NCERT Solutions – Chapter 1: Chemical Reactions and Equations

Subject: Science | Class: 10 | Chapter 1

Chemical Reactions and Equations is the first chapter of Class 10 Science. It helps students understand how substances react to form new products, the role of equations in representing these reactions, and the importance of balancing them. This chapter also introduces key types of reactions such as combination, decomposition, displacement, and redox reactions.

In this page, you will find NCERT Solutions for Class 10 Science Chapter 1 written in a clear, simple, and detailed manner. Each question is explained with examples and equations so that students can easily understand the logic behind every concept.

• ✅ Based on the latest NCERT textbook (2025 Edition)
• ✅ Step-by-step explanations of each question
• ✅ Easy language for self-study and quick revision
• ✅ Useful for CBSE, ICSE, and State Boards

Let’s begin our study of this chapter by understanding what a chemical reaction is and how we represent it using chemical equations.

Question 1:

Why should magnesium ribbon be cleaned before burning?

Answer: Magnesium ribbon is cleaned by rubbing with sandpaper to remove the thin layer of magnesium oxide formed on its surface due to its reaction with oxygen in air. This ensures that the pure magnesium metal burns completely in air, producing a bright white flame and forming white magnesium oxide (MgO).

Question 2:

Write the balanced chemical equation with state symbols for the following reactions:

• Hydrogen gas combines with nitrogen to form ammonia.
• Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
• Barium chloride reacts with sodium sulphate to give barium sulphate and sodium chloride.

Answer:

• N₂(g) + 3H₂(g) → 2NH₃(g)
• 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
• BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Question 3:

What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: A balanced chemical equation has an equal number of atoms of each element on both sides of the arrow. Balancing ensures the Law of Conservation of Mass—matter is neither created nor destroyed during a chemical reaction. Thus, the total mass of reactants equals the total mass of products.

Question 4:

Translate the following statements into chemical equations and balance them:

• Hydrogen gas combines with chlorine gas to give hydrogen chloride gas.
• Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
• Barium chloride reacts with aluminium sulphate to give barium sulphate and aluminium chloride.
• Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer:

• H₂(g) + Cl₂(g) → 2HCl(g)
• 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
• 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
• 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

Question 5:

Write a balanced chemical equation for the following reaction and identify the type of reaction:

Zinc reacts with dilute sulphuric acid to produce hydrogen gas and zinc sulphate.

Answer:

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

This is a displacement reaction because zinc displaces hydrogen from sulphuric acid to form zinc sulphate.

Question 6:

What are the different types of chemical reactions?

Answer: The major types of chemical reactions are:

• Combination Reaction: Two or more substances combine to form a single product.
  Example: 2H₂ + O₂ → 2H₂O
• Decomposition Reaction: A single compound breaks down into simpler substances.
  Example: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
• Displacement Reaction: One element replaces another in a compound.
  Example: Fe + CuSO₄ → FeSO₄ + Cu
• Double Displacement Reaction: Exchange of ions between two compounds.
  Example: AgNO₃ + NaCl → AgCl + NaNO₃
• Redox Reaction: Both oxidation and reduction occur simultaneously.
  Example: Zn + CuO → ZnO + Cu

Question 7:

What is a decomposition reaction? Explain the different types of decomposition reactions with examples.

Answer: A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more simpler substances when heat, light, or electricity is supplied.

• Thermal decomposition: Decomposition by heat.
  Example: CaCO₃(s) → CaO(s) + CO₂(g)
• Electrolytic decomposition: Decomposition by passing electric current.
  Example: 2H₂O(l) → 2H₂(g) + O₂(g)
• Photochemical decomposition: Decomposition by light energy.
  Example: 2AgCl(s) → 2Ag(s) + Cl₂(g) (in sunlight)

Decomposition reactions are the opposite of combination reactions and are usually endothermic as they absorb energy to break bonds.

Question 8:

What can you observe when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube?

Answer: When potassium iodide solution is added to lead nitrate solution, a yellow precipitate of lead iodide (PbI₂) is formed. This is a double displacement reaction and also a precipitation reaction.

Chemical Equation: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

Question 9:

Explain the following terms with one example each:

• Oxidation
• Reduction

Answer:

• Oxidation: It is the process in which a substance gains oxygen or loses hydrogen.
  Example: 2Mg + O₂ → 2MgO
• Reduction: It is the process in which a substance gains hydrogen or loses oxygen.
  Example: CuO + H₂ → Cu + H₂O

Both oxidation and reduction occur simultaneously in a reaction, hence such reactions are called redox reactions.

Question 10:

A shiny brown-colored element ‘X’ on heating in air becomes black in color. Name the element ‘X’ and the black-colored compound formed.

Answer: The brown-colored element ‘X’ is Copper (Cu). When heated in air, it reacts with oxygen to form black copper oxide (CuO).

Chemical Equation: 2Cu + O₂ → 2CuO

The black color of the product indicates the formation of copper(II) oxide.

Question 11:

Why do we apply paint on iron articles?

Answer: Paint is applied on iron articles to prevent rusting. Paint acts as a protective coating that prevents the direct contact of iron with air, moisture, and oxygen — the main factors responsible for corrosion or rust formation (Fe₂O₃·xH₂O).

Question 12:

Oil and fat containing food items are flushed with nitrogen. Why?

Answer: Oil and fat-based food items are easily oxidized when exposed to air, leading to rancidity (foul smell and taste). To prevent oxidation, such packets are flushed with nitrogen gas, which is inert and prevents contact with oxygen.

Question 13:

Explain the following observations:

• The shining surface of some metals becomes dull when exposed to air.
• Iron nails kept in damp cloth rust.
• Grapes get fermented.

Answer:

• Dulling of metal surface: Metals such as copper and silver react with gases in the atmosphere (oxygen, carbon dioxide, sulphur compounds) to form a dull, corroded layer of oxides or sulphides on their surface.
• Rusting of iron: Iron reacts with oxygen and moisture to form reddish-brown hydrated ferric oxide (rust), represented as Fe₂O₃·xH₂O.
• Fermentation of grapes: The sugars present in grapes are converted into alcohol by the action of yeast (anaerobic respiration), producing ethanol and carbon dioxide.

All these changes involve chemical reactions — oxidation in metals and biological oxidation in fermentation.

Question 14:

What is corrosion? State the conditions necessary for rusting of iron.

Answer: Corrosion is the slow deterioration of metals when they react with substances present in air, moisture, or chemicals. In case of iron, corrosion is known as rusting.

Chemical Reaction: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (Rust)

• Conditions required for rusting:

If either oxygen or moisture is absent, rusting does not take place.

Question 15:

What is rancidity? How can it be prevented?

Answer: Rancidity is the process by which oils and fats in food materials get oxidized when exposed to air, leading to an unpleasant smell and taste.

• Ways to prevent rancidity:

Question 16:

Why are decomposition reactions called the opposite of combination reactions? Write equations for both types of reactions.

Answer: In a combination reaction, two or more substances combine to form a single product. Whereas in a decomposition reaction, a single compound breaks down into two or more simpler substances. Hence, they are opposite to each other.

• Combination Reaction: 2H₂ + O₂ → 2H₂O
• Decomposition Reaction: 2H₂O → 2H₂ + O₂

One reaction forms a compound while the other breaks it down — making them reverse processes.

Question 17:

Identify the type of reaction and balance the following equations:

• NaOH + HCl → NaCl + H₂O
• CaCO₃ → CaO + CO₂
• 3H₂ + N₂ → 2NH₃
• Zn + H₂SO₄ → ZnSO₄ + H₂

Answer:

• Neutralization Reaction: NaOH + HCl → NaCl + H₂O
• Thermal Decomposition: CaCO₃ → CaO + CO₂
• Combination Reaction: 3H₂ + N₂ → 2NH₃
• Displacement Reaction: Zn + H₂SO₄ → ZnSO₄ + H₂

Question 18:

What do you understand by exothermic and endothermic reactions? Give one example of each.

Answer:

• Exothermic Reaction: Reactions that release heat energy.
  Example: C + O₂ → CO₂ + Heat
• Endothermic Reaction: Reactions that absorb heat energy.
  Example: CaCO₃ → CaO + CO₂ (on heating)

Exothermic reactions increase temperature, while endothermic reactions decrease it.

1. Chemical Reactions and Equations
2. Acids, Bases and Salts
3. Metals and Non‑Metals
4. Carbon and Its Compounds
5. Life Processes
6. Control and Coordination
7. How Do Organisms Reproduce?
8. Heredity and Evolution
9. Light – Reflection and Refraction
10. The Human Eye and the Colourful World
11. Electricity
12. Magnetic Effects of Electric Current
13. Our Environment

Conclusion: Key Takeaways from Chapter 1 – Chemical Reactions and Equations

• Chemical reactions involve the transformation of reactants into products through rearrangement of atoms.
• Every reaction must be represented by a balanced chemical equation to obey the Law of Conservation of Mass.
• Main types of reactions: Combination, Decomposition, Displacement, Double Displacement, and Redox.
• Corrosion and rancidity are everyday examples of chemical reactions that occur naturally.
• Balancing equations and recognizing reaction types form the foundation for studying chemistry in higher classes.

Tip for Students: Always write chemical equations with correct symbols and state symbols (s, l, g, aq). Practice balancing equations daily to strengthen your basics for board exams.

We hope these Class 10 Science Chapter 1 Solutions help you in your preparation. Stay tuned for more chapters and notes from NotesWave — your digital study companion!