🔬 Summary: Chemical Reactions

A chemical reaction is a process in which one or more substances (reactants) are transformed into one or more different substances (products). During this process, chemical bonds are broken and new bonds are formed, resulting in a new arrangement of atoms.

⚗️ Key Concepts

• Reactants: Substances that take part in the reaction
• Products: New substances formed after the reaction
• Chemical Equation: Representation of a reaction using symbols
• Balanced Equation: Equal number of atoms on both sides
• Exothermic Reaction: Releases heat (e.g., combustion)
• Endothermic Reaction: Absorbs heat (e.g., photosynthesis)
• Catalyst: A substance that speeds up a reaction without being consumed
• Precipitation Reaction: Formation of an insoluble solid
• Redox Reaction: Involves oxidation and reduction

🧪 Types of Chemical Reactions

1. Combination Reaction – Two or more reactants form one product
2. Decomposition Reaction – One reactant breaks into simpler products
3. Displacement Reaction – More reactive element displaces less reactive one
4. Double Displacement Reaction – Exchange of ions between reactants
5. Oxidation-Reduction (Redox) Reaction – Loss and gain of electrons

❓ 50 Important Questions and Answers

📘 A. Multiple Choice Questions (Q1–Q20)

Which of the following is a combination reaction?

✅ A) CaO + H₂O → Ca(OH)₂

The reaction 2KClO₃ → 2KCl + 3O₂ is:

✅ B) Decomposition

Rusting of iron is a:

✅ C) Redox reaction

Which reaction is endothermic?

✅ A) Photosynthesis

In a chemical equation, the substances written on the left are:

✅ B) Reactants

Which gas is evolved when Zn reacts with HCl?

✅ A) Hydrogen

Catalyst used in the decomposition of hydrogen peroxide is:

✅ C) Manganese dioxide

The formation of curd is:

✅ A) Chemical change

The reaction of baking soda with lemon juice produces:

✅ D) CO₂

Which one is a displacement reaction?

✅ B) Zn + CuSO₄ → ZnSO₄ + Cu

What is observed when silver chloride is exposed to sunlight?

✅ C) It turns grey

2Mg + O₂ → 2MgO is:

✅ A) Combination and redox

The substance which increases the rate of a reaction is called:

✅ C) Catalyst

Which of these is not a physical change?

✅ B) Burning of paper

The reaction that absorbs heat from surroundings is:

✅ D) Endothermic

What is the precipitate formed in: BaCl₂ + Na₂SO₄ → ?

✅ C) BaSO₄

Example of a decomposition reaction is:

✅ A) 2H₂O₂ → 2H₂O + O₂

CuO + H₂ → Cu + H₂O is an example of:

✅ B) Redox

The equation H₂ + Cl₂ → 2HCl is:

✅ A) Combination

Which one is exothermic?

✅ D) Neutralization

📗 B. Short Answer Questions (Q21–Q40)

Define a chemical reaction.

Ans: A process where substances change into new substances with different properties.

What is a balanced chemical equation?

Ans: An equation with equal atoms of each element on both sides.

Write an example of a displacement reaction.

Ans: Zn + CuSO₄ → ZnSO₄ + Cu

What is a catalyst?

Ans: A substance that speeds up a reaction without being used up.

What happens when lead nitrate is heated?

Ans: It decomposes into lead oxide, nitrogen dioxide, and oxygen.

Give an example of a combination reaction.

Ans: C + O₂ → CO₂

Why is respiration a redox reaction?

Ans: Glucose is oxidized and oxygen is reduced.

What is oxidation?

Ans: Addition of oxygen or loss of electrons.

What is reduction?

Ans: Loss of oxygen or gain of electrons.

Name a reaction involving precipitation.

Ans: AgNO₃ + NaCl → AgCl↓ + NaNO₃

What happens when iron reacts with water and oxygen?

Ans: Rust (Fe₂O₃·xH₂O) forms.

State any two observations that show a chemical change has occurred.

Ans: (1) Color change, (2) Gas evolution

What is thermal decomposition?

Ans: Breaking down of compounds using heat.

What is a redox reaction?

Ans: A reaction involving both oxidation and reduction.

Write the equation for rusting.

Ans: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃

What is meant by neutralization reaction?

Ans: Acid + Base → Salt + Water

How is lime water prepared?

Ans: By adding water to calcium oxide (CaO + H₂O → Ca(OH)₂)

What is chemical change?

Ans: A change that results in formation of a new substance.

Why does AgCl turn grey in sunlight?

Ans: Due to decomposition into Ag and Cl₂

Name two physical changes.

Ans: Melting of ice, boiling of water

📙 C. Long Answer / Descriptive (Q41–Q50)

1. What are the different types of chemical reactions with examples?
2. Explain redox reactions with at least two examples.
3. Describe the characteristics of a chemical reaction.
4. How do we balance a chemical equation? Give steps.
5. Write an activity to show that a chemical change has occurred.
6. Differentiate between combination and decomposition reactions.
7. What is the role of heat, light, or electricity in chemical reactions?
8. Explain displacement and double displacement reactions with equations.
9. Discuss corrosion and its prevention.
10. Why is it necessary to balance chemical equations?

✅ Conclusion

Understanding chemical reactions is the foundation of chemistry. From burning fuels to digesting food, these reactions are part of everyday life. With this summary and 50+ questions, you’re ready for school exams, NTSE, Olympiads, and competitive entrance tests.