📘 Chapter 1: Chemical Reactions and Equations (Class 10 Science – Full Notes)
🔷 I. Introduction
A chemical reaction is a process in which new substances with new properties are formed. It involves a chemical change.
Examples:
- Rusting of iron
- Digestion of food
- Respiration
- Photosynthesis
🔷 II. Chemical Equation
A chemical equation represents a chemical reaction using chemical formulas of reactants and products.
Example:
- Word Equation: Magnesium + Oxygen → Magnesium oxide
- Chemical Equation: Mg + O₂ → MgO
🔷 III. Balanced Chemical Equation
According to the Law of Conservation of Mass, the total mass of reactants = total mass of products.
Thus, we balance the number of atoms of each element on both sides.
Example:
- Unbalanced: H₂ + O₂ → H₂O
- Balanced: 2H₂ + O₂ → 2H₂O
🔷 IV. Types of Chemical Reactions
🔹 1. Combination Reaction
Two or more reactants combine to form a single product.
General Form: A + B → AB
Example: CaO + H₂O → Ca(OH)₂ (Slaked lime, exothermic reaction)
🔹 2. Decomposition Reaction
A single compound breaks down into two or more simpler substances.
General Form: AB → A + B
Example:
CaCO₃ → CaO + CO₂ (Thermal decomposition)
Types:
- Thermal decomposition (heat):
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ - Photolytic decomposition (light):
2AgCl → 2Ag + Cl₂ (white to grey in sunlight) - Electrolytic decomposition (electricity):
2H₂O → 2H₂ + O₂
🔹 3. Displacement Reaction
A more reactive element displaces a less reactive one from its compound.
General Form: A + BC → AC + B
Example:
Fe + CuSO₄ → FeSO₄ + Cu
(Zn > Fe > Cu in reactivity)
🔹 4. Double Displacement Reaction
Exchange of ions between two compounds.
General Form: AB + CD → AD + CB
Example:
Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl
(white ppt of BaSO₄ forms)
🔹 5. Redox Reaction
Simultaneous oxidation and reduction.
- Oxidation: Gain of oxygen / loss of electrons
- Reduction: Loss of oxygen / gain of electrons
Example:
CuO + H₂ → Cu + H₂O
(CuO is reduced, H₂ is oxidized)
🔷 V. Oxidation and Reduction (In Daily Life)
🔸 Oxidation:
- Burning of fuel
- Rusting of iron
- Rancidity of food
🔸 Reduction:
- Extraction of metals from ores (e.g., Fe₂O₃ + 3CO → 2Fe + 3CO₂)
🔷 VI. Effects of Oxidation in Everyday Life
🔹 1. Corrosion:
- Oxidation of metals due to moisture and air
- Iron → rusts (Fe₂O₃·xH₂O)
- Silver → black coating (Ag₂S)
- Copper → green patina (CuCO₃·Cu(OH)₂)
Prevention:
- Painting
- Galvanizing
- Alloying
- Oiling/greasing
🔹 2. Rancidity:
- Oxidation of oils/fats in food
- Bad taste and smell
Prevention:
- Use of antioxidants (BHA, BHT)
- Airtight packaging
- Refrigeration
- Nitrogen flushing (chips packets)
🔷 VII. Important Chemical Equations (Class 10 Level)
| Reaction Type | Equation |
|---|---|
| Thermal Decomposition | CaCO₃ → CaO + CO₂ |
| Photolysis | 2AgBr → 2Ag + Br₂ (light exposure) |
| Displacement | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double Displacement | Na₂CO₃ + CaCl₂ → CaCO₃ ↓ + 2NaCl |
| Redox | CuO + H₂ → Cu + H₂O |
🔷 VIII. Balancing Equations – Rules
- Write skeletal equation.
- Count atoms on both sides.
- Start with the most complex molecule.
- Use coefficients (not subscripts).
- Balance metals → non-metals → hydrogen → oxygen.
🔷 IX. MCQs / Exam-Style Questions
Q1. Identify the type of reaction:
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
Answer: Decomposition (Thermal)
Q2. Why is respiration considered an exothermic process?
Because it releases energy:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Q3. Write the balanced equation for:
Al + HCl → ?
2Al + 6HCl → 2AlCl₃ + 3H₂
🔷 X. Mind Map – Class 10 Reactions
sqlCopyEdit CHEMICAL REACTIONS
|
---------------------------------
| | | | | |
Comb. Decomp Displ. Double Redox Oxidation
| | | |
Thermal Photo Electro Ions Exchange
🧠 HOTS (Higher Order Thinking Skill)
Q: Why do silver chloride and silver bromide turn grey in sunlight?
Ans: Due to photochemical decomposition into silver metal and halogen.
Q: Why are food items packed in nitrogen gas?
Ans: Nitrogen prevents oxidation of oils/fats, thereby preventing rancidity.
📘 Chapter 1: Chemical Reactions and Equations – MCQs (Class 10)
🔹 Section A: Basic Conceptual MCQs
- Which of the following is a chemical change?
a) Melting of ice
b) Dissolving sugar in water
c) Rusting of iron
d) Boiling of water
✅ Answer: c - What happens when calcium oxide reacts with water?
a) An acidic solution is formed
b) It forms calcium hydroxide and releases heat
c) It forms a salt
d) Gas is evolved
✅ Answer: b - The balanced equation for:
Fe + H₂O → ?
a) Fe + H₂O → FeO + H₂
b) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
c) Fe + 2H₂O → Fe(OH)₂ + H₂
d) Fe + 3H₂O → Fe₂O₃ + 3H₂
✅ Answer: b - Which of the following reactions is an example of a decomposition reaction?
a) NaCl + AgNO₃ → AgCl + NaNO₃
b) CaCO₃ → CaO + CO₂
c) H₂ + Cl₂ → 2HCl
d) Zn + CuSO₄ → ZnSO₄ + Cu
✅ Answer: b - Which one is a redox reaction?
a) H₂ + Cl₂ → 2HCl
b) AgNO₃ + NaCl → AgCl + NaNO₃
c) CaCO₃ → CaO + CO₂
d) Zn + HCl → ZnCl₂ + H₂
✅ Answer: d
🔹 Section B: Application-Based MCQs
- Which gas is evolved when zinc reacts with dilute sulphuric acid?
a) Oxygen
b) Hydrogen
c) Nitrogen
d) Carbon dioxide
✅ Answer: b - What is the color change observed when silver chloride is exposed to sunlight?
a) White to grey
b) White to brown
c) Yellow to white
d) Colorless to blue
✅ Answer: a - Which type of reaction is:
Zn + CuSO₄ → ZnSO₄ + Cu
a) Double displacement
b) Combination
c) Decomposition
d) Displacement
✅ Answer: d - Which of the following is used in the prevention of rancidity?
a) Oxygen
b) Nitrogen
c) Chlorine
d) Carbon dioxide
✅ Answer: b - Which of the following reactions is exothermic?
a) CaO + H₂O → Ca(OH)₂
b) Decomposition of AgBr
c) Electrolysis of water
d) Photosynthesis
✅ Answer: a
🔹 Section C: Assertion-Reason Type MCQs
- Assertion: Rusting is a redox reaction.
Reason: Iron loses electrons and gets oxidized.
a) Both A and R are true, and R is the correct explanation.
b) Both A and R are true, but R is not the correct explanation.
c) A is true, R is false.
d) A is false, R is true.
✅ Answer: a - Assertion: Cooking oil turns rancid in air.
Reason: Oil gets reduced due to presence of moisture.
a) Both A and R are true, and R is the correct explanation.
b) Both A and R are true, but R is not the correct explanation.
c) A is true, R is false.
d) A is false, R is true.
✅ Answer: c
🔹 Section D: Advanced Concept MCQs
- Which of the following represents a double displacement reaction?
a) Zn + H₂SO₄ → ZnSO₄ + H₂
b) Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
c) CaCO₃ → CaO + CO₂
d) H₂ + O₂ → H₂O
✅ Answer: b - Which of these is a photochemical decomposition reaction?
a) ZnCO₃ → ZnO + CO₂
b) 2AgCl → 2Ag + Cl₂
c) 2KClO₃ → 2KCl + 3O₂
d) CaCO₃ → CaO + CO₂
✅ Answer: b - What type of reaction is respiration?
a) Endothermic decomposition
b) Exothermic combination
c) Photochemical combination
d) Electrolytic decomposition
✅ Answer: b
🔹 Section E: Matching and Assertion-Based Questions
- Match the following:
| Column A | Column B |
|---|---|
| 1. Rusting | A. Oxidation |
| 2. AgBr + sunlight | B. Photolysis |
| 3. Zn + CuSO₄ | C. Displacement |
| 4. BaCl₂ + Na₂SO₄ | D. Double displacement |
Correct Match:
1→A, 2→B, 3→C, 4→D
✅ Answer: 1-A, 2-B, 3-C, 4-D
🔹 Section F: Higher Order Thinking MCQs
- Why are chips packets flushed with nitrogen?
a) To kill bacteria
b) To make chips crispy
c) To prevent oxidation of oils
d) To increase the pressure inside
✅ Answer: c - What will happen when iron nails are dipped in copper sulphate solution?
a) Color of solution changes to green
b) No change occurs
c) Blue color intensifies
d) Iron dissolves completely
✅ Answer: a - Which is the correct representation of a balanced chemical equation?
a) Mg + O₂ → MgO
b) 2Mg + O₂ → 2MgO
c) Mg + 2O₂ → MgO₂
d) Mg₂ + O → MgO
✅ Answer: b - Which is not a characteristic of a chemical reaction?
a) Change in state
b) Change in color
c) Evolution of light
d) Change in mass of individual elements
✅ Answer: d
🔹 Section G: Fill-in-the-Blank Style MCQs
- In the reaction 2H₂ + O₂ → 2H₂O, oxygen is being _______.
a) Oxidized
b) Reduced
c) Displaced
d) Burned
✅ Answer: b - The product of CaO + H₂O is _______.
a) CaCl₂
b) Ca(OH)₂
c) CaO₂
d) HCaO
✅ Answer: b - The reaction Fe + CuSO₄ → FeSO₄ + Cu is a ______ reaction.
a) Combination
b) Displacement
c) Redox
d) Both b and c
✅ Answer: d - Which one is not a redox reaction?
a) 2AgNO₃ + Cu → Cu(NO₃)₂ + 2Ag
b) 2KClO₃ → 2KCl + 3O₂
c) H₂ + Cl₂ → 2HCl
d) NaOH + HCl → NaCl + H₂O
✅ Answer: d - Which reaction gives a white precipitate?
a) Na₂CO₃ + CaCl₂ → ?
b) Zn + HCl → ?
c) Cu + O₂ → ?
d) Fe + H₂O → ?
✅ Answer: a