Class 10 Science – Chapter 1: Chemical Reactions and Equations

This chapter teaches how substances change into new substances and how to write and balance chemical equations. It also introduces the main types of chemical reactions and oxidation effects in everyday life.

1. What is a Chemical Reaction?
   A chemical reaction is a process in which one or more substances (reactants) change into new substances (products) with different properties.
   Examples:

   • Burning of magnesium ribbon

   • Rusting of iron

   • Digestion of food

   • Cooking food

   • Respiration
     Signs of a Chemical Reaction:

   • Change in colour

   • Change in temperature

   • Evolution of gas

   • Formation of precipitate (solid)

   • Change in state

2. Chemical Equation
   A chemical equation represents a chemical reaction using symbols and formulae.

   • Word Equation:
     Magnesium + Oxygen → Magnesium Oxide

   • Chemical Equation:
     $Mg + O_2 → MgO$ (Unbalanced)

   • Balanced Equation:
     $2Mg + O_2 → 2MgO$

3. Balanced Chemical Equation
   A balanced chemical equation has the same number of atoms of each element on both sides.
   Why is balancing necessary? Because of the Law of Conservation of Mass: Matter can neither be created nor destroyed during a chemical reaction.
   Rules:

   • Do not change chemical formulae.

   • Change only the coefficients.

4. Types of Chemical Reactions
   A. Combination Reaction
   Two or more substances combine to form one product.
   Example:
   $2Mg + O<em>2 → 2MgO$ $CaO + H</em>2O → Ca(OH)<em>2 + ext{Heat}$ Quick lime + Water → Slaked lime B. Decomposition Reaction One compound breaks into two or more simpler substances. Types: (i) Thermal decomposition (Heat): $CaCO</em>3 → CaO + CO<em>2$ (ii) Electrolytic decomposition (Electricity): $2H</em>2O → 2H<em>2 + O</em>2$
   (iii) Photolytic decomposition (Light):
   $2AgCl → 2Ag + Cl<em>2$ $2AgBr → 2Ag + Br</em>2$
   These reactions are used in photography.
   C. Displacement Reaction
   A more reactive element displaces a less reactive element.
   Example:
   $Zn + CuSO<em>4 → ZnSO</em>4 + Cu$
   Iron + Copper sulphate:
   $Fe + CuSO<em>4 → FeSO</em>4 + Cu$
   D. Double Displacement Reaction
   Exchange of ions between two compounds.
   Example:
   $Na<em>2SO</em>4 + BaCl<em>2 → BaSO</em>4↓ + 2NaCl$
   White precipitate of barium sulphate is formed.
   Neutralization Reaction:
   Acid + Base → Salt + Water
   $NaOH + HCl → NaCl + H_2O$
   E. Redox Reaction
   Oxidation and reduction occur together.

   • Oxidation: Gain of oxygen / Loss of hydrogen
     Example:
     $2Cu + O_2 → 2CuO$

   • Reduction: Loss of oxygen / Gain of hydrogen
     Example:
     $CuO + H<em>2 → Cu + H</em>2O$

5. Oxidation in Everyday Life
   Corrosion: Slow destruction of metals due to reaction with air and moisture.
   Examples:

   • Rusting of iron

   • Green coating on copper

   • Black coating on silver
     Prevention:

   • Painting

   • Galvanization

   • Oiling

   • Greasing
     Rancidity: Oxidation of oils and fats causing bad smell and taste.
     Prevention:

   • Refrigeration

   • Airtight containers

   • Nitrogen gas packing

   • Antioxidants

Important Chemical Equations:

• $2Mg + O_2 → 2MgO$

• $CaO + H<em>2O → Ca(OH)</em>2$

• $CaCO<em>3 → CaO + CO</em>2$

• $2AgCl → 2Ag + Cl_2$

• $2AgBr → 2Ag + Br_2$

• $2H<em>2O → 2H</em>2 + O_2$

• $Zn + CuSO<em>4 → ZnSO</em>4 + Cu$

• $Fe + CuSO<em>4 → FeSO</em>4 + Cu$

• $BaCl<em>2 + Na</em>2SO<em>4 → BaSO</em>4 + 2NaCl$

• $NaOH + HCl → NaCl + H_2O$

• $CuO + H<em>2 → Cu + H</em>2O$

Important Definitions:

• Reactants: Substances taking part in a reaction.

• Products: New substances formed.

• Precipitate: Insoluble solid formed.

• Catalyst: Substance that changes the rate of reaction without being consumed.

• Oxidation: Gain of oxygen or loss of hydrogen.

• Reduction: Loss of oxygen or gain of hydrogen.

Frequently Asked Board Questions:

• What is a chemical reaction?

• Why should chemical equations be balanced?

• State the law of conservation of mass.

• Differentiate between combination and decomposition reactions.

• Explain displacement and double displacement reactions with examples.

• What are oxidation and reduction?

• What is corrosion? How can it be prevented?

• What is rancidity? How can it be prevented?