CLASS 10 SCIENCE
Chapter 1: Chemical Reactions and Equations
1. Chemical Reaction
A chemical reaction is a process in which one or more substances change into new substances with
different properties. The starting substances are called reactants, and the substances formed are called
products.
Example: Magnesium burns in oxygen to form magnesium oxide.
2Mg + O₂ → 2MgO
2. Signs of a Chemical Reaction
• Change in colour
• Change in state
• Evolution of a gas
• Change in temperature
• Formation of a precipitate (an insoluble solid)
Examples: Magnesium burns with a dazzling white flame; iron forms reddish-brown rust; zinc reacts with
dilute acid and releases hydrogen gas.
3. Chemical Equation
A chemical equation represents a chemical reaction using symbols and formulae. In an equation,
reactants are written on the left and products on the right, with an arrow (→) between them.
Word equation: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Chemical equation:
Zn + H₂SO₄ → ZnSO₄ + H₂
4. Balanced Chemical Equation
An equation is balanced when the number of atoms of each element is equal on both sides. It follows the
Law of Conservation of Mass: mass can neither be created nor destroyed in a chemical reaction.
Remember: Balance equations by changing coefficients placed before formulae. Never change the small
numbers (subscripts) inside a chemical formula.
Class 10 Science • Page 1
, BALANCING EQUATIONS
5. Example of Balancing
Unbalanced equation: Mg + O₂ → MgO
Balanced equation: 2Mg + O₂ → 2MgO
Atom check: Left side has 2 Mg and 2 O atoms. Right side also has 2 Mg and 2 O atoms.
6. State Symbols
(s) = solid (l) = liquid (g) = gas (aq) = aqueous solution (dissolved in water)
Example: 2H₂(g) + O₂(g) → 2H₂O(l)
7. Types of Chemical Reactions
A. Combination Reaction
Two or more reactants combine to form a single product.
CaO + H₂O → Ca(OH)₂ + heat
Quicklime + Water → Slaked lime + Heat
This is an exothermic reaction because heat is released.
B. Decomposition Reaction
A single compound breaks down into two or more simpler substances. It requires energy in the form of
heat, light or electricity.
Thermal decomposition (heat):
CaCO₃ —heat→ CaO + CO₂
Electrolytic decomposition (electricity):
2H₂O —electricity→ 2H₂ + O₂
Photolytic decomposition (sunlight):
2AgCl —sunlight→ 2Ag + Cl₂
An endothermic reaction absorbs energy.
Class 10 Science • Page 2
Chapter 1: Chemical Reactions and Equations
1. Chemical Reaction
A chemical reaction is a process in which one or more substances change into new substances with
different properties. The starting substances are called reactants, and the substances formed are called
products.
Example: Magnesium burns in oxygen to form magnesium oxide.
2Mg + O₂ → 2MgO
2. Signs of a Chemical Reaction
• Change in colour
• Change in state
• Evolution of a gas
• Change in temperature
• Formation of a precipitate (an insoluble solid)
Examples: Magnesium burns with a dazzling white flame; iron forms reddish-brown rust; zinc reacts with
dilute acid and releases hydrogen gas.
3. Chemical Equation
A chemical equation represents a chemical reaction using symbols and formulae. In an equation,
reactants are written on the left and products on the right, with an arrow (→) between them.
Word equation: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Chemical equation:
Zn + H₂SO₄ → ZnSO₄ + H₂
4. Balanced Chemical Equation
An equation is balanced when the number of atoms of each element is equal on both sides. It follows the
Law of Conservation of Mass: mass can neither be created nor destroyed in a chemical reaction.
Remember: Balance equations by changing coefficients placed before formulae. Never change the small
numbers (subscripts) inside a chemical formula.
Class 10 Science • Page 1
, BALANCING EQUATIONS
5. Example of Balancing
Unbalanced equation: Mg + O₂ → MgO
Balanced equation: 2Mg + O₂ → 2MgO
Atom check: Left side has 2 Mg and 2 O atoms. Right side also has 2 Mg and 2 O atoms.
6. State Symbols
(s) = solid (l) = liquid (g) = gas (aq) = aqueous solution (dissolved in water)
Example: 2H₂(g) + O₂(g) → 2H₂O(l)
7. Types of Chemical Reactions
A. Combination Reaction
Two or more reactants combine to form a single product.
CaO + H₂O → Ca(OH)₂ + heat
Quicklime + Water → Slaked lime + Heat
This is an exothermic reaction because heat is released.
B. Decomposition Reaction
A single compound breaks down into two or more simpler substances. It requires energy in the form of
heat, light or electricity.
Thermal decomposition (heat):
CaCO₃ —heat→ CaO + CO₂
Electrolytic decomposition (electricity):
2H₂O —electricity→ 2H₂ + O₂
Photolytic decomposition (sunlight):
2AgCl —sunlight→ 2Ag + Cl₂
An endothermic reaction absorbs energy.
Class 10 Science • Page 2
