Class 9 · ScienceNCERT 2026 Revised Syllabus
Chapter 9: Atomic Foundations of Matter (0906CH09)
Exploration · Atomic number, mass number, ions, chemical formulae, bonding.
Chapter code: 0906ch09 · Book: Exploration · Class 9 Science
Find complete NCERT solutions for Class 9 Science, Exploration Chapter 9. Every question has a clear, step-by-step answer for chapter code 0906CH09.
Pause and Ponder(24 questions)
1
A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain.
School Level Solution
Ans.
No. The law applies to the total closed system. Burning ethanol forms gaseous products such as carbon dioxide and water vapour that leave the open beaker, so the residue in the beaker is not the total mass of products.
2
When 20 g of hydrogen reacts completely with 160 g of oxygen, how much water is formed according to the Law of Conservation of Mass?
School Level Solution
Ans.
Mass of reactants = 20 + 160 = 180 g. By conservation of mass, mass of water formed = 180 g.
3
A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing 20 g of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions?
School Level Solution
Ans.
If sulfur is 40% and oxygen 60%, then oxygen:sulfur = 60:40 = 3:2. For 20 g sulfur, oxygen = 20×(3/2) = 30 g.
4
Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3:4. How much oxygen will combine with 9 g of carbon to form carbon monoxide?
School Level Solution
Ans.
O:C = 4:3. For 9 g carbon, oxygen = 9×(4/3) = 12 g.
5
The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.
School Level Solution
Ans.
In a compound, elements are chemically combined in a definite stoichiometric ratio, so composition is fixed. In a mixture, substances are only physically combined and can be present in variable proportions.
6
Students X and Y, both prepared an oxide of copper by combining copper and oxygen in the ratios of 4:1 and 8:2, respectively. Do their results justify the Law of Constant Proportions? Explain.
School Level Solution
Ans.
8:2 simplifies to 4:1. Thus both samples have the same copper:oxygen mass ratio, consistent with the Law of Definite Proportions.
7
Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g of water. Reason (R): According to Dalton’s Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.
School Level Solution
Ans.
The mass calculation in A is consistent with conservation of mass for water. R, as worded, is not a correct statement of Dalton’s theory because the “simple whole number ratio” specifically describes combination of atoms, not a general mass-ratio statement.
8
Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule (N₂).
School Level Solution
Ans.
Each nitrogen atom needs three more electrons to complete its octet. The two nitrogen atoms therefore share three pairs of electrons, forming N≡N; each N retains one lone pair.
9
The atomic number of fluorine is 9. Explain the formation of the fluorine molecule (F₂).
School Level Solution
Ans.
Fluorine has configuration 2,7 and needs one more electron for a stable octet. Two fluorine atoms share one electron each, forming a shared pair: F–F.
10
Show the formation of the following molecules: (i) Carbon dioxide (CO₂) (ii) Hydrogen sulfide (H₂S) (iii) Ammonia (NH₃).
School Level Solution
Ans.
Carbon forms two double bonds with oxygen in O=C=O. Sulfur has two unpaired valence electrons available for bonding and forms H–S–H. Nitrogen has three unpaired valence electrons and forms three N–H bonds in NH₃, leaving one lone pair on N.
11
Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.
School Level Solution
Ans.
Neon has electronic configuration 2,8. Its outermost shell is complete, so it is already stable and normally does not need to gain, lose or share electrons to achieve an octet.
12
What kind of ion will oxygen (O) form?
School Level Solution
Ans.
Oxygen (Z=8) has configuration 2,6 and tends to gain two electrons to complete its octet, forming O²⁻.
13
Fill in the blanks. Among magnesium and chlorine, magnesium atom can give two electrons to become Mg²⁺. However, chlorine can take only one electron to become ____________. Now, __________ ion of magnesium and __________ ions of chlorine combine to give magnesium chloride.
School Level Solution
Ans.
Chlorine gains one electron to form Cl⁻. Therefore one Mg²⁺ combines with two Cl⁻ ions to form MgCl₂.
14
Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.
School Level Solution
Ans.
Potassium: K → K⁺ + e⁻, then K⁺ + Cl⁻ → KCl. Calcium: Ca → Ca²⁺ + 2e⁻, then Ca²⁺ + 2Cl⁻ → CaCl₂. Lewis/electron-dot diagrams should show the electron transfer and the resulting stable ions.
15
Illustrate how sodium sulfide (Na₂S) is formed.
School Level Solution
Ans.
Each sodium atom loses one electron: 2Na → 2Na⁺ + 2e⁻. Sulfur gains two electrons: S + 2e⁻ → S²⁻. The ions combine in a 2:1 ratio to give Na₂S.
16
Name the following: (i) CO₂ (ii) NO₂ (iii) SF₆ (iv) PCl₃.
School Level Solution
Ans.
Apply the prefixes for the number of atoms: CO₂ carbon dioxide; NO₂ nitrogen dioxide; SF₆ sulfur hexafluoride; PCl₃ phosphorus trichloride.
17
Write the formula for the following: (i) Sodium hydrogencarbonate (ii) Sulfur dioxide (iii) Ferric chloride (iv) Cuprous oxide.
School Level Solution
Ans.
Sodium hydrogencarbonate = NaHCO₃; sulfur dioxide = SO₂; ferric chloride (Fe³⁺, Cl⁻) = FeCl₃; cuprous oxide (Cu⁺, O²⁻) = Cu₂O.
18
Write the formulae for the compounds formed from the following pairs of ions: (i) Fe³⁺ and OH⁻ (ii) K⁺ and CO₃²⁻.
School Level Solution
Ans.
Balance charges: Fe³⁺ needs three OH⁻ → Fe(OH)₃. Two K⁺ balance one CO₃²⁻ → K₂CO₃.
19
What type of chemical bond is present in a solid compound that does not conduct electricity in the solid state but conducts electricity when dissolved in water?
School Level Solution
Ans.
In a solid ionic compound, ions are fixed in a crystal lattice and cannot move, so it does not conduct. When dissolved in water, the ions become mobile and the solution conducts electricity.
20
Metal M, with two electrons in its valence shell (M shell), reacts with oxygen to form a compound that is slightly soluble in water. Predict its: (i) formula (ii) type of bond (iii) electrical conductivity of its aqueous solution.
School Level Solution
Ans.
M has valency 2 and oxygen forms O²⁻, so the simplest formula is MO. The bond is ionic. In water, the dissolved ions make the solution electrically conductive (to the extent that the compound dissolves).
21
Find the molecular mass of nitric acid (HNO₃). Atomic mass — H = 1 u; N = 14 u; O = 16 u.
School Level Solution
Ans.
HNO₃ = 1 + 14 + 3×16 = 63 u.
22
Find the molecular mass of methane (CH₄). Atomic mass — C = 12 u; H = 1 u.
School Level Solution
Ans.
CH₄ = 12 + 4×1 = 16 u.
23
Find the formula unit mass of potassium chloride (KCl). Atomic mass — K = 39 u; Cl = 35.5 u.
School Level Solution
Ans.
KCl = 39 + 35.5 = 74.5 u.
24
Find the formula unit mass of magnesium hydroxide, Mg(OH)₂. Atomic mass — Mg = 24 u; O = 16 u; H = 1 u.
School Level Solution
Ans.
Mg(OH)₂ = 24 + 2×(16+1) = 58 u.
Activities — How to Do(3 questions)
1
Activity 9.1 (physical change): weigh water in a beaker (tared balance), add a spatula of salt, record the mass, then stir till dissolved. What do you observe about mass, and what does it show?
School Level Solution
Ans.
Reading after dissolving equals the sum of water and salt masses. Observation: salt disappears visually but mass is conserved — dissolving is a physical change (no new substance), so mass cannot change.
2
Activity 9.2 (vinegar + baking soda): Setup 1 — mix openly on a balance and compare readings. Setup 2 — same reaction with a balloon sealing the flask. Are initial and final readings same in each case? Why the difference?
School Level Solution
Ans.
Setup 1 (open): final reading is lower — CO₂ gas escapes into air. Setup 2 (balloon sealed): readings are equal — gas is trapped and weighed too. Lesson: mass is conserved in a chemical change only if nothing escapes the system.
3
Activity 9.3 (verify the law): weigh flasks A (sodium sulphate solution) and B (barium chloride solution) together, pour B into A, mix, and weigh again. What do you observe, and what law does it verify?
School Level Solution
Ans.
On mixing, a white precipitate of barium sulphate appears, yet the balance reading does not change (closed system, nothing escapes). This verifies the law of conservation of mass: atoms only rearrange in a chemical reaction.