CLASS 10 CHEMISTRY
Chapter 1 — Chemical Reactions and Equations
NCERT-based detailed notes
Important reactions • observations • definitions • exam questions
1. Chemical Reaction
A chemical reaction is a process in which one or more substances undergo a change to form one or more
new substances.
Examples:
2Mg + O₂ → 2MgO
2H₂ + O₂ → 2H₂O
2. Signs of a Chemical Reaction
Change in state • Change in colour • Evolution of a gas • Change in temperature • Formation of a
precipitate • Change in smell.
Magnesium ribbon burns with a dazzling white flame and forms white magnesium oxide.
2Mg + O₂ → 2MgO
3. Chemical Equation
A chemical equation represents a chemical reaction using symbols and formulae.
Unbalanced: H₂ + O₂ → H₂O
Balanced: 2H₂ + O₂ → 2H₂O
Equations are balanced according to the Law of Conservation of Mass: mass can neither be created nor
destroyed in a chemical reaction.
4. Balancing a Chemical Equation
Example: Fe + H₂O → Fe₃O₄ + H₂
Balance Fe: 3Fe + H₂O → Fe₃O₄ + H₂
Balance O: 3Fe + 4H₂O → Fe₃O₄ + H₂
Balance H: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
5. Combination Reaction
Two or more substances combine to form a single product. General form: A + B → AB.
Example 1: 2Mg + O₂ → 2MgO
Example 2: CaO + H₂O → Ca(OH)₂
Key point: a combination reaction forms one product.
6. Decomposition Reaction
A single compound breaks down into two or more simpler substances. It may require heat, light or
electricity.
Class 10 Chemistry • Chapter 1 Page 1
Chapter 1 — Chemical Reactions and Equations
NCERT-based detailed notes
Important reactions • observations • definitions • exam questions
1. Chemical Reaction
A chemical reaction is a process in which one or more substances undergo a change to form one or more
new substances.
Examples:
2Mg + O₂ → 2MgO
2H₂ + O₂ → 2H₂O
2. Signs of a Chemical Reaction
Change in state • Change in colour • Evolution of a gas • Change in temperature • Formation of a
precipitate • Change in smell.
Magnesium ribbon burns with a dazzling white flame and forms white magnesium oxide.
2Mg + O₂ → 2MgO
3. Chemical Equation
A chemical equation represents a chemical reaction using symbols and formulae.
Unbalanced: H₂ + O₂ → H₂O
Balanced: 2H₂ + O₂ → 2H₂O
Equations are balanced according to the Law of Conservation of Mass: mass can neither be created nor
destroyed in a chemical reaction.
4. Balancing a Chemical Equation
Example: Fe + H₂O → Fe₃O₄ + H₂
Balance Fe: 3Fe + H₂O → Fe₃O₄ + H₂
Balance O: 3Fe + 4H₂O → Fe₃O₄ + H₂
Balance H: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
5. Combination Reaction
Two or more substances combine to form a single product. General form: A + B → AB.
Example 1: 2Mg + O₂ → 2MgO
Example 2: CaO + H₂O → Ca(OH)₂
Key point: a combination reaction forms one product.
6. Decomposition Reaction
A single compound breaks down into two or more simpler substances. It may require heat, light or
electricity.
Class 10 Chemistry • Chapter 1 Page 1
