Class 10 Science Chapter 1 – Chemical Reactions and Equations 🧪

Complete Notes

Introduction – Chemistry Kyu Important Hai?

Chemistry hamari daily life ka hissa hai! Khana pakane se lekar medicine banane tak, har jagah chemical reactions hoti rehti hain.

Chemistry ke Applications:

• Agriculture: Fertilizers (urea, ammonium nitrate), pesticides
• Medicine: Aspirin, paracetamol, antibiotics banane mein
• Industry: Plastics, metals, petrochemicals
• Daily Life: Cooking, cleaning, cosmetics

1.1 Chemical Reactions Kya Hain? class 10 science chapter 1 notes

Definition:

Chemical Reaction = Jab substances apni composition change karke new substances banate hain

Examples:

• Burning: C + O₂ → CO₂ (carbon dioxide banna)
• Rusting: Fe + O₂ → Fe₂O₃ (iron ka rust hona)
• Cooking: Roti pakana, daal banana
• Digestion: Khana digest hona stomach mein

Chemical Reaction ke Signs:

1. Color change – Leaves ka green se brown hona
2. Gas evolution – Antacid tablet paani mein fizz karna
3. Precipitate formation – Solid particles banna
4. Temperature change – Heat absorb ya release hona
5. Light emission – Burning magnesium ribbon

1.2 Chemical Equations

What is Chemical Equation?

Chemical Equation = Chemical reaction ka symbolic representation

Word Equation vs Chemical Equation:

Word Equation: Magnesium + Oxygen → Magnesium Oxide

Chemical Equation: Mg + O₂ → MgO

Balanced vs Unbalanced Equations:

Unbalanced: Mg + O₂ → MgO

• Left side: 1 Mg, 2 O
• Right side: 1 Mg, 1 O
• Not balanced!

Balanced: 2Mg + O₂ → 2MgO

• Left side: 2 Mg, 2 O
• Right side: 2 Mg, 2 O
• Balanced!

1.3 Balancing Chemical Equations

Why Balance Karna Zaroori Hai?

Law of Conservation of Mass ke according: “Mass can neither be created nor destroyed in chemical reactions”

Steps to Balance Equations:

Step 1: Unbalanced equation likhna Step 2: Atoms count karna both sides Step 3: Coefficients add karna Step 4: Recheck karna

Example:

Unbalanced: H₂ + O₂ → H₂O

Step-by-step balancing:

• H atoms: Left = 2, Right = 2 ✓
• O atoms: Left = 2, Right = 1 ✗

Balanced: 2H₂ + O₂ → 2H₂O

• H atoms: Left = 4, Right = 4 ✓
• O atoms: Left = 2, Right = 2 ✓

Balancing Tips:

1. Metals first balance karo
2. Non-metals phir karo
3. Hydrogen aur Oxygen last mein karo
4. Smallest whole numbers use karo

1.4 Types of Chemical Reactions

1) Combination Reaction (संयोजन अभिक्रिया)

Definition: Do ya zyada reactants milkar ek product banate hain General Form: A + B → AB

Examples:

• C + O₂ → CO₂ (carbon burning)
• 2Mg + O₂ → 2MgO (magnesium burning)
• CaO + H₂O → Ca(OH)₂ (quicklime + water)

2) Decomposition Reaction (अपघटन अभिक्रिया)

Definition: Ek compound break hokar do ya zyada products banata hai General Form: AB → A + B

Types:

• Thermal Decomposition: Heat se breakdown
• Electrolytic Decomposition: Electricity se breakdown
• Photolytic Decomposition: Light se breakdown

Examples:

• 2HgO → 2Hg + O₂ (thermal – heat se)
• 2H₂O → 2H₂ + O₂ (electrolytic – electricity se)
• 2AgCl → 2Ag + Cl₂ (photolytic – light se)

3) Displacement Reaction (विस्थापन अभिक्रिया)

Definition: Ek element dusre element ko compound se displace kar deta hai General Form: A + BC → AC + B

Examples:

• Zn + CuSO₄ → ZnSO₄ + Cu (zinc displaces copper)
• Fe + CuSO₄ → FeSO₄ + Cu (iron displaces copper)

Activity Series: Reactivity ke according metals ka order K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au

4) Double Displacement Reaction (द्विविस्थापन अभिक्रिया)

Definition: Do compounds apne ions exchange karte hain General Form: AB + CD → AD + CB

Examples:

• AgNO₃ + NaCl → AgCl + NaNO₃ (white precipitate)
• BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl (white precipitate)

5) Redox Reactions (अपचयोपचय अभिक्रिया)

Definition: Oxidation aur Reduction saath mein hote hain

Oxidation: Oxygen gain ya hydrogen loss Reduction: Oxygen loss ya hydrogen gain

Example: CuO + H₂ → Cu + H₂O

• CuO reduced (oxygen loss)
• H₂ oxidized (oxygen gain)

1.5 Effects of Oxidation in Daily Life

1) Corrosion

Definition: Metals ka air aur moisture se reaction

Examples:

• Rusting of Iron: 4Fe + 3O₂ + 6H₂O → 2Fe₂O₃.3H₂O
• Black coating on silver: Ag + H₂S → Ag₂S + H₂
• Green coating on copper: Cu ka copper carbonate ban jana

Prevention Methods:

• Painting (oxygen se contact rokna)
• Galvanization (zinc coating)
• Alloying (stainless steel banana)

2) Rancidity

Definition: Fats aur oils ka oxidation hoke bad smell aana

Examples:

• Butter ka rancid ho jana
• Chips ka taste kharab ho jana

Prevention Methods:

• Antioxidants: BHT, BHA add karna
• Nitrogen packaging: Oxygen-free environment
• Refrigeration: Low temperature storage
• Vacuum packaging: Air remove karna

1.6 Important Terms and Definitions

Key Terms:

• Reactants: Jo substances react karti hain (left side)
• Products: Jo substances baniti hain (right side)
• Catalyst: Reaction speed badhane wala substance
• Precipitate: Insoluble solid jo solution mein banta hai
• Endothermic: Heat absorb karne wali reaction
• Exothermic: Heat release karne wali reaction

Symbols Used:

• → : Yields/produces
• + : Plus/and
• (s) : Solid state
• (l) : Liquid state
• (g) : Gas state
• (aq) : Aqueous solution
• Δ : Heat is supplied
• ↑ : Gas evolved
• ↓ : Precipitate formed

1.7 Solved Examples

Example 1: Balancing Equation

Balance: Al + O₂ → Al₂O₃

Solution:

• Al atoms: Left = 1, Right = 2
• O atoms: Left = 2, Right = 3

Balanced: 4Al + 3O₂ → 2Al₂O₃

Example 2: Type of Reaction

Identify: Zn + H₂SO₄ → ZnSO₄ + H₂

Solution:

• Zn displaces H₂ from H₂SO₄
• Type: Displacement Reaction

Example 3: Oxidation-Reduction

Identify: 2Cu + O₂ → 2CuO

Solution:

• Cu gains oxygen → Oxidation
• O₂ provides oxygen → Oxidizing agent

1.8 Practice Questions

Multiple Choice Questions:

1. Which gas is evolved when zinc reacts with hydrochloric acid? a) O₂ b) H₂ c) CO₂ d) N₂
2. What type of reaction is photosynthesis? a) Combination b) Decomposition c) Displacement d) Redox
3. Which of the following is an oxidizing agent? a) H₂ b) CO c) O₂ d) H₂S

Short Answer Questions:

1. Why should magnesium ribbon be cleaned before burning?
2. What is the difference between displacement and double displacement?
3. Give one example each of thermal, electrolytic and photolytic decomposition.

Long Answer Questions:

1. Explain corrosion with examples and prevention methods.
2. What are redox reactions? Give examples with identification of oxidizing and reducing agents.

1.9 Memory Tricks and Tips 🧠

Balancing Equations:

• “MHNHO” – Metals, Hydrogen, Non-metals, Hydrogen, Oxygen (balancing order)

Activity Series:

• “Please Send Charlie’s Monkeys And Zebras In Lead Hunting Parties” (K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au)

Types of Reactions:

• “CDDR” – Combination, Decomposition, Displacement, Double displacement, Redox

Oxidation-Reduction:

• “OIL RIG” – Oxidation Is Loss, Reduction Is Gain (of electrons)

1.10 Important Points for Exam 📚

High Weightage Topics:

1. Balancing chemical equations (3-5 marks)
2. Types of reactions with examples (3-5 marks)
3. Oxidation-reduction identification (2-3 marks)
4. Corrosion and rancidity (3-5 marks)

Common Mistakes to Avoid:

1. Not balancing equations before identifying reaction type
2. Confusing displacement with double displacement
3. Wrong identification of oxidizing/reducing agents
4. Not writing state symbols properly

Exam Strategy:

• Practice balancing at least 20 equations daily
• Learn activity series by heart
• Understand concepts rather than rote learning
• Write chemical formulas correctly

Chapter Summary 📖

Chapter 1 chemistry ki foundation hai! Isme jo concepts clear honge, wahi aage ke chapters mein kaam aayenge.

Key Takeaways:

• Chemical equations balance karna essential skill hai
• Types of reactions identify karna important hai
• Oxidation-reduction daily life mein bahut common hai
• Corrosion aur rancidity practical applications hain

Practice Strategy:

• Daily 10-15 equations balance karna
• Examples ke saath concepts samajhna
• Real-life examples se connect karna
• Previous year questions solve karna

All the best! Ye chapter master kar lo to chemistry journey smooth ho jayegi!

Remember: “Chemistry is not about memorizing, it’s about understanding patterns and logic!”