Class 10 Science — Chapter 2
Acids, Bases and Salts (Complete Detailed Revision Notes)
1. Introduction to Acids and Bases
====================================
* Acids (अम्ल):
- Taste: Sour (खट्टा).
- Litmus Test: Turns Blue litmus paper to Red.
- Ions: Releases Hydrogen ions (H+) in water.
- Examples: Strong Acids (HCl, H₂SO₄), Weak Acids (Acetic acid, Citric acid).
* Bases (क्षारक):
- Taste: Bitter (कड़वा) and feel soapy/slippery to touch.
- Litmus Test: Turns Red litmus paper to Blue.
- Ions: Releases Hydroxide ions (OH-) in water.
- Examples: Strong Bases (NaOH, KOH), Weak Bases (NH₄OH).
- Note: Water-soluble bases are called Alkalis.
2. Indicators (सच
ू क) & Their Changes
========================================
Indicators change their color or odor in acidic or basic mediums.
1. Litmus Solution: Natural indicator (purple color from Lichen plant).
- Acid = Red, Base = Blue.
2. Turmeric (हल्दी): Natural indicator.
- Acid = No change, Base = Reddish-brown.
3. Phenolphthalein: Synthetic indicator.
- Acid = Colorless, Base = Pink.
4. Methyl Orange: Synthetic indicator.
- Acid = Red, Base = Yellow.
5. Olfactory Indicators (गंधीय सचू क): Change their smell. Example: Onion, Vanilla, Clove oil.
3. Chemical Properties of Acids and Bases
=============================================
* Reaction with Metals:
- Acid + Metal → Salt + Hydrogen Gas
- Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑ (Pop sound test for H₂ gas).
* Neutralization Reaction (उदासीनीकरण अभिक्रिया):
- When an acid and a base react together, they cancel each other's effect and form salt and
water.
- Acid + Base → Salt + Water
- Example: HCl (Acid) + NaOH (Base) → NaCl (Salt) + H₂O (Water)
* Reaction of Metallic Oxides:
- Metallic oxides are Basic in nature.
- Metal Oxide + Acid → Salt + Water
4. The pH Scale (Strength of Acids/Bases)
, ===========================================
* pH scale ranges from 0 to 14 (p stands for 'potenz' meaning power).
- pH = 7 -> Neutral solution (Pure Water).
- pH < 7 -> Acidic solution (Lower pH means stronger acid).
- pH > 7 -> Basic solution (Higher pH means stronger base).
* Importance of pH in Daily Life:
1. Our body works efficiently between pH 7.0 to 7.8.
2. Acid Rain: When rain water pH is less than 5.6. It damages aquatic life.
3. Tooth Decay: Starts when mouth pH falls below 5.5. (Bacteria produce acid from sugar).
4. Soil pH: Plants need a specific pH range for healthy growth.
5. Honeybee Sting: Releases methanoic acid causing pain. Baking soda (base) gives relief.
5. Important Chemical Compounds from Common Salt
====================================================
1. Sodium Hydroxide / Caustic Soda (NaOH):
- Prepared by Chlor-Alkali process (passing electricity through brine/NaCl solution).
- Equation: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
2. Bleaching Powder (CaOCl₂):
- Formed by action of chlorine on dry slaked lime [Ca(OH)₂].
- Uses: Disinfecting drinking water, bleaching cotton in textile industries.
3. Baking Soda (NaHCO₃ - Sodium Hydrogen Carbonate):
- Uses: Making crispy pakoras, ingredient in antacids, fire extinguishers.
4. Washing Soda (Na₂CO₃·10H₂O):
- Formed by heating baking soda and recrystallization.
- Uses: Cleaning agent in houses, removing permanent hardness of water.
5. Plaster of Paris / POP (CaSO₄·½H₂O):
- Obtained by heating Gypsum (CaSO₄·2H₂O) at 373K.
- Uses: Supporting fractured bones, making toys and decorative materials.
6. Top Important Board Exam Questions
=========================================
Q1. Why does an aqueous solution of an acid conduct electricity?
Ans: The aqueous solution of an acid conducts electricity because it dissociates into
hydrogen ions (H+), which are responsible for the flow of electric current.
Q2. Why does dry HCl gas not change the color of dry litmus paper?
Ans: Dry HCl gas does not form H+ ions in the absence of water. Since color change
depends on H+ ions, dry litmus paper does not change color.
Q3. What is the difference between a strong acid and a weak acid?
Acids, Bases and Salts (Complete Detailed Revision Notes)
1. Introduction to Acids and Bases
====================================
* Acids (अम्ल):
- Taste: Sour (खट्टा).
- Litmus Test: Turns Blue litmus paper to Red.
- Ions: Releases Hydrogen ions (H+) in water.
- Examples: Strong Acids (HCl, H₂SO₄), Weak Acids (Acetic acid, Citric acid).
* Bases (क्षारक):
- Taste: Bitter (कड़वा) and feel soapy/slippery to touch.
- Litmus Test: Turns Red litmus paper to Blue.
- Ions: Releases Hydroxide ions (OH-) in water.
- Examples: Strong Bases (NaOH, KOH), Weak Bases (NH₄OH).
- Note: Water-soluble bases are called Alkalis.
2. Indicators (सच
ू क) & Their Changes
========================================
Indicators change their color or odor in acidic or basic mediums.
1. Litmus Solution: Natural indicator (purple color from Lichen plant).
- Acid = Red, Base = Blue.
2. Turmeric (हल्दी): Natural indicator.
- Acid = No change, Base = Reddish-brown.
3. Phenolphthalein: Synthetic indicator.
- Acid = Colorless, Base = Pink.
4. Methyl Orange: Synthetic indicator.
- Acid = Red, Base = Yellow.
5. Olfactory Indicators (गंधीय सचू क): Change their smell. Example: Onion, Vanilla, Clove oil.
3. Chemical Properties of Acids and Bases
=============================================
* Reaction with Metals:
- Acid + Metal → Salt + Hydrogen Gas
- Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑ (Pop sound test for H₂ gas).
* Neutralization Reaction (उदासीनीकरण अभिक्रिया):
- When an acid and a base react together, they cancel each other's effect and form salt and
water.
- Acid + Base → Salt + Water
- Example: HCl (Acid) + NaOH (Base) → NaCl (Salt) + H₂O (Water)
* Reaction of Metallic Oxides:
- Metallic oxides are Basic in nature.
- Metal Oxide + Acid → Salt + Water
4. The pH Scale (Strength of Acids/Bases)
, ===========================================
* pH scale ranges from 0 to 14 (p stands for 'potenz' meaning power).
- pH = 7 -> Neutral solution (Pure Water).
- pH < 7 -> Acidic solution (Lower pH means stronger acid).
- pH > 7 -> Basic solution (Higher pH means stronger base).
* Importance of pH in Daily Life:
1. Our body works efficiently between pH 7.0 to 7.8.
2. Acid Rain: When rain water pH is less than 5.6. It damages aquatic life.
3. Tooth Decay: Starts when mouth pH falls below 5.5. (Bacteria produce acid from sugar).
4. Soil pH: Plants need a specific pH range for healthy growth.
5. Honeybee Sting: Releases methanoic acid causing pain. Baking soda (base) gives relief.
5. Important Chemical Compounds from Common Salt
====================================================
1. Sodium Hydroxide / Caustic Soda (NaOH):
- Prepared by Chlor-Alkali process (passing electricity through brine/NaCl solution).
- Equation: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
2. Bleaching Powder (CaOCl₂):
- Formed by action of chlorine on dry slaked lime [Ca(OH)₂].
- Uses: Disinfecting drinking water, bleaching cotton in textile industries.
3. Baking Soda (NaHCO₃ - Sodium Hydrogen Carbonate):
- Uses: Making crispy pakoras, ingredient in antacids, fire extinguishers.
4. Washing Soda (Na₂CO₃·10H₂O):
- Formed by heating baking soda and recrystallization.
- Uses: Cleaning agent in houses, removing permanent hardness of water.
5. Plaster of Paris / POP (CaSO₄·½H₂O):
- Obtained by heating Gypsum (CaSO₄·2H₂O) at 373K.
- Uses: Supporting fractured bones, making toys and decorative materials.
6. Top Important Board Exam Questions
=========================================
Q1. Why does an aqueous solution of an acid conduct electricity?
Ans: The aqueous solution of an acid conducts electricity because it dissociates into
hydrogen ions (H+), which are responsible for the flow of electric current.
Q2. Why does dry HCl gas not change the color of dry litmus paper?
Ans: Dry HCl gas does not form H+ ions in the absence of water. Since color change
depends on H+ ions, dry litmus paper does not change color.
Q3. What is the difference between a strong acid and a weak acid?
