NCERT Solutions for Class 10 Science Chapter 1 – Chemical Reactions and Equations

Key Concepts Overview

Chemical reactions are processes where substances (reactants) are transformed into new substances (products) through the breaking and forming of chemical bonds. Understanding these reactions is fundamental to chemistry.

What is a Chemical Equation?

A chemical equation is a symbolic representation of a chemical reaction. It shows:

• The reactants (substances that react)
• The products (substances that are formed)
• The physical states (solid, liquid, gas, aqueous)
• The conditions required for the reaction

Writing and Balancing Chemical Equations

A balanced chemical equation has an equal number of atoms of each element on both sides. This follows the Law of Conservation of Mass.

Example – Burning of Magnesium:

Word equation: Magnesium + Oxygen to Magnesium Oxide

Unbalanced: Mg + O2 to MgO

Balanced: 2Mg + O2 to 2MgO

Types of Chemical Reactions

1. Combination Reaction

Two or more substances combine to form a single product.

General form: A + B to AB

Example: CaO + H2O to Ca(OH)2 (Quick lime reacts with water to form slaked lime)

2. Decomposition Reaction

A single compound breaks down into two or more simpler substances.

Types of Decomposition:

• Thermal decomposition: Breaking down by heat
• Electrolytic decomposition: Breaking down by electricity
• Photolytic decomposition: Breaking down by light

3. Displacement Reaction

A more reactive element displaces a less reactive element from its compound.

Example: Fe + CuSO4 to FeSO4 + Cu (Iron displaces copper)

4. Double Displacement Reaction

Exchange of ions between two compounds to form new compounds.

Example: Na2SO4 + BaCl2 to BaSO4 + 2NaCl

5. Oxidation and Reduction Reactions (Redox)

Oxidation: Addition of oxygen or removal of hydrogen

Reduction: Addition of hydrogen or removal of oxygen

Both reactions occur simultaneously in redox reactions.

NCERT Exercise Solutions

Page 6 – Intext Questions

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Answer: Magnesium ribbon should be cleaned before burning to remove the layer of magnesium oxide that forms on its surface when exposed to air. This oxide layer prevents the burning of magnesium ribbon. Cleaning with sandpaper removes this layer, allowing magnesium to burn properly with a bright white flame.

Q2. Write the balanced equation for: Hydrogen + Chlorine to Hydrogen chloride

Answer: H2 + Cl2 to 2HCl

Page 10 – Intext Questions

Q1. A solution of substance X is used for whitewashing. Name the substance X.

Answer: The substance X is Calcium hydroxide (slaked lime). Its chemical formula is Ca(OH)2. When slaked lime reacts with carbon dioxide in air, it forms calcium carbonate which gives a shiny finish to walls.

Important Questions for Board Exams

1 Mark Questions

1. What is a chemical equation?
2. Define oxidation reaction with example.
3. What type of reaction is respiration?

3 Mark Questions

1. Explain the different types of decomposition reactions with examples.
2. What is a redox reaction? Identify the oxidizing and reducing agents.
3. Why do we apply paint on iron articles?

Key Points to Remember

• Always balance chemical equations before solving numerical problems
• Remember the reactivity series for displacement reactions
• Oxidation and reduction always occur together
• Corrosion is an oxidation reaction
• Rancidity can be prevented by adding antioxidants

Continue Learning: Explore Chapter 2 – Acids, Bases and Salts for more NCERT Solutions.