📝 Important Questions · Class 9 Science
Chapter 9: Atomic Foundations of Matter
15 Short Answer Questions + 10 Long Answer Questions | Theory & Application Both
2–3 Marks SAQs
5 Marks LAQs
Numericals Included
CBSE/NCERT Pattern
Laws of Chemistry
Bonding Concepts
5 Marks LAQs
Numericals Included
CBSE/NCERT Pattern
Laws of Chemistry
Bonding Concepts
📚 Contents
How to Use This Q&A Sheet
- All questions are based strictly on Chapter 9: Atomic Foundations of Matter.
- Short Answer Questions (SAQs) carry 2–3 marks. Keep answers to 3–5 sentences.
- Long Answer Questions (LAQs) carry 5 marks. Always write step-by-step solutions for numericals.
- Questions are labelled [Theoretical] or [Practical/Application] so you know what kind of answer is expected.
- Bold terms in answers are key words — always include them in your exam answers.
Exam Strategy
Attempt SAQs first to secure easy marks. For numericals, always write the formula first, then substitute values, then state the final answer with the correct unit.
Attempt SAQs first to secure easy marks. For numericals, always write the formula first, then substitute values, then state the final answer with the correct unit.
Short Answer Questions — 15 Questions
🧪 Chemistry Questions (Q1–Q10)
Q1. [Theoretical] State the Law of Conservation of Mass. Who proposed it and in which year? (2 Marks)
Ans: The Law of Conservation of Mass states that matter can neither be created nor destroyed during a chemical reaction. In other words, the total mass of the reactants is always equal to the total mass of the products. This law was proposed by Antoine Lavoisier in 1789. Lavoisier is also known as the Father of Modern Chemistry.
Q2. [Practical/Application] In an experiment, 4 g of calcium carbonate reacts with 2.92 g of hydrochloric acid to produce carbon dioxide, water, and calcium chloride. If carbon dioxide produced is 1.76 g and water is 0.72 g, find the mass of calcium chloride formed. (3 Marks)
Ans: We use the Law of Conservation of Mass — total mass of reactants = total mass of products.
Given: Mass of CaCO₃ = 4.0 g | Mass of HCl = 2.92 g | CO₂ = 1.76 g | H₂O = 0.72 g
Total mass of reactants = 4.0 + 2.92 = 6.92 g
Total mass of products = 1.76 + 0.72 + mass of CaCl₂ = 6.92 g
Mass of CaCl₂ = 6.92 − 1.76 − 0.72
∴ Mass of Calcium Chloride = 4.44 g
Total mass of reactants = 4.0 + 2.92 = 6.92 g
Total mass of products = 1.76 + 0.72 + mass of CaCl₂ = 6.92 g
Mass of CaCl₂ = 6.92 − 1.76 − 0.72
∴ Mass of Calcium Chloride = 4.44 g
Q3. [Theoretical] State the Law of Constant Proportions (Law of Definite Proportions). Give one example to illustrate it. (2 Marks)
Ans: The Law of Constant Proportions, proposed by Joseph Proust, states that in any compound formed by two or more elements, the elements are always combined in a fixed ratio by mass, regardless of its source or method of preparation. For example, water (H₂O) always contains hydrogen and oxygen in a mass ratio of 1:8, whether it is obtained from a river, borewell, or ocean.
Q4. [Practical/Application] Sodium chloride (NaCl) contains sodium and chlorine in the mass ratio 23:35.5. If 46 g of sodium reacts completely, how much chlorine is required to form NaCl? (3 Marks)
Ans: Using the Law of Constant Proportions, the ratio of chlorine to sodium must remain constant.
Given: Mass ratio Na:Cl = 23:35.5 | Mass of Na = 46 g
Mass of Cl = (35.5 ÷ 23) × 46
Mass of Cl = 1.543 × 46
∴ Mass of Chlorine required = 71 g
Mass of Cl = (35.5 ÷ 23) × 46
Mass of Cl = 1.543 × 46
∴ Mass of Chlorine required = 71 g
Q5. [Theoretical] List any four postulates of Dalton’s Atomic Theory. (2 Marks)
Ans: According to Dalton’s Atomic Theory (proposed in 1808):
- All matter is made up of very tiny particles called atoms, which participate in chemical reactions.
- Atoms are indivisible and cannot be created or destroyed in a chemical reaction.
- Atoms of a given element are identical in mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in simple whole number ratios to form compounds.
Q6. [Theoretical] What is a covalent bond? How is a single bond different from a double bond? (2 Marks)
Ans: A covalent bond is formed when two atoms share one or more pairs of electrons to attain a stable electronic configuration (octet). A single bond is formed when two atoms share one pair of electrons, represented as a single line (e.g., H—H). A double bond is formed when two atoms share two pairs of electrons, represented by two lines (e.g., O=O). Double bonds are stronger than single bonds.
Q7. [Theoretical] Define an ionic bond. Explain with the example of sodium chloride (NaCl). (3 Marks)
Ans: An ionic bond is the electrostatic force of attraction between oppositely charged ions formed by the transfer of electrons from one atom to another. In sodium chloride (NaCl): the sodium atom (Na) has one valence electron, which it loses to become a positively charged sodium cation (Na⁺). The chlorine atom (Cl) has seven valence electrons and gains one electron to become a negatively charged chloride anion (Cl⁻). These oppositely charged ions attract each other, forming the ionic compound NaCl.
Q8. [Practical/Application] Why does the reading of the weighing balance differ before and after the reaction in Experimental Set-up 1 (vinegar + baking soda in open flask) but NOT in Experimental Set-up 2? (3 Marks)
Ans: In Experimental Set-up 1, baking soda is added directly to vinegar in an open conical flask. The carbon dioxide gas produced during the reaction escapes into the atmosphere, so the final mass recorded is less than the initial mass. This makes it appear that mass is not conserved.
In Experimental Set-up 2, the balloon is tied to the mouth of the flask, so the carbon dioxide produced is trapped inside the balloon and does not escape. The total mass remains the same — confirming the Law of Conservation of Mass.
In Experimental Set-up 2, the balloon is tied to the mouth of the flask, so the carbon dioxide produced is trapped inside the balloon and does not escape. The total mass remains the same — confirming the Law of Conservation of Mass.
Q9. [Theoretical] What is a molecule? How is a molecule of an element different from a molecule of a compound? (2 Marks)
Ans: A molecule is an electrically neutral entity consisting of more than one atom that is capable of independent existence and shows all the properties of that substance. A molecule of an element is formed when atoms of the same element combine (e.g., H₂, O₂, Cl₂). A molecule of a compound is formed when atoms of different elements combine in a fixed ratio (e.g., HCl, H₂O).
Q10. [Practical/Application] Write the chemical formula for: (i) Calcium chloride (ii) Aluminium oxide (iii) Magnesium hydroxide. (3 Marks)
Ans: Using the criss-cross method with valencies:
- Calcium chloride: Ca²⁺ and Cl⁻ → criss-cross gives CaCl₂
- Aluminium oxide: Al³⁺ and O²⁻ → criss-cross gives Al₂O₃
- Magnesium hydroxide: Mg²⁺ and OH⁻ → criss-cross gives Mg(OH)₂ (bracket used since there are two polyatomic OH⁻ ions)
Exam Tip
Always simplify subscripts if they have a common factor. Also, use brackets when a polyatomic ion appears more than once.
Always simplify subscripts if they have a common factor. Also, use brackets when a polyatomic ion appears more than once.
⚗️ More Chemistry Questions (Q11–Q15)
Q11. [Theoretical] Name the following covalent compounds: (i) CO (ii) CO₂ (iii) PCl₃ (iv) N₂O₄ (2 Marks)
Ans: Using the prefix system for naming covalent compounds:
- CO → Carbon monoxide (mono is dropped for the first element but used for the second)
- CO₂ → Carbon dioxide
- PCl₃ → Phosphorus trichloride
- N₂O₄ → Dinitrogen tetroxide
Q12. [Theoretical] Why do ionic compounds not conduct electricity in the solid state, but conduct electricity when dissolved in water? (2 Marks)
Ans: In the solid state, ions in ionic compounds are held in fixed positions in a crystal lattice by strong electrostatic forces and cannot move freely. Since electrical conduction requires the movement of charged particles, ionic solids do not conduct electricity. When dissolved in water, the ionic compound dissociates into free-moving cations and anions. These freely moving ions carry electric current through the solution, making it a good conductor of electricity.
Q13. [Practical/Application] Find the molecular mass of water (H₂O) and carbon dioxide (CO₂). (Atomic masses: H = 1 u, O = 16 u, C = 12 u) (3 Marks)
Ans:
Molecular mass of H₂O:
= (1 u × 2) + (16 u × 1)
= 2 + 16
∴ Molecular mass of H₂O = 18 u
= (1 u × 2) + (16 u × 1)
= 2 + 16
∴ Molecular mass of H₂O = 18 u
Molecular mass of CO₂:
= (12 u × 1) + (16 u × 2)
= 12 + 32
∴ Molecular mass of CO₂ = 44 u
Q14. [Theoretical] What is an ionic bond? State two properties by which ionic compounds differ from covalent compounds. (2 Marks)
Ans: An ionic bond is the force of electrostatic attraction between oppositely charged ions formed by the transfer of electrons.
⚡ Ionic Compounds
- Soluble in water; insoluble in kerosene/petrol
- High melting and boiling points
- Conduct electricity when dissolved in water
🔗 Covalent Compounds
- Generally insoluble in water; soluble in kerosene/petrol
- Low melting and boiling points
- Generally do not conduct electricity
Q15. [Practical/Application] A compound contains 40% sulfur and 60% oxygen by mass. A sample of this compound contains 20 g of sulfur. What mass of oxygen must be present? (3 Marks)
Ans: By the Law of Constant Proportions, the ratio of sulfur to oxygen must always be 40:60 = 2:3.
Given: Sulfur = 40%, Oxygen = 60% | Mass of S in sample = 20 g
Mass of O = (60 ÷ 40) × 20
= 1.5 × 20
∴ Mass of Oxygen = 30 g
Mass of O = (60 ÷ 40) × 20
= 1.5 × 20
∴ Mass of Oxygen = 30 g
Long Answer Questions — 10 Questions
🧪 Chemistry Questions (Q1–Q6)
Q1. [Theoretical] State and explain the Law of Conservation of Mass. Describe an experiment to verify it using vinegar and baking soda. (5 Marks)
Ans: The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction — the total mass of reactants equals the total mass of products.
- Experiment (Set-up 2): Place a conical flask containing about 20 mL of vinegar on a weighing balance. Place baking soda in a balloon attached to the mouth of the flask.
- Initial Reading: Record the total mass of vinegar + baking soda (in the balloon) before the reaction. Note this as the initial mass.
- Reaction: Lift the balloon to allow baking soda to fall into the vinegar. A brisk effervescence occurs as carbon dioxide (CO₂) is produced. The CO₂ inflates the balloon but does not escape.
- Final Reading: Record the mass of the conical flask + balloon after the reaction. This is the final mass.
- Observation: The initial mass equals the final mass. The chemical reaction is: Vinegar + Baking soda → CO₂ + other substances
- Conclusion: Since no matter was lost or gained, the experiment confirms the Law of Conservation of Mass.
Remember
In Set-up 1 (open flask), mass appears to decrease because CO₂ escapes. Always use a closed system to correctly verify this law.
In Set-up 1 (open flask), mass appears to decrease because CO₂ escapes. Always use a closed system to correctly verify this law.
Q2. [Practical/Application] 12 g of carbon combines with 32 g of oxygen to form 44 g of carbon dioxide. If 3.6 g of carbon reacts completely with oxygen, how much carbon dioxide is produced? Also verify the Law of Conservation of Mass for the original reaction. (5 Marks)
Ans:
Given: 12 g C + 32 g O → 44 g CO₂
Part 1 — Verify Law of Conservation of Mass:
Total mass of reactants = 12 + 32 = 44 g
Total mass of products = 44 g (CO₂)
Mass of reactants = Mass of products ✓ Law verified.
Part 1 — Verify Law of Conservation of Mass:
Total mass of reactants = 12 + 32 = 44 g
Total mass of products = 44 g (CO₂)
Mass of reactants = Mass of products ✓ Law verified.
Part 2 — Find CO₂ for 3.6 g of C:
12 g of C produces → 44 g of CO₂
1 g of C produces → 44 ÷ 12 g of CO₂
3.6 g of C produces → (44 ÷ 12) × 3.6
= 3.667 × 3.6
∴ CO₂ produced = 13.2 g
Exam Tip
In such problems, always write out the unitary method clearly. Examiners award marks for showing each step, not just the final answer.
In such problems, always write out the unitary method clearly. Examiners award marks for showing each step, not just the final answer.
Q3. [Theoretical] State Dalton’s Atomic Theory. How does it explain the Law of Conservation of Mass and the Law of Constant Proportions? (5 Marks)
Ans: Dalton’s Atomic Theory (proposed 1808) consists of the following postulates:
- All matter is made up of very tiny particles called atoms, which participate in chemical reactions.
- Atoms are indivisible particles — they cannot be created or destroyed in a chemical reaction.
- Atoms of a given element are identical in mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in the ratio of simple whole numbers to form compounds.
- The relative number and kinds of atoms are constant in a given compound.
How it explains the two laws:
- Law of Conservation of Mass: Since atoms are indivisible and cannot be created or destroyed, they simply rearrange during a chemical reaction. The total number of atoms before and after the reaction remains the same → so mass is conserved.
- Law of Constant Proportions: Since atoms of the same element are identical and atoms always combine in fixed whole number ratios, every molecule of a compound will always have the same ratio of elements by mass.
Scientist Spotlight
John Dalton was born in England. He moved to Manchester in 1793 to teach mathematics and chemistry. He presented his atomic theory in 1808, which became a turning point in the study of matter.
John Dalton was born in England. He moved to Manchester in 1793 to teach mathematics and chemistry. He presented his atomic theory in 1808, which became a turning point in the study of matter.
Q4. [Theoretical] Explain the formation of a water molecule (H₂O) and a hydrogen chloride molecule (HCl) using the concept of covalent bonding. Draw electron-sharing diagrams in description. (5 Marks)
Ans: A covalent bond is formed by the sharing of electrons between atoms to achieve a stable electronic configuration.
- Formation of HCl: Hydrogen (atomic no. 1) has 1 electron in its K-shell and needs 1 more to complete its duplet. Chlorine (atomic no. 17) has 7 valence electrons and needs 1 more for an octet. Each atom shares one electron to form a single covalent bond. Formula: H—Cl
- Formation of H₂O: Oxygen (atomic no. 8) has 6 valence electrons and needs 2 more electrons to complete its octet. One oxygen atom shares one electron each with two hydrogen atoms. This forms two single covalent bonds. Formula: H₂O
- Key Terms: The shared pair of electrons is called a bonding pair. Unshared pairs are called lone pairs.
- Bond Representation: HCl → H—Cl (single bond); H₂O → H—O—H (two single bonds)
Note for Diagrams
In your exam answer, draw circles for each atom, show the valence shell electrons as dots (·), and circle the shared pair of electrons to indicate the covalent bond.
In your exam answer, draw circles for each atom, show the valence shell electrons as dots (·), and circle the shared pair of electrons to indicate the covalent bond.
Q5. [Theoretical] Explain the formation of an ionic bond using the example of sodium chloride (NaCl). Include: electron transfer, ion formation, and properties of the compound formed. (5 Marks)
Ans:
- Sodium (Na) — atomic number 11, electronic configuration: 2, 8, 1. It has 1 valence electron, which it tends to lose to achieve the stable configuration of neon (2, 8).
- Loss of electron by Na: Na atom → loses 1 electron → becomes Na⁺ (sodium cation). Na⁺ has 11 protons and 10 electrons, giving it a +1 charge.
- Chlorine (Cl) — atomic number 17, electronic configuration: 2, 8, 7. It has 7 valence electrons and needs 1 more to complete its octet.
- Gain of electron by Cl: Cl atom → gains 1 electron → becomes Cl⁻ (chloride anion). Cl⁻ has 17 protons and 18 electrons, giving it a −1 charge.
- Bond Formation: The oppositely charged Na⁺ and Cl⁻ ions attract each other through electrostatic force, forming the ionic compound NaCl. Reaction: Na + Cl → Na⁺ + Cl⁻ → NaCl
- Crystal Structure: NaCl does not form individual molecules. It forms a 3-D crystal lattice where each Na⁺ is surrounded by 6 Cl⁻ ions and each Cl⁻ is surrounded by 6 Na⁺ ions.
Properties of NaCl (Ionic Compound)
High melting point | Soluble in water | Conducts electricity when dissolved | Insoluble in kerosene and petrol
High melting point | Soluble in water | Conducts electricity when dissolved | Insoluble in kerosene and petrol
Q6. [Practical/Application] Find the formula unit mass of: (i) Calcium nitrate Ca(NO₃)₂ (ii) Magnesium hydroxide Mg(OH)₂. (Atomic masses: Ca=40, N=14, O=16, Mg=24, H=1) (5 Marks)
Ans: Formula unit mass is the sum of atomic masses of all atoms present in one formula unit of an ionic compound.
Part 1: Formula unit mass of Ca(NO₃)₂
Atoms present: 1 Ca, 2 N, 6 O
= (40 × 1) + (14 × 2) + (16 × 6)
= 40 + 28 + 96
∴ Formula unit mass of Ca(NO₃)₂ = 164 u
Atoms present: 1 Ca, 2 N, 6 O
= (40 × 1) + (14 × 2) + (16 × 6)
= 40 + 28 + 96
∴ Formula unit mass of Ca(NO₃)₂ = 164 u
Part 2: Formula unit mass of Mg(OH)₂
Atoms present: 1 Mg, 2 O, 2 H
= (24 × 1) + (16 × 2) + (1 × 2)
= 24 + 32 + 2
∴ Formula unit mass of Mg(OH)₂ = 58 u
Common Mistake
In Ca(NO₃)₂, students often count only 3 oxygen atoms. Remember the bracket means the whole NO₃ group is multiplied by 2, so there are 6 oxygen atoms.
In Ca(NO₃)₂, students often count only 3 oxygen atoms. Remember the bracket means the whole NO₃ group is multiplied by 2, so there are 6 oxygen atoms.
⚗️ Bonding & Properties Questions (Q7–Q10)
Q7. [Theoretical] Compare ionic compounds and covalent compounds on the basis of: (i) solubility in water (ii) electrical conductivity (iii) melting and boiling points (iv) solubility in kerosene and petrol. Give one example of each type. (5 Marks)
Ans:
🔷 Ionic Compounds
- Solubility in water: Generally soluble in water (e.g., NaCl dissolves easily)
- Electrical conductivity: Do not conduct in solid state; conduct when dissolved in water (ions become free to move)
- Melting/Boiling points: High — strong inter-ionic attractions require more energy to break
- Kerosene/Petrol: Insoluble in non-polar solvents
- Example: Sodium chloride (NaCl), Copper sulfate (CuSO₄)
🔶 Covalent Compounds
- Solubility in water: Most are insoluble in water (sugar is an exception)
- Electrical conductivity: Generally do not conduct electricity (no free ions)
- Melting/Boiling points: Low — weak intermolecular forces
- Kerosene/Petrol: Generally soluble in non-polar solvents
- Example: Camphor, Naphthalene
Q8. [Practical/Application] Write chemical formulae for the following compounds using the criss-cross method: (i) Aluminium nitrate (ii) Iron(III) oxide (Ferric oxide) (iii) Aluminium sulfate (iv) Calcium carbonate. (5 Marks)
Ans: Using the criss-cross method — write cation first, then anion; crossover the charge numbers as subscripts; simplify if needed.
- Aluminium nitrate: Al³⁺ and NO₃⁻ → criss-cross → Al(NO₃)₃ [brackets used as NO₃ is polyatomic and appears 3 times]
- Ferric oxide (Iron III oxide): Fe³⁺ and O²⁻ → criss-cross → subscripts 2 and 3 → Fe₂O₃
- Aluminium sulfate: Al³⁺ and SO₄²⁻ → criss-cross → Al₂(SO₄)₃ [brackets used as SO₄ is polyatomic and appears 3 times]
- Calcium carbonate: Ca²⁺ and CO₃²⁻ → both have the same valency → subscripts cancel → simplify Ca₂(CO₃)₂ → CaCO₃
Exam Tip
Always check if the subscripts have a common factor. If yes, divide both by it. E.g., Mg₂O₂ simplifies to MgO. Also, never write brackets when there is only 1 polyatomic ion.
Always check if the subscripts have a common factor. If yes, divide both by it. E.g., Mg₂O₂ simplifies to MgO. Also, never write brackets when there is only 1 polyatomic ion.
Q9. [Theoretical] Explain the formation of: (i) a hydrogen molecule (H₂) (ii) a chlorine molecule (Cl₂) (iii) an oxygen molecule (O₂) through covalent bonding. State the type of bond in each. (5 Marks)
Ans:
- Hydrogen molecule (H₂): Each H atom (atomic no. 1) has 1 electron in its K-shell and needs 1 more for stability. Two H atoms each share 1 electron, forming a single covalent bond. Represented as H—H.
- Chlorine molecule (Cl₂): Each Cl atom (atomic no. 17) has 7 valence electrons and needs 1 more for an octet. Two Cl atoms each share 1 electron, forming a single covalent bond. Represented as Cl—Cl.
- Oxygen molecule (O₂): Each O atom (atomic no. 8) has 6 valence electrons and needs 2 more for an octet. Two O atoms each share 2 electrons, forming a double covalent bond (two pairs of shared electrons). Represented as O=O.
- Summary of bond types: H₂ → Single bond | Cl₂ → Single bond | O₂ → Double bond
- Key principle: In all cases, sharing electrons lowers the total energy of the system, making the molecule more stable than the individual atoms.
🌟 Did You Know?
Nitrogen (N₂) has 5 valence electrons and needs 3 more — so it forms a triple bond (N≡N). It is one of the strongest bonds in chemistry, which is why N₂ gas is very stable and unreactive under normal conditions!
Nitrogen (N₂) has 5 valence electrons and needs 3 more — so it forms a triple bond (N≡N). It is one of the strongest bonds in chemistry, which is why N₂ gas is very stable and unreactive under normal conditions!
Q10. [Practical/Application] 5.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water, and some sodium acetate. (i) Find the mass of sodium acetate formed. (ii) Verify whether the Law of Conservation of Mass holds. (5 Marks)
Ans:
Given: Na₂CO₃ = 5.3 g | Acetic acid = 6.0 g | CO₂ = 2.2 g | H₂O = 0.9 g
Step 1 — Total mass of reactants:
= 5.3 + 6.0 = 11.3 g
Step 2 — Find mass of sodium acetate:
Total mass of products = Total mass of reactants = 11.3 g
Mass of sodium acetate = 11.3 − 2.2 − 0.9
∴ Mass of Sodium Acetate = 8.2 g
Step 3 — Verify Law of Conservation of Mass:
Total mass of products = 2.2 + 0.9 + 8.2 = 11.3 g
Total mass of reactants = 11.3 g
Mass of reactants = Mass of products = 11.3 g ✓
∴ Law of Conservation of Mass is verified.
Key Conclusion
The total mass before and after a chemical reaction is always equal. This is true for every chemical reaction, whether products are solid, liquid, or gas — as long as the system is closed.
The total mass before and after a chemical reaction is always equal. This is true for every chemical reaction, whether products are solid, liquid, or gas — as long as the system is closed.
Formula & Key Terms Quick Reference
Law of Conservation of Mass: Total mass of reactants = Total mass of products
Law of Constant Proportions: Elements in a compound are ALWAYS in a fixed mass ratio
Molecular Mass = Sum of atomic masses of all atoms in one molecule
Formula Unit Mass = Sum of atomic masses of all atoms in one formula unit (ionic compound)
Criss-Cross Rule: Swap the valencies as subscripts to write the chemical formula
Single Bond = One shared electron pair | Double Bond = Two shared electron pairs
| Key Term | Meaning | Example |
|---|---|---|
| Atom | Smallest particle of an element | Na, Cl, O |
| Molecule | Smallest particle of a substance that can exist independently | H₂O, CO₂, O₂ |
| Cation | Positively charged ion (atom loses electrons) | Na⁺, Ca²⁺, Al³⁺ |
| Anion | Negatively charged ion (atom gains electrons) | Cl⁻, O²⁻, SO₄²⁻ |
| Covalent bond | Bond formed by sharing of electrons | H₂, HCl, H₂O |
| Ionic bond | Bond formed by transfer of electrons | NaCl, MgO |
| Polyatomic ion | Ion made of two or more atoms | OH⁻, NO₃⁻, SO₄²⁻ |
| Formula unit | Simplest whole number ratio of ions in an ionic compound | NaCl, CaCl₂ |
Common Exam Mistakes to Avoid
Mistake 1: Not writing a closed system for Conservation of Mass
Students forget that if gas escapes in an open system, mass appears to decrease — this does NOT mean the law is violated. Always mention that the system must be closed.
Students forget that if gas escapes in an open system, mass appears to decrease — this does NOT mean the law is violated. Always mention that the system must be closed.
Mistake 2: Forgetting to simplify the formula after criss-crossing
If you get Mg₂O₂ after criss-crossing, simplify it to MgO. Always divide subscripts by their highest common factor.
If you get Mg₂O₂ after criss-crossing, simplify it to MgO. Always divide subscripts by their highest common factor.
Mistake 3: Writing molecular mass for ionic compounds
Ionic compounds (like NaCl) do NOT have molecules — they form crystals. Always write formula unit mass, not molecular mass, for ionic compounds.
Ionic compounds (like NaCl) do NOT have molecules — they form crystals. Always write formula unit mass, not molecular mass, for ionic compounds.
Mistake 4: Miscounting atoms in formula unit mass calculations
In Ca(NO₃)₂, the bracket × 2 applies to the whole NO₃ group — so there are 2 N and 6 O atoms, not 1 N and 3 O atoms. Always expand brackets first.
In Ca(NO₃)₂, the bracket × 2 applies to the whole NO₃ group — so there are 2 N and 6 O atoms, not 1 N and 3 O atoms. Always expand brackets first.
Mistake 5: Confusing cation and anion order in naming
In ionic compound names, always write the cation name first, then the anion. E.g., “Sodium chloride” — NOT “Chloride sodium.”
In ionic compound names, always write the cation name first, then the anion. E.g., “Sodium chloride” — NOT “Chloride sodium.”
Mistake 6: Using “mono-” prefix for the first element in covalent naming
CO is “carbon monoxide,” NOT “monocarbon monoxide.” The mono- prefix is only omitted for the FIRST element, but used for the second.
CO is “carbon monoxide,” NOT “monocarbon monoxide.” The mono- prefix is only omitted for the FIRST element, but used for the second.
Mistake 7: Confusing ionic and covalent bond formation
Covalent bonds involve SHARING of electrons (between non-metals). Ionic bonds involve TRANSFER of electrons (between a metal and a non-metal). Never mix these up.
Covalent bonds involve SHARING of electrons (between non-metals). Ionic bonds involve TRANSFER of electrons (between a metal and a non-metal). Never mix these up.
Quick Revision Summary
Law of Conservation of Mass
Mass of reactants = Mass of products. Proposed by Lavoisier, 1789. Matter can neither be created nor destroyed.
Law of Constant Proportions
Elements in a compound always combine in a fixed mass ratio. Proposed by Joseph Proust. E.g., H:O in water = 1:8 always.
Dalton’s Atomic Theory
Atoms are tiny, indivisible, identical within an element, different across elements, and combine in whole number ratios. (1808)
Covalent Bond
Formed by sharing electrons. Single bond = 1 shared pair. Double bond = 2 pairs. Found in H₂, O₂, H₂O, HCl.
Ionic Bond
Formed by electron transfer. Metal loses electrons (cation), non-metal gains (anion). Electrostatic attraction holds them. E.g., NaCl.
Molecular & Formula Unit Mass
Molecular mass → covalent compounds (add all atomic masses). Formula unit mass → ionic compounds (add atomic masses of formula unit).
Properties: Ionic vs Covalent
Ionic: soluble in water, conduct electricity in solution, high MP/BP. Covalent: soluble in kerosene/petrol, do not conduct, low MP/BP.
Criss-Cross Rule
Write cation + anion → swap their valencies as subscripts → simplify → use brackets for polyatomic ions appearing more than once.
Final Exam Strategy
For numericals on Conservation of Mass: clearly write “Total mass of reactants” and “Total mass of products” and show they are equal. For formula writing: always show the criss-cross steps — you get step marks even if the final formula is wrong. For bonding: always mention the valence electrons and why sharing/transfer occurs. Good luck! 🌟
For numericals on Conservation of Mass: clearly write “Total mass of reactants” and “Total mass of products” and show they are equal. For formula writing: always show the criss-cross steps — you get step marks even if the final formula is wrong. For bonding: always mention the valence electrons and why sharing/transfer occurs. Good luck! 🌟
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