Welcome to our ultimate guide on Class 9 Science Chapter 3: Atoms and Molecules! Whether you’re a student aiming to ace your 2025 CBSE exams, a parent supporting your child, or a teacher seeking resources, this blog post has it all. We’ll cover the in-text questions and answers, exercise solutions, and detailed notes for Chapter 3, breaking down the concepts of atoms, molecules, and chemical formulas into simple, digestible explanations. By the end, you’ll have a solid grasp of this foundational Chemistry chapter—perfect for scoring high and building a strong science base!
Chapter 3 introduces the building blocks of matter—atoms and molecules—and explains how they combine to form everything around us. From understanding atomic mass to writing chemical formulas, this chapter is key to excelling in Class 9 Science and beyond. Let’s dive into the Class 9 Science Chapter 3 questions, answers, and notes!
Why Study Class 9 Science Chapter 3?
Before we explore the content, let’s see why this chapter matters. Atoms and molecules are the essence of Chemistry, explaining how substances form and react. This knowledge is not only crucial for your exams (carrying around 20-25 marks in the CBSE syllabus) but also connects to real-world applications like medicine, engineering, and environmental science. Mastering this chapter in 2025 will set you up for success in higher classes and competitive exams like NTSE or Olympiads.
Now, let’s break this down into three sections: Notes, In-Text Questions and Answers, and Exercise Solutions.
Class 9 Science Chapter 3 Notes
1. Laws of Chemical Combination
- Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction. For example, if 12 g of carbon reacts with 32 g of oxygen to form 44 g of CO₂, the total mass remains constant.
- Law of Definite Proportions: A compound always contains elements in a fixed ratio by mass. In water (H₂O), hydrogen and oxygen are always in a 1:8 mass ratio.
2. Atoms
- Definition: An atom is the smallest unit of an element that retains its properties. It’s indivisible in chemical reactions.
- Atomic Mass: Measured in atomic mass units (amu), where 1 amu = 1/12th the mass of a carbon-12 atom. Examples: Hydrogen = 1 u, Oxygen = 16 u.
- Symbols: Elements are represented by symbols (e.g., H for hydrogen, O for oxygen).
3. Molecules
- Definition: A molecule is a group of atoms bonded together, representing the smallest unit of a compound or element that can exist independently.
- Types:
- Elemental Molecules: O₂ (oxygen gas), N₂ (nitrogen gas).
- Compound Molecules: H₂O (water), CO₂ (carbon dioxide).
4. Atomicity
- The number of atoms in a molecule. Examples:
- Monoatomic: He (helium) – 1 atom.
- Diatomic: O₂ – 2 atoms.
- Polyatomic: O₃ (ozone) – 3 atoms.
5. Molecular Mass
- The sum of atomic masses of all atoms in a molecule. Example: H₂O = (2 × 1) + 16 = 18 u.
6. Ions
- Charged particles formed by gaining or losing electrons.
- Cations: Positive (e.g., Na⁺).
- Anions: Negative (e.g., Cl⁻).
7. Writing Chemical Formulas
- Rules:
- Write the symbol of the positive ion first, then the negative ion (e.g., NaCl).
- Use valency to balance charges (e.g., Ca²⁺ + O²⁻ = CaO).
- For polyatomic ions, use brackets if needed (e.g., Ca(OH)₂).
8. Mole Concept
- Mole: A unit representing 6.022 × 10²³ particles (Avogadro’s number).
- Molar Mass: Mass of 1 mole of a substance in grams (e.g., H₂O = 18 g/mol).
- Formula: Number of moles = Mass / Molar Mass.
These notes summarize the chapter’s core ideas—refer to them while solving questions!
In-Text Questions and Answers: Page 35
Question 1: In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
Answer:
- Reactants: Sodium carbonate (5.3 g) + Ethanoic acid (6 g) = 5.3 + 6 = 11.3 g.
- Products: Carbon dioxide (2.2 g) + Water (0.9 g) + Sodium ethanoate (8.2 g) = 2.2 + 0.9 + 8.2 = 11.3 g.
Since the mass of reactants (11.3 g) equals the mass of products (11.3 g), this agrees with the law of conservation of mass.
Question 2: Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Answer:
- Ratio of H:O in water = 1:8.
- For 1 g of hydrogen, 8 g of oxygen is needed.
- For 3 g of hydrogen: 3 × 8 = 24 g of oxygen.
Thus, 24 g of oxygen is required.
In-Text Questions and Answers: Page 38
Question 1: Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Answer:
The postulate: “Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.” This explains why the total mass remains constant, as atoms rearrange without changing their total mass.
Question 2: Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Answer:
The postulate: “Atoms of a given element are identical in mass and properties, and compounds are formed by a fixed ratio of atoms.” This ensures a compound always has elements in a definite mass ratio.
In-Text Questions and Answers: Page 40
Question 1: Define the atomic mass unit.
Answer:
An atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom. It’s a standard unit for measuring the mass of atoms and molecules. 1 amu ≈ 1.66 × 10⁻²⁴ g.
Question 2: Why is it not possible to see an atom with the naked eye?
Answer:
Atoms are extremely small (size ~10⁻¹⁰ m), far below the resolution of the human eye (which detects objects > 0.1 mm). They can only be observed using advanced tools like scanning tunneling microscopes.
Exercise Questions and Answers: Page 44
Question 1: Calculate the molecular mass of the following:
(a) H₂O
(b) CO₂
(c) CH₄
Answer:
(a) H₂O = (2 × 1) + 16 = 18 u.
(b) CO₂ = 12 + (2 × 16) = 12 + 32 = 44 u.
(c) CH₄ = 12 + (4 × 1) = 12 + 4 = 16 u.
Question 2: Calculate the mass of:
(a) 0.5 mole of N₂ gas
(b) 0.5 mole of N atoms
(c) 3.011 × 10²³ molecules of CO₂
Answer:
(a) Molar mass of N₂ = 14 × 2 = 28 g/mol.
Mass = 0.5 × 28 = 14 g.
(b) Molar mass of N = 14 g/mol.
Mass = 0.5 × 14 = 7 g.
(c) 1 mole of CO₂ = 6.022 × 10²³ molecules, molar mass = 44 g/mol.
Moles = (3.011 × 10²³) / (6.022 × 10²³) ≈ 0.5 mol.
Mass = 0.5 × 44 = 22 g.
Question 3: Calculate the number of particles in:
(a) 46 g of Na atoms
(b) 8 g of O₂ molecules
(c) 0.1 mole of H₂
Answer:
(a) Molar mass of Na = 23 g/mol.
Moles = 46 / 23 = 2 mol.
Particles = 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ atoms.
(b) Molar mass of O₂ = 32 g/mol.
Moles = 8 / 32 = 0.25 mol.
Particles = 0.25 × 6.022 × 10²³ = 1.5055 × 10²³ molecules.
(c) Particles = 0.1 × 6.022 × 10²³ = 6.022 × 10²² molecules.
Question 4: Write the chemical formulas for:
(a) Calcium carbonate
(b) Magnesium chloride
(c) Sodium hydroxide
Answer:
(a) Ca²⁺ + CO₃²⁻ = CaCO₃.
(b) Mg²⁺ + 2Cl⁻ = MgCl₂.
(c) Na⁺ + OH⁻ = NaOH.
Question 5: What is the mass of 10 moles of sodium sulphite (Na₂SO₃)?
Answer:
Molar mass of Na₂SO₃ = (2 × 23) + 32 + (3 × 16) = 46 + 32 + 48 = 126 g/mol.
Mass = 10 × 126 = 1260 g.
Tips to Master Chapter 3
- Memorize Key Terms: Learn definitions of atom, molecule, mole, and atomic mass.
- Practice Numericals: Focus on molecular mass, mole concept, and particle calculations.
- Understand Laws: Relate Dalton’s theory to the laws of chemical combination.
- Revise Formulas: Practice writing chemical formulas using valency rules.
Class 9 Science Chapter 3: Atoms and Molecules unlocks the microscopic world of matter, from atoms to moles. With these notes, questions, and exercise solutions, you’re well-equipped to excel in your 2025 CBSE exams. This chapter connects theory to calculations, making it both fascinating and scoring.
For more resources, check NCERT solutions and sample papers online. Have questions? Drop them in the comments—we’re here to help! Keep studying and stay curious!
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