Welcome to Board Guru! In this comprehensive study guide, we will cover Class 10 Science Chapter 1: Chemical Reactions and Equations with step-by-step definitions, chemical formulas, balanced reactions, and key concept explanations designed for CBSE and State Board exams.
1. Introduction to Chemical Reactions
What is a Physical Change?
A change in which only the physical properties (like state, shape, or size) of a substance change, and no new chemical substance is formed. It is usually reversible.
- Example: Melting of ice into water, boiling of water.
What is a Chemical Change?
A change in which one or more new substances with entirely new chemical properties are formed. It is usually irreversible.
- Example: Rusting of iron, souring of milk, digestion of food in our body.
Definition of Chemical Reaction
A Chemical Reaction is a chemical change in which initial substances (reactants) react chemically with each other to transform into new substances (products) having different properties.
Key Components of a Reaction:
- Reactants (अभिकारक): The starting chemical substances that undergo a change during a chemical reaction (written on the Left-Hand Side - LHS).
- Products (उत्पाद): The new chemical substances formed as a result of a chemical reaction (written on the Right-Hand Side - RHS).
Example: When Magnesium ribbon is burnt in oxygen, it burns with a dazzling white flame and forms Magnesium Oxide.
Magnesium (2Mg) + Oxygen (O2) → Magnesium Oxide (2MgO)
Observations to Confirm a Chemical Reaction:
- Change in State: Liquid to gas or solid formation.
- Change in Color: Rusting turns shiny iron into a reddish-brown color.
- Evolution of Gas: Release of gases like Hydrogen (H2) or Carbon dioxide (CO2).
- Change in Temperature: Exothermic (release of heat) or Endothermic (absorption of heat) changes.
- Formation of Precipitate: Insoluble solid particles settling down in a liquid solution.
2. Chemical Equations & Balancing Rules
Definition of Word Equation
A simple way to represent a chemical reaction using the names of reactants and products separated by an arrow (→).
Definition of Chemical Equation
A shorthand representation of a chemical reaction using the chemical symbols and formulas of the reactants and products.
Skeleton Chemical Equation (Unbalanced)
An equation in which the number of atoms of one or more elements is NOT equal on both sides (LHS and RHS).
Mg + O2 → MgO (Unbalanced: 2 Oxygen atoms on LHS, only 1 on RHS)
Balanced Chemical Equation
An equation in which the total number of atoms of each element is equal on both the reactant side and the product side.
Law of Conservation of Mass
This law states that "Mass can neither be created nor destroyed in a chemical reaction." Therefore, the total mass of the reactants must equal the total mass of the products, which requires balancing every chemical equation.
Balanced Examples:
2Mg + O2 → 2MgOZn + H2SO4 → ZnSO4 + H2 ↑
3. Detailed Types of Chemical Reactions
1. Combination Reaction (संयोजन अभिक्रिया)
Definition: A chemical reaction in which two or more simpler substances (elements or compounds) combine together to form a single new product.
General Format: A + B → AB
Example: Slaking of Lime. Quick Lime (Calcium Oxide) reacts vigorously with water to form Slaked Lime (Calcium Hydroxide) releasing heat.
CaO (s) + H2O (l) → Ca(OH)2 (aq) + Heat
2. Decomposition Reaction (वियोजन अभिक्रिया)
Definition: A chemical reaction in which a single compound breaks down into two or more simpler elements or compounds. This reaction requires energy.
A. Thermal Decomposition (Heat Energy):
Decomposition carried out by heating a substance.
2FeSO4 (s) + Heat → Fe2O3 (s) + SO2 (g) + SO3 (g)CaCO3 (s) + Heat → CaO (s) + CO2 (g)
B. Electrolytic Decomposition (Electrical Energy):
Decomposition carried out by passing an electric current.
2H2O (l) + Electricity → 2H2 (g) + O2 (g)
C. Photolytic Decomposition (Light Energy):
Decomposition carried out in the presence of sunlight.
2AgCl (s) + Sunlight → 2Ag (s) + Cl2 (g)(Used in Black & White Photography)
3. Displacement Reaction (विस्थापन अभिक्रिया)
Definition: A chemical reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution.
Example: Iron nail placed in Copper Sulphate solution (Blue color turns Green).
Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)
4. Double Displacement Reaction (द्विविस्थापन अभिक्रिया)
Definition: A chemical reaction in which two different ionic compounds react by exchanging their ions to form two new compounds.
Precipitate Definition: An insoluble solid that separates out from a liquid solution during a chemical reaction.
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) ↓ + 2NaCl (aq)
5. Redox Reaction (Oxidation & Reduction)
- Oxidation (उपचयन): Addition of oxygen OR removal of hydrogen.
- Reduction (अपचयन): Addition of hydrogen OR removal of oxygen.
- Redox Reaction: Reaction in which both Oxidation and Reduction happen simultaneously.
CuO + H2 + Heat → Cu + H2O
- Substance Oxidized: H2 → H2O
- Substance Reduced: CuO → Cu
- Oxidizing Agent: CuO
- Reducing Agent: H2
4. Classification Based on Heat Exchange
1. Exothermic Reaction (ऊष्माक्षेपी अभिक्रिया)
Chemical reactions in which heat energy is released along with products.
- Respiration:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy - Burning of Natural Gas:
CH4 + 2O2 → CO2 + 2H2O + Heat
2. Endothermic Reaction (ऊष्माशोषी अभिक्रिया)
Chemical reactions in which heat, light, or electrical energy is absorbed from surroundings.
- Examples: Photosynthesis, All Decomposition reactions.
5. Effects of Oxidation in Daily Life
A. Corrosion (संक्षारण)
Definition: The slow eating up of metals by the action of air, moisture, or acids on their surface over time.
- Examples: Rusting of iron (reddish-brown), black coating on silver, green coating on copper.
- Prevention: Painting, Oiling, Greasing, and Galvanization (coating iron with Zinc).
B. Rancidity (विकृतगंधिता)
Definition: Aerial oxidation of fats and oils in food materials resulting in unpleasant smell and taste.
- Prevention: Flushing chip bags with Nitrogen gas, adding antioxidants, storing food in airtight containers.
📝 Quick Summary Checklist for Students
- Law of Conservation of Mass: Reactant Mass = Product Mass
- Precipitate Indicator: Downward arrow (↓)
- Gas Evolution Indicator: Upward arrow (↑)
- Exothermic: Heat released (→ Product + Heat)
- Endothermic: Heat absorbed (Reactant + Heat →)
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