Medium CBSE Notes

Chapter 2: Exploring Substances – Acidic, Basic and Neutral

Introduction: In our daily life, we come across many different substances such as lemon juice, soap, milk, vinegar, and water. Some of these taste sour, some are bitter, and some do not have any specific taste. These differences are not just in taste but also in their chemical nature.

To understand these differences, scientists have classified substances into three main categories — acidic, basic, and neutral. This classification helps us study their properties and uses in a better way.

What are Acids? Acids are substances that usually have a sour taste. They are commonly found in many natural sources like fruits.

• Lemon contains citric acid
• Vinegar contains acetic acid
• Curd contains lactic acid

Example: When you taste lemon, you feel sourness. This sour taste is due to the presence of acid.

What are Bases? Bases are substances that usually have a bitter taste and feel soapy when touched.

• Soap solution
• Washing soda
• Baking soda

Example: Soap feels slippery when we touch it. This shows it is basic in nature.

What are Neutral Substances? Neutral substances are those which are neither acidic nor basic.

• Pure water
• Salt solution (in some cases)

These substances do not show properties of acids or bases.

Important Properties of Acids:

• Sour in taste
• Turn blue litmus paper red
• React with metals to produce hydrogen gas

Important Properties of Bases:

• Bitter in taste
• Turn red litmus paper blue
• Feel soapy to touch

Key Point: Taste should not be used to identify acids or bases directly as some substances can be harmful.

Indicators (संसूचक): Indicators are special substances that help us identify whether a substance is acidic, basic, or neutral. They show different colours when added to acids and bases, which makes identification easy.

Indicators are very useful because we cannot always taste or touch substances to identify their nature. Some substances can be harmful, so indicators provide a safe method.

Types of Indicators:

• Natural Indicators: These are obtained from natural sources like plants.
• Artificial Indicators: These are made in laboratories.

Natural Indicators:

• Litmus: It is obtained from lichens. It turns red in acids and blue in bases.
• Turmeric: It remains yellow in acids but turns reddish-brown in bases.
• China rose: It turns dark pink in acids and green in bases.

Example: When turmeric is applied on soap, it changes colour. This shows that soap is basic.

Artificial Indicators:

• Phenolphthalein: It is colourless in acids and turns pink in bases.
• Methyl orange: It turns red in acids and yellow in bases.

These indicators are widely used in laboratories for accurate results.

Neutralization Reaction: When an acid reacts with a base, they neutralize each other and form salt and water. This process is called neutralization.

Acid + Base → Salt + Water

Example: When hydrochloric acid reacts with sodium hydroxide, it forms sodium chloride (salt) and water.

This reaction is very useful in daily life and helps in controlling harmful effects of acids and bases.

Applications of Neutralization in Daily Life:

• Indigestion: Excess acid in the stomach causes pain. It is neutralized by taking antacids.
• Soil Treatment: Acidic soil is treated with bases like lime to improve fertility.
• Tooth Decay: Toothpaste (basic in nature) neutralizes acids produced by bacteria.
• Insect Bites: Ant bite (acidic) is relieved by applying base like baking soda.

Key Point: Neutralization is an important process that helps maintain balance in nature and in our body.

Strength of Acids and Bases: Not all acids and bases are equally strong. Some are strong while others are weak. The strength depends on how effectively they show their acidic or basic properties.

Strong acids show more effect and can be harmful, while weak acids show less effect. Similarly, strong bases are more reactive compared to weak bases.

Example: Hydrochloric acid is a strong acid, while citric acid found in lemon is a weak acid.

However, at this level, we mainly identify acids and bases based on their properties and indicators rather than their strength.

Safety Precautions while Handling Acids and Bases: Since some acids and bases can be harmful, it is important to handle them carefully.

• Do not taste chemicals directly.
• Avoid direct contact with skin.
• Use protective equipment like gloves if needed.
• Always follow instructions during experiments.

These precautions help prevent accidents and ensure safe use of chemicals.

Acids and Bases in Daily Life: Acids and bases are not only found in laboratories but are also present in many things we use daily.

• Curd and fruits contain acids.
• Soap and detergents are basic in nature.
• Toothpaste is basic and helps protect teeth.
• Soft drinks contain mild acids.

This shows that acids and bases play an important role in our daily life.

Common Misconceptions:

• All acids are dangerous — ❌ (Some are mild like lemon juice)
• All bases are safe — ❌ (Strong bases can be harmful)
• Taste is the best way to identify — ❌ (Unsafe method)

Key Point: Always use indicators and proper methods to identify acids and bases instead of relying on taste or touch.

Final Understanding: Acids, bases, and neutral substances are an important part of chemistry. Their identification using indicators and their reactions like neutralization help us understand many natural and daily life processes.

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