Class 9 Science Exploration Chapter 9 MCQs – Atomic Foundations of Matter – practice will strengthen your grip on atomic foundations. The chapter starts with the Law of Conservation of Mass and the Law of Constant Proportions, then explains Dalton’s atomic theory. It goes on to show how atoms combine through covalent bonds (single and double) and ionic bonds, how covalent and ionic compounds are named and how chemical formulae are written by the criss-cross method. It also compares the properties of ionic and covalent compounds and shows how to calculate molecular mass and formula unit mass. These MCQs with answers follow the new NCERT Exploration textbook and combine concept checks with numerical practice.
Class 9 Science Exploration Chapter 9 MCQs with Answers
Class 9 Science Chapter 9 MCQs Set 1
Q1. What does the law of conservation of mass state for a chemical reaction?
[A]. Atoms disappear during reaction
[B]. Only solids retain their mass
[C]. Mass is neither created nor destroyed
[D]. Products always weigh less than reactants
Q2. Why can an open flask lose measured mass when vinegar reacts with baking soda?
[A]. All the liquid becomes massless
[B]. Carbon dioxide escapes
[C]. The balance creates matter
[D]. Atoms are destroyed
Q3. Why is a balloon attached to the flask when checking mass conservation in a gas-producing reaction?
[A]. To destroy the gas
[B]. To increase the temperature
[C]. To absorb all the liquid
[D]. To retain the gas within the weighed system
Q4. Twenty grams of hydrogen reacts completely with 160 g oxygen. What mass of water forms?
[A]. 3200 g
[B]. 140 g
[C]. 180 g
[D]. 160 g
Q5. What does the law of constant proportions state?
[A]. All mixtures have identical composition
[B]. Every compound contains equal masses of its elements
[C]. Atoms can combine in any mass ratio in one compound
[D]. A compound contains its elements in a fixed ratio by mass
Class 9 Science Chapter 9 MCQs Set 2
Q6. Hydrogen and oxygen occur in water in the mass ratio 1:8. How much oxygen combines with 3 g hydrogen?
[A]. 24 g
[B]. 8 g
[C]. 11 g
[D]. 3 g
Q7. NaCl contains sodium and chlorine in the mass ratio 23:35.5. How much chlorine combines with 46 g sodium?
[A]. 71 g
[B]. 46 g
[C]. 58.5 g
[D]. 35.5 g
Q8. Why does the law of constant proportions not generally apply to mixtures?
[A]. Mixtures are always gases
[B]. Their components can be mixed in variable proportions
[C]. Mixtures contain no atoms
[D]. Mixtures always have only one component
Q9. Which idea explains conservation of mass during ordinary chemical reactions?
[A]. Atoms are converted into empty space
[B]. Atoms rearrange without being created or destroyed
[C]. All atoms become the same element
[D]. New atoms appear from heat
Q10. What is a covalent bond?
[A]. Attraction between two isolated positive ions
[B]. A bond formed through shared electron pairs
[C]. A bond caused by sharing neutrons
[D]. A bond formed by destroying electrons
Class 9 Science Chapter 9 MCQs Set 3
Q11. How many electron pairs are shared in a single covalent bond?
[A]. Three
[B]. Two
[C]. Four
[D]. One
Q12. What kind of bond joins the two atoms in an oxygen molecule, O₂?
[A]. Ionic bond
[B]. Double covalent bond
[C]. Single covalent bond
[D]. No bond
Q13. How many shared electron pairs join the two nitrogen atoms in N₂?
[A]. Two
[B]. Four
[C]. Three
[D]. One
Q14. What does the formula H₂O indicate about one water molecule?
[A]. Two hydrogen molecules and one oxygen atom
[B]. Two hydrogen atoms and one oxygen atom
[C]. One hydrogen atom and two oxygen atoms
[D]. Two oxygen molecules and one hydrogen atom
Q15. What is the name of CO₂?
[A]. Carbon monoxide
[B]. Carbon dioxide
[C]. Calcium dioxide
[D]. Cobalt oxide
Class 9 Science Chapter 9 MCQs Set 4
Q16. A sodium atom loses one electron. What does it become?
[A]. A chloride anion
[B]. A neutral neon atom
[C]. A sodium cation, Na⁺
[D]. A sodium anion, Na⁻
Q17. A chlorine atom gains one electron. What does it become?
[A]. Cl⁻
[B]. A neutral argon atom
[C]. Cl⁺
[D]. A proton
Q18. What holds sodium and chloride ions together in sodium chloride?
[A]. A shared pair of neutrons
[B]. Gravitational attraction alone
[C]. Electrostatic attraction between opposite charges
[D]. Repulsion between opposite charges
Q19. Which statement describes solid sodium chloride correctly?
[A]. It consists only of separate NaCl molecules
[B]. Its ions are randomly isolated
[C]. It contains only uncharged sodium atoms
[D]. Its ions form a regular three-dimensional crystal arrangement
Q20. What is the correct formula of magnesium chloride, formed from Mg²⁺ and Cl⁻?
[A]. MgCl₂
[B]. MgCl
[C]. Mg₂Cl
[D]. Mg₂Cl₂
Class 9 Science Chapter 9 MCQs Set 5
Q22. What is the formula of aluminium oxide formed from Al³⁺ and O²⁻?
[A]. AlO
[B]. Al₂O
[C]. Al₂O₃
[D]. Al₃O₂
Q23. Why is magnesium oxide written MgO instead of Mg₂O₂?
[A]. The simplest charge-balanced ratio is 1:1
[B]. Oxygen always has valency one
[C]. The ions have no charges
[D]. Magnesium contains only one electron
Q24. Which formula correctly represents magnesium hydroxide?
[A]. MgO₂H
[B]. Mg₂OH
[C]. MgOH₂
[D]. Mg(OH)₂
Q25. What is the formula of aluminium sulfate, formed from Al³⁺ and SO₄²⁻?
[A]. Al₃(SO₄)₂
[B]. Al₂(SO₄)₃
[C]. AlSO₄
[D]. Al₂SO₄
Class 9 Science Chapter 9 MCQs Set 6
Q26. Why does solid sodium chloride not conduct electricity under ordinary conditions?
[A]. Its ions have lost all charge
[B]. It contains only free electrons
[C]. Its ions are held in fixed positions
[D]. It contains no charged particles
Q27. Why does sodium chloride solution conduct electricity while sugar solution does not?
[A]. Salt becomes a metal in water
[B]. Sugar cannot dissolve in water
[C]. All dissolved substances conduct equally
[D]. Salt supplies mobile ions, whereas sugar does not
Q28. What is the molecular mass of CO₂? Use C = 12 u and O = 16 u.
[A]. 44 u
[B]. 60 u
[C]. 32 u
[D]. 28 u
Q29. What is the formula unit mass of Na₂O? Use Na = 23 u and O = 16 u.
[A]. 62 u
[B]. 46 u
[C]. 39 u
[D]. 78 u
Q30. What is the formula unit mass of Ca(NO₃)₂? Use Ca = 40 u, N = 14 u and O = 16 u.
[A]. 164 u
[B]. 148 u
[C]. 102 u
[D]. 124 u
Essential Points from Chapter 9 Atomic Foundations of Matter
- Law of Conservation of Mass (Lavoisier, 1789): the total mass of the reactants equals the total mass of the products. In an open container, escaping gas makes the mass appear to drop.
- Law of Constant Proportions (Proust): a compound always contains its elements in a fixed ratio by mass, whatever its source. For example, hydrogen and oxygen are always in a 1:8 ratio in water.
- Dalton’s atomic theory says atoms are indivisible, atoms of one element are identical, and atoms combine in simple whole-number ratios to form compounds.
- A molecule is a neutral group of two or more atoms that can exist independently. Noble gases such as helium exist as single atoms because they are already stable.
- A covalent bond forms by sharing electrons. H₂ and Cl₂ have single bonds, and O₂ has a double bond (O=O). HCl and H₂O are covalent compounds.
- Covalent compounds are named with prefixes: mono, di, tri, tetra, penta, hexa. Examples include CO (carbon monoxide), PCl₃ (phosphorus trichloride) and N₂O₄ (dinitrogen tetroxide).
- An ionic bond is the electrostatic attraction between a cation and an anion formed by electron transfer, as in Na⁺Cl⁻. Ionic compounds form 3-D crystals, not molecules.
- To write a formula, cross over the valencies or charges and reduce to the simplest ratio. Use brackets for more than one polyatomic ion, as in Mg(OH)₂ and Al₂(SO₄)₃.
- Ionic compounds dissolve in water, conduct electricity only when dissolved or molten, and have high melting points. Covalent compounds generally do not conduct electricity and have low melting points and sugar dissolves in water but still does not conduct.
- Molecular mass applies to covalent compounds, for example H₂O = 18 u and CO₂ = 44 u. Formula unit mass applies to ionic compounds, for example Na₂O = 62 u and Ca(NO₃)₂ = 164 u.
Handy Tricks for Bonding and Formula MCQs
- If an atom has fewer than 4 valence electrons, it usually loses them and forms a cation. If it has more than 4, it usually gains electrons and forms an anion.
- Do not write the subscript 1 and do not write the ion charges in the final formula.
- When hydrogen comes first in a formula, no prefix is used, so H₂S is hydrogen sulfide.
- Check that the total positive and negative charges are equal before you choose a formula option.
- In conservation of mass numericals, add all the reactant masses and all the product masses and compare the two totals.
These Class 9 Science Exploration Chapter 9 MCQs connect atomic structure with how real substances form and behave. Questions based on the baking soda and vinegar balloon experiment, cinnabar’s fixed composition, salt water conducting electricity and the solubility of camphor and naphthalene make abstract ideas easy to picture. Regular practice with formula writing and mass calculations will speed up your problem solving considerably. Keep the table of common ions with you while revising, because most formula-based questions rely on it.
Chapter 9 Solutions for Deeper Practice
After finishing this quiz, check our NCERT Solutions for Class 9 Science Exploration Chapter 9 for clear, worked answers to every textbook exercise. They cover electron-transfer and electron-sharing diagrams, formula completion tables, formula unit mass of fertiliser compounds, conservation of mass numericals and assertion-reason questions. Using the solutions together with these MCQs builds a strong base in “Atomic Foundations of Matter” for both school exams and higher classes.
