Class 10 Science Chapter 2: Acids, Bases and Salts | Complete NCERT Notes with Exam Focus
📊 Chapter Difficulty & Exam Importance
Exam Weight: 8-10 marks (GUARANTEED major question + practicals)
Difficulty Level:
- Understanding Concepts: ✅ EASY (Real-life examples make it simple)
- Chemical Reactions: ⚡ MEDIUM (Need to memorize equations)
- Numericals (pH): ✅ EASY (Simple calculations)
Board Exam Pattern:
- 1-2 mark questions: Definitions, properties, uses, pH values
- 3 mark questions: Chemical reactions, differences, practical applications
- 5 mark questions: Preparation of compounds, properties with reactions, uses
- Practical-based questions: 3-5 marks (indicators, neutralization, preparation of salts)
Why This Chapter is Super Important:
- Easiest chemistry chapter to score full marks
- Directly linked to practicals (guaranteed questions!)
- Daily life applications (easy to remember)
- Simple reactions (no complex mechanisms)
- pH scale questions are free marks
- Questions are HIGHLY predictable
🎯 What You'll Master in This Chapter
By the end of these notes, you'll confidently:
- ✅ Identify acids, bases, and salts instantly
- ✅ Write all chemical reactions perfectly
- ✅ Understand pH scale completely
- ✅ Explain real-life applications
- ✅ Prepare for practical exams
- ✅ Solve numerical problems easily
- ✅ Score 9-10 marks guaranteed!
🍋 Part 1: Introduction - Acids, Bases and Salts Around Us
What Are These Substances?
Look around your kitchen right now:
- 🍋 Lemon juice - ACID
- 🧼 Soap - BASE
- 🧂 Table salt - SALT
- 🥛 Milk of magnesia - BASE
- 🍊 Orange juice - ACID
- 🧴 Baking soda - BASE
Everything you taste, use, or touch belongs to one of these three categories!
Historical Discovery:
- Ancient people noticed sour taste = acids
- Bitter, slippery substances = bases
- When mixed together = salt + water
Early Understanding of Acids and Bases
Traditional Definitions (Before we knew chemistry):
ACIDS:
- Sour taste (like lemon, tamarind)
- Turn blue litmus red
- React with metals to release hydrogen
BASES:
- Bitter taste (like soap, neem)
- Slippery/soapy feel
- Turn red litmus blue
But these definitions had PROBLEMS:
- Can't taste dangerous acids (like sulfuric acid!)
- Not scientific enough
- Didn't explain WHY they behaved this way
That's why scientists created better definitions!
🧪 Part 2: Scientific Definitions
Arrhenius Theory (First Scientific Definition)
Svante Arrhenius (1884) said:
ACID: Substance that releases H⁺ ions when dissolved in water
HCl → H⁺ + Cl⁻
(Hydrochloric acid gives hydrogen ions)BASE: Substance that releases OH⁻ ions when dissolved in water
NaOH → Na⁺ + OH⁻
(Sodium hydroxide gives hydroxide ions)Simple Rule:
- H⁺ ions = Acidic
- OH⁻ ions = Basic
This explained a LOT but still had limitations (what about acids/bases not in water?)
Common Acids and Their Sources
Natural Acids (Found in Nature):
| Acid | Chemical Formula | Found In |
|---|---|---|
| Acetic acid | CH₃COOH | Vinegar, sour curd |
| Citric acid | C₆H₈O₇ | Citrus fruits (lemon, orange) |
| Lactic acid | C₃H₆O₃ | Sour milk, yogurt |
| Tartaric acid | C₄H₆O₆ | Tamarind, grapes |
| Formic acid | HCOOH | Ant sting, bee sting |
| Oxalic acid | C₂H₂O₄ | Tomatoes, spinach |
Mineral Acids (Made in Labs/Industries):
| Acid | Chemical Formula | Common Name | Use |
|---|---|---|---|
| Hydrochloric acid | HCl | Muriatic acid | Stomach acid, cleaning |
| Sulfuric acid | H₂SO₄ | Oil of vitriol | Car batteries, fertilizers |
| Nitric acid | HNO₃ | Aqua fortis | Fertilizers, explosives |
Exam Tip: Questions often ask "Name the acid found in lemon" - Answer: Citric acid!
Common Bases and Their Sources
Natural Bases:
| Base | Chemical Formula | Found In/Use |
|---|---|---|
| Calcium hydroxide | Ca(OH)₂ | Lime water, whitewash |
| Magnesium hydroxide | Mg(OH)₂ | Milk of magnesia (antacid) |
| Ammonium hydroxide | NH₄OH | Window cleaners |
Strong Bases (Alkalis - soluble in water):
| Base | Chemical Formula | Common Name | Use |
|---|---|---|---|
| Sodium hydroxide | NaOH | Caustic soda | Soap making, drain cleaners |
| Potassium hydroxide | KOH | Caustic potash | Soap, batteries |
| Calcium hydroxide | Ca(OH)₂ | Slaked lime | Whitewash, neutralizing acids |
Important Term - ALKALI:
- All alkalis are bases
- But all bases are NOT alkalis
- Alkali = Base that dissolves in water
Example:
- NaOH = Alkali (dissolves in water)
- Cu(OH)₂ = Base but NOT alkali (doesn't dissolve)
🌈 Part 3: Indicators - Detecting Acids and Bases
What Are Indicators?
Definition: Substances that show different colors in acidic and basic solutions
Why We Need Them:
- Can't taste unknown substances (dangerous!)
- Need quick, safe way to identify
- Show exact nature of solution
Natural Indicators
1. Litmus Solution
- Source: Extracted from lichens (a plant)
- Most common indicator in labs
| Solution Type | Red Litmus | Blue Litmus |
|---|---|---|
| Acidic | Stays red | Turns red |
| Basic | Turns blue | Stays blue |
| Neutral | No change | No change |
Memory Trick: "Blue litmus for Bases, Red litmus for Really acids"
2. Turmeric (Haldi)
- Yellow in acid/neutral
- Reddish-brown in base
- Fun fact: When you wash soap with turmeric stain, it turns red!
3. Red Cabbage Extract
- Red/Pink in acids
- Green/Yellow in bases
4. China Rose (Gudhal/Hibiscus)
- Dark pink/magenta in acid
- Green in base
5. Phenolphthalein (Lab Indicator)
- Colorless in acid
- Pink in base
- Most important for titration experiments!
6. Methyl Orange (Lab Indicator)
- Red in acid
- Yellow in base
Universal Indicator
Special mixture of many indicators!
What it does:
- Shows DIFFERENT colors for DIFFERENT pH values
- Not just "acid or base" - shows HOW acidic or HOW basic!
Color Chart:
- Red/Orange = Strong acid (pH 0-4)
- Yellow = Weak acid (pH 5-6)
- Green = Neutral (pH 7)
- Blue = Weak base (pH 8-10)
- Purple/Violet = Strong base (pH 11-14)
Exam Favorite: Questions show universal indicator color, ask pH value!
⚗️ Part 4: Chemical Properties of Acids
Property 1: Reaction with Metals
General Reaction:
Acid + Metal → Salt + Hydrogen gas (H₂↑)Examples:
1. Zinc + Hydrochloric acid:
Zn + 2HCl → ZnCl₂ + H₂↑
(Zinc chloride + Hydrogen gas)2. Magnesium + Sulfuric acid:
Mg + H₂SO₄ → MgSO₄ + H₂↑
(Magnesium sulfate + Hydrogen gas)Test for Hydrogen Gas:
- Bring burning splinter near gas
- Produces "pop" sound
- Burns with pale blue flame
Important Notes:
- NOT all metals react (Gold, Silver, Platinum don't react)
- More reactive metal = faster reaction
- Always produces HYDROGEN gas (characteristic of acids!)
Exam Trick: If question asks "which gas evolved?", answer is ALWAYS hydrogen when metal + acid!
Property 2: Reaction with Metal Carbonates/Bicarbonates
General Reaction:
Acid + Carbonate → Salt + Water + Carbon dioxide (CO₂↑)Examples:
1. Sodium carbonate + Hydrochloric acid:
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑2. Calcium carbonate (Marble/Limestone) + Sulfuric acid:
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂↑3. Sodium bicarbonate (Baking soda) + Acetic acid (Vinegar):
NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂↑
(This is the volcano experiment in science fairs!)Test for Carbon Dioxide:
- Pass gas through lime water
- Lime water turns milky/cloudy
- Chemical reason: CO₂ reacts with Ca(OH)₂ to form white CaCO₃
Ca(OH)₂ + CO₂ → CaCO₃ ↓ + H₂O
(Lime water) (White precipitate)Practical Exam Alert: This is ALWAYS asked! Remember lime water test!
Property 3: Reaction with Bases (Neutralization)
General Reaction:
Acid + Base → Salt + WaterThis is called NEUTRALIZATION - acid and base cancel each other!
Examples:
1. Hydrochloric acid + Sodium hydroxide:
HCl + NaOH → NaCl + H₂O
(Common salt + Water)2. Sulfuric acid + Potassium hydroxide:
H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
(Potassium sulfate + Water)What's Happening:
H⁺ (from acid) + OH⁻ (from base) → H₂O (water!)- H⁺ and OH⁻ neutralize each other
- Result is neutral water
- The remaining ions form salt
Real-Life Application:
- Antacids contain bases (Mg(OH)₂) to neutralize excess stomach acid (HCl)
- Bee sting (contains formic acid) → Apply baking soda (base) to neutralize
- Soil treatment: Acidic soil → Add lime (Ca(OH)₂) to neutralize
Property 4: Reaction with Metal Oxides
General Reaction:
Acid + Metal Oxide → Salt + WaterExample:
CuO + 2HCl → CuCl₂ + H₂O
(Copper oxide + HCl → Copper chloride + Water)Why This Matters:
- Metal oxides are BASIC in nature
- React similar to bases
- This proves: Metal oxides = Basic oxides
🧴 Part 5: Chemical Properties of Bases
Property 1: Reaction with Acids (Neutralization)
Already covered above!
Base + Acid → Salt + WaterProperty 2: Reaction with Metals
Only some bases react with metals!
Example: Sodium hydroxide + Zinc:
2NaOH + Zn → Na₂ZnO₂ + H₂↑
(Sodium zincate + Hydrogen gas)Example: Sodium hydroxide + Aluminum:
2NaOH + 2Al + 2H₂O → 2NaAlO₂ + 3H₂↑
(Sodium aluminate + Hydrogen gas)Important: This reaction is RARE - only few reactive metals with strong alkalis!
Property 3: Reaction with Non-metal Oxides
General Reaction:
Base + Non-metal oxide → Salt + WaterExample: Sodium hydroxide + Carbon dioxide:
2NaOH + CO₂ → Na₂CO₃ + H₂O
(Sodium carbonate + Water)Example: Calcium hydroxide + Carbon dioxide:
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
(This is why lime water turns milky!)Why This Matters:
- Non-metal oxides are ACIDIC in nature
- React with bases like acids do
- This proves: Non-metal oxides = Acidic oxides
💧 Part 6: What Happens When Acids/Bases Dissolve in Water?
Acids in Water
When acid dissolves in water:
HCl → H⁺ + Cl⁻But here's the truth:
- H⁺ ions are very reactive
- They CANNOT exist alone in water
- They immediately combine with H₂O molecules
Actual reaction:
HCl + H₂O → H₃O⁺ + Cl⁻H₃O⁺ = Hydronium ion (This is the real acidic species!)
Why Water is Essential:
- Acids show acidic properties ONLY in water
- Dry HCl gas doesn't turn blue litmus red
- Water helps ionization
Experiment Proof:
- Take dry blue litmus paper
- Add dry HCl gas - NO change!
- Add water to litmus, then HCl gas - Turns RED!
Conclusion: Water is essential for acidic properties!
Bases in Water
When base dissolves in water:
NaOH → Na⁺ + OH⁻OH⁻ ions = Hydroxide ions (responsible for basic properties)
Dilution Effect:
- When you add water to acid/base, H⁺ or OH⁻ ions get diluted
- Concentration decreases
- Solution becomes less acidic/basic
🌡️ Part 7: Strength of Acids and Bases - The pH Scale
What is pH?
pH = "Power of Hydrogen"
Definition: Measure of how acidic or basic a solution is
pH Scale:
- Ranges from 0 to 14
- 7 = Neutral (pure water)
- 0-6 = Acidic (lower = more acidic)
- 8-14 = Basic (higher = more basic)
Mathematically:
pH = -log₁₀[H⁺](Don't worry, you don't need to calculate this in Class 10!)
Understanding pH Values
Strong Acids (pH 0-3):
- HCl (pH ≈ 1)
- H₂SO₄ (pH ≈ 1)
- HNO₃ (pH ≈ 1)
- Stomach acid (pH ≈ 1-2)
Weak Acids (pH 4-6):
- Lemon juice (pH ≈ 2-3)
- Vinegar (pH ≈ 3)
- Tomato juice (pH ≈ 4)
- Coffee (pH ≈ 5)
Neutral (pH 7):
- Pure water
- Blood (pH ≈ 7.4, slightly basic!)
- Milk (pH ≈ 6.5-6.7, slightly acidic)
Weak Bases (pH 8-10):
- Baking soda solution (pH ≈ 9)
- Toothpaste (pH ≈ 9)
Strong Bases (pH 11-14):
- Lime water (pH ≈ 12)
- NaOH solution (pH ≈ 13-14)
- Drain cleaners (pH ≈ 14)
Memory Trick: "0-6-7-8-14"
- 0-6: Acids (sour like lemon - pH 2-3)
- 7: Neutral (pure water)
- 8-14: Bases (bitter like soap - pH 9-10)
Importance of pH in Daily Life
1. Human Body:
- Stomach: pH 1-2 (very acidic - helps digest food)
- Blood: pH 7.35-7.45 (must stay constant!)
- Mouth: pH should be above 5.5 (below = tooth decay starts)
2. Agriculture:
- Most plants grow best at pH 6.5-7.5
- If soil too acidic → Add lime (Ca(OH)₂)
- If soil too basic → Add organic matter
3. Aquatic Life:
- Fish survive best at pH 6.5-8.5
- Acid rain (pH < 5.6) kills fish
- Rivers polluted with acids/bases harm ecosystem
4. Digestion:
- Stomach produces HCl (pH 1-2) to digest food
- If excess acid → Heartburn! → Take antacid (base)
5. Bee/Ant Stings:
- Contain formic acid (painful!)
- Apply baking soda (weak base) to neutralize
6. Industrial Processes:
- Paper making, textile dyeing, food processing all need specific pH
🧂 Part 8: Salts - The Result of Neutralization
What Are Salts?
Definition: Ionic compounds formed when acid and base neutralize each other
General Formation:
Acid + Base → Salt + WaterStructure:
- Positive ion (cation) from BASE
- Negative ion (anion) from ACID
Example:
HCl + NaOH → NaCl + H₂O
↓ ↓ ↓
Acid Base Salt
(Cl⁻) (Na⁺) (Na⁺Cl⁻)Family of Salts
Salts can be grouped by the acid they come from:
1. Chlorides (from HCl):
- NaCl (Table salt)
- KCl (Used in medicine)
- NH₄Cl (Used in dry cells)
2. Sulfates (from H₂SO₄):
- CaSO₄·2H₂O (Gypsum - used in plaster)
- MgSO₄·7H₂O (Epsom salt - laxative)
- CuSO₄·5H₂O (Blue vitriol - fungicide)
3. Nitrates (from HNO₃):
- NaNO₃ (Chile saltpeter - fertilizer)
- KNO₃ (Potassium nitrate - gunpowder, fertilizer)
4. Carbonates (from H₂CO₃):
- Na₂CO₃·10H₂O (Washing soda)
- NaHCO₃ (Baking soda)
- CaCO₃ (Limestone, marble)
Types of Salts Based on pH
1. Neutral Salts (pH = 7)
- Formed from: Strong acid + Strong base
- Examples:
- NaCl (HCl + NaOH)
- K₂SO₄ (H₂SO₄ + KOH)
2. Acidic Salts (pH < 7)
- Formed from: Strong acid + Weak base
- Examples:
- NH₄Cl (HCl + NH₄OH)
- (NH₄)₂SO₄
3. Basic Salts (pH > 7)
- Formed from: Weak acid + Strong base
- Examples:
- CH₃COONa (CH₃COOH + NaOH)
- Na₂CO₃
Memory Trick: "Strength Decides pH"
- Strong + Strong = Neutral (equal strength, cancel out)
- Strong acid + Weak base = Acidic (acid wins!)
- Weak acid + Strong base = Basic (base wins!)
Important Salts and Their Uses
1. Sodium Chloride (NaCl) - Common Salt
Properties:
- White crystalline solid
- Soluble in water
- Essential for life (electrolyte balance)
Uses:
- Food seasoning and preservation
- Raw material for NaOH, Na₂CO₃, HCl
- Melting ice on roads (lowers freezing point)
Extraction:
- Sea water evaporation
- Rock salt mining
2. Sodium Hydroxide (NaOH) - Caustic Soda
Preparation: Chlor-Alkali Process
2NaCl + 2H₂O → (electricity) → 2NaOH + Cl₂↑ + H₂↑Products:
- At anode: Cl₂ gas (chlorine)
- At cathode: H₂ gas (hydrogen)
- Near cathode: NaOH solution
Uses:
- Soap and detergent manufacturing
- Paper industry
- Textile industry
- Petroleum refining
Exam Note: Chlor-alkali process is a 3-mark question favorite!
3. Bleaching Powder (CaOCl₂)
Preparation:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
(Dry slaked lime + Chlorine gas)Properties:
- White powder
- Strong smell of chlorine
- Releases chlorine when exposed to air
Uses:
- Disinfecting drinking water
- Bleaching cotton and linen
- Oxidizing agent in industries
Why it Works: Releases chlorine which kills bacteria!
4. Baking Soda (NaHCO₃) - Sodium Bicarbonate
Preparation:
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
(This is Solvay process - complex!)Simple version:
Na₂CO₃ + H₂O + CO₂ → 2NaHCO₃Properties:
- White crystalline solid
- Mild base (pH ≈ 9)
- Releases CO₂ when heated
Uses:
- Baking: Releases CO₂ which makes cake fluffy
2NaHCO₃ + Heat → Na₂CO₃ + H₂O + CO₂↑- Fire extinguisher: CO₂ gas puts out fire
- Antacid: Neutralizes excess stomach acid
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑- Bee sting treatment: Neutralizes formic acid
Exam Trick: If question mentions "baking" or "fire extinguisher" → Answer is baking soda!
5. Washing Soda (Na₂CO₃·10H₂O) - Sodium Carbonate Decahydrate
Preparation:
Step 1: Heat baking soda
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑
Step 2: Add water (recrystallization)
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂OProperties:
- White crystalline solid
- Loses water on exposure to air (efflorescence)
- Solution is basic (pH ≈ 11)
Uses:
- Removing permanent hardness of water
- Glass, soap, paper industry
- Cleaning agent
How it Softens Hard Water:
Na₂CO₃ + CaCl₂ → 2NaCl + CaCO₃↓
(Washing soda + Hard water salt → Soft water + Precipitate)6. Plaster of Paris (CaSO₄·½H₂O)
Preparation:
CaSO₄·2H₂O + Heat (373K) → CaSO₄·½H₂O + 1½H₂O
(Gypsum) (Plaster of Paris)Important: Heat should be 373K (100°C) exactly!
- Below 373K → Doesn't form properly
- Above 373K → Forms dead burnt plaster (useless!)
Properties:
- White powder
- When mixed with water, becomes hard solid
Setting Reaction:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
(Plaster) (Gypsum - hard mass)Uses:
- Making casts for broken bones
- Making statues and decorative materials
- Dentistry (making teeth moulds)
- Making blackboard chalk
Exam Favorite: Questions ask temperature (373K) and why it's used in fractures!
💎 Part 9: Crystals of Salts - Water of Crystallization
What is Water of Crystallization?
Definition: Fixed number of water molecules present in one formula unit of a salt
Examples:
1. Copper Sulfate:
CuSO₄·5H₂O
(5 water molecules per formula unit)- Blue crystals
- When heated, loses water → Becomes white powder
- When water added to white powder → Becomes blue again!
2. Gypsum:
CaSO₄·2H₂O
(2 water molecules)3. Washing Soda:
Na₂CO₃·10H₂O
(10 water molecules!)Important: The water is part of crystal structure!
- NOT just wet salt
- Chemically bonded in definite proportion
- Gives color to some salts (like blue to copper sulfate)
Exam Question Pattern: "What is the chemical formula of hydrated copper sulfate?" Answer: CuSO₄·5H₂O (must mention 5H₂O!)
📝 Important Chemical Equations Summary
Must memorize these for exams:
1. Neutralization:
HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O2. Metal + Acid:
Zn + 2HCl → ZnCl₂ + H₂↑
Mg + H₂SO₄ → MgSO₄ + H₂↑3. Carbonate + Acid:
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂↑4. Lime water test:
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
(milky white)5. Chlor-alkali process:
2NaCl + 2H₂O → 2NaOH + Cl₂↑ + H₂↑6. Bleaching powder:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O7. Baking soda heating:
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑8. Washing soda preparation:
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O9. Plaster of Paris:
CaSO₄·2H₂O + Heat → CaSO₄·½H₂O + 1½H₂O
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (setting)10. Blue vitriol dehydration:
CuSO₄·5H₂O + Heat → CuSO₄ + 5H₂O
(Blue) (White)🧪 Part 10: Practical Experiments (Lab Work)
Experiment 1: Testing Acids and Bases with Indicators
Aim: To test various solutions with litmus paper
Materials: HCl, NaOH, lemon juice, soap solution, red/blue litmus
Procedure:
- Take small amounts of each solution in test tubes
- Dip red litmus paper in each
- Dip blue litmus paper in each
- Record color changes
Observations:
| Solution | Red Litmus | Blue Litmus | Nature |
|---|---|---|---|
| HCl | No change | Turns red | Acidic |
| Lemon juice | No change | Turns red | Acidic |
| NaOH | Turns blue | No change | Basic |
| Soap solution | Turns blue | No change | Basic |
Conclusion: Litmus can identify acidic and basic nature of solutions
Experiment 2: Reaction of Zinc with HCl
Aim: To show acids produce H₂ gas with metals
Materials: Zinc granules, dilute HCl, test tube, matchstick
Procedure:
- Take zinc granules in test tube
- Add dilute HCl
- Bring burning matchstick near mouth
Observation:
- Bubbles appear (effervescence)
- Gas burns with "pop" sound
Chemical Equation:
Zn + 2HCl → ZnCl₂ + H₂↑Conclusion: Acids react with metals to produce hydrogen gas
Experiment 3: Reaction of Na₂CO₃ with HCl
Aim: To show acids produce CO₂ with carbonates
Materials: Na₂CO₃, dilute HCl, lime water
Procedure:
- Take Na₂CO₃ in test tube
- Add dilute HCl
- Pass gas through lime water
