Class 7 , Subject : Science Chapter 2 Acidic , Basic and Neutralization

 

Acid

An acid is a substance that has a sour taste and turns blue litmus paper red. 

Examples:

• Hydrochloric acid
• Sulfuric acid
• Citric acid

Base

A base is a substance that has a bitter taste and feels soapy or slippery, and turns red litmus paper blue.

Examples:

• Sodium hydroxide
• Calcium hydroxide

Extra (Important Point)

• Bases that are soluble in water are called alkalis.

Indicator :-

An indicator is a substance that changes its color to show whether a substance is an acid or a base.

Examples of Indicators:

• Litmus → turns red in acid and blue in base
• Turmeric → turns reddish-brown in base
• Phenolphthalein → colorless in acid and pink in base
• Rose → In acid  dark pink / red and Green in Base

 Olfactory Indicators (Smell-based) 

These indicators change their smell in acids and bases.

Examples:

• Onion
• Vanilla essence
• Clove oil

Behavior:

• In acid → smell remains same
• In base → smell disappears or changes

Classification of Substances: Acidic, Basic, and Neutral

Substances are classified based on their chemical nature and effect on indicators.

1. Acidic Substances

• Substances that produce H⁺ (hydrogen ions) in solution
• Taste: Sour
• Turn blue litmus → red

Examples: Lemon juice, vinegar, curd, tamarind

2. Basic (Alkaline) Substances

• Substances that produce OH⁻ (hydroxide ions) in solution
• Taste: Bitter, feel soapy/slippery
• Turn red litmus → blue

Examples: Soap, baking soda, lime water

3. Neutral Substances

• Substances that have no acidic or basic properties
• Do not change the color of litmus paper

Examples: Pure water, sugar solution, salt solution.

Neutralisation Reaction

A neutralisation reaction is a chemical reaction in which an acid reacts with a base to form salt and water.

General Equation:

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

Example:

• Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

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

  

Explanation:

• Acid provides H⁺ ions
• Base provides OH⁻ ions
• These combine to form water (H₂O)
• Remaining ions form salt

Neutralisation in Daily Life

Neutralisation helps reduce the effect of excess acid or base in many real-life situations:

1. Ant Bite

• Ant sting contains acid (formic acid)
• Relief is obtained by applying a mild base like baking soda or calamine
• This neutralises the acid and reduces irritation

2. Soil Treatment

• Too acidic soil → treated with bases like quicklime or slaked lime
• Too basic soil → treated with organic matter (like compost), which releases acids
• This helps plants grow better by maintaining proper pH 

3. Industrial Waste

• Factory waste may contain harmful acids or bases
• Before releasing into water bodies, it is neutralised
  • Acidic waste → treated with base
  • Basic waste → treated with acid
• This prevents environmental damage

Preparation of Natural Indicators:-

1. Turmeric Indicator

Preparation:

• Take turmeric powder and mix it with water to make a paste
• Soak paper strips in it and dry them (turmeric paper)

Use:

• Acid → No change (yellow)
• Base → Turns reddish-brown

2. China Rose (Hibiscus) Indicator

Preparation:

• Take petals of China rose flower
• Crush and soak them in warm water
• Filter the solution

Use:

• Acid → Dark pink/red
• Base → Green

3. Red Cabbage Indicator

Preparation:

• Chop red cabbage leaves
• Boil them in water
• Cool and filter the liquid

Use:

• Acid → Red/Pink
• Neutral → Purple
• Base → Green/Yellow

Profile of Acharya Prafulla Chandra Ray

Acharya P.C. Ray was a famous Indian scientist, teacher, and entrepreneur, known as the “Father of Indian Chemistry.”

Basic Information

• Full Name: Prafulla Chandra Ray
• Born: 2 August 1861
• Place: Khulna (now in Bangladesh)
• Died: 16 June 1944

Education & Career

• Studied at University of Calcutta
• Later went to University of Edinburgh for higher studies
• Became a professor of chemistry at Presidency College

Major Contributions

• Discovered Mercurous Nitrite, an important chemical compound
• Founded Bengal Chemicals & Pharmaceuticals (India’s first chemical company)
• Promoted scientific research and industrial development in India

Books & Work

• Wrote the famous book: A History of Hindu Chemistry

Importance

• Played a key role in developing modern chemistry education in India
• Inspired many students to pursue science
• Supported Swadeshi movement by encouraging Indian industries