This MCQ module is based on: Laws of Chemical Combination
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Laws of Chemical Combination
🎓 Class 9
Science
CBSE
Theory
Ch 9 — Atomic Foundations of Matter
⏱ ~15 min
🌐 Language: [gtranslate]
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This assessment will be based on: Laws of Chemical Combination
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Introduction: Why Does Mass Behave So Predictably?
Light a matchstick, burn a sheet of paper, dissolve a spoon of sugar in water, or rust an iron nail in damp air — chemistry is everywhere around us. For centuries, people watched these changes but could not explain why they always followed certain numerical rules. Why does a fixed amount of hydrogen react with a fixed amount of oxygen to make water — never some random ratio? Why does the total mass before and after a reaction stay the same, even if the substances look completely different?
By the late eighteenth century, careful weighing experiments by Antoine Lavoisier and Joseph Proust began to reveal the hidden rules. These rules were later given a particle-level explanation by John Dalton, whose atomic theory turned chemistry into a quantitative science.
Big Idea: Chemical reactions are not random — they obey strict numerical laws because matter is made of indivisible atoms that combine in fixed whole-number ratios.
9.1 Law of Conservation of Mass
The first quantitative law of chemistry was established around 1789 by the French chemist Antoine Laurent Lavoisier. He carried out a series of careful experiments inside sealed vessels, weighing reactants before a reaction and products after, and found that the total mass never changed.
Law of Conservation of Mass. In any chemical reaction, the total mass of the reactants equals the total mass of the products. Mass can neither be created nor destroyed during a chemical change.
One of Lavoisier's classic experiments studied the formation of water from hydrogen and oxygen. When 2 g of hydrogen gas combined with 16 g of oxygen gas inside a sealed apparatus, exactly 18 g of water was formed — no atoms vanished, none appeared from nowhere.
2 H2 + O2 → 2 H2O
(2 × 2 g) + (32 g) → (2 × 18 g)
4 g + 32 g → 36 g ✓ Mass conserved
(2 × 2 g) + (32 g) → (2 × 18 g)
4 g + 32 g → 36 g ✓ Mass conserved
⚖️ Lavoisier's Mass Balance — Step through the experiment L3 Apply
Walk through Lavoisier's classic 2H₂ + O₂ → 2H₂O experiment. Click Before reaction, the = sign, then After reaction in order to apply the law of conservation of mass and check the numbers yourself.
Click Before reaction → = → After reaction in order to walk step-by-step through Lavoisier's mass-balance check.
Activity — Verifying conservation of mass
Activity 9.1 — Sodium Sulphate + Barium ChlorideL3 Apply
Predict first: If a clear solution of sodium sulphate is mixed with a clear solution of barium chloride, a white solid suddenly appears. Will the total mass on the balance go up, go down, or stay the same?
- Take a small conical flask. Pour about 10 mL of sodium sulphate solution (Na2SO4) into a small ignition tube and lower it into the flask without spilling.
- Pour about 10 mL of barium chloride solution (BaCl2) into the flask, around the ignition tube.
- Cork the flask tightly so nothing can escape.
- Place the corked flask on a balance and note the reading carefully.
- Tilt the flask so the two solutions mix. A milky white precipitate of barium sulphate appears.
- Place the flask back on the balance and read again.
Observation: The balance reading is identical before and after mixing.
Reaction: Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2 NaCl(aq)
Conclusion: Even though a new white solid has formed and looks very different from the starting solutions, the total mass is unchanged. The atoms have only rearranged into new combinations — none were created or destroyed. This confirms the law of conservation of mass.
Reaction: Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2 NaCl(aq)
Conclusion: Even though a new white solid has formed and looks very different from the starting solutions, the total mass is unchanged. The atoms have only rearranged into new combinations — none were created or destroyed. This confirms the law of conservation of mass.
Why a sealed flask? If the reaction releases a gas (e.g., burning magnesium ribbon in open air or fizzing of an antacid), some product escapes and the open balance reading drops. The law still holds — but you must trap every product to confirm it.
9.2 Law of Constant (Definite) Proportions
Around 1799, the French chemist Joseph Louis Proust went one step further. He showed that a particular compound, no matter how it is made or where it comes from, always contains the same elements in the same fixed mass ratio.
Law of Constant Proportions. A pure chemical compound always contains its constituent elements combined in the same fixed proportion by mass, irrespective of the source or method of preparation.
Proust's classic example — water
Take pure water from a river, from a lab synthesis, from melted polar ice, or from the breath of an animal — the result is always the same: hydrogen and oxygen in a fixed mass ratio of 1 : 8.
In every 9 g of water: 1 g hydrogen + 8 g oxygen
In every 18 g of water: 2 g hydrogen + 16 g oxygen
In every 36 g of water: 4 g hydrogen + 32 g oxygen
Mass ratio H : O is always \( 1:8 \).
In every 18 g of water: 2 g hydrogen + 16 g oxygen
In every 36 g of water: 4 g hydrogen + 32 g oxygen
Mass ratio H : O is always \( 1:8 \).
Worked example — checking constant proportions
Q: 3.0 g of carbon burns in 8.0 g of oxygen to give 11.0 g of CO2. Predict the mass of CO2 formed when 9 g of carbon burns in excess oxygen.
A: From the data, ratio C : O is \(3:8\). For 9 g C, oxygen needed \(= 9 \times \dfrac{8}{3} = 24\) g. Total CO2 mass \(= 9 + 24 = 33\) g.
A: From the data, ratio C : O is \(3:8\). For 9 g C, oxygen needed \(= 9 \times \dfrac{8}{3} = 24\) g. Total CO2 mass \(= 9 + 24 = 33\) g.
9.3 Dalton's Atomic Theory (1808)
Lavoisier's and Proust's laws were experimental facts but had no underlying explanation. The English schoolteacher and scientist John Dalton proposed a simple particle picture that beautifully accounted for both laws. His 1808 theory had five core postulates.
The five postulates
- All matter is made of atoms. Atoms are extremely tiny particles that can take part in a chemical reaction but cannot be broken into anything simpler by chemical means.
- All atoms of a given element are alike. They have identical mass and identical chemical properties.
- Atoms of different elements differ. They have different masses and different chemical properties.
- Atoms combine in small whole-number ratios. When atoms of different elements join to form a compound, they always do so in fixed simple ratios — 1:1, 1:2, 2:3, and so on.
- Atoms are neither created nor destroyed in a chemical reaction. A reaction is simply a rearrangement of atoms.
How Dalton's theory explains the two laws
Postulate 5 directly explains the law of conservation of mass: if no atom is created or destroyed and each atom has a fixed mass, then total mass cannot change. Postulate 4 explains the law of constant proportions: because atoms always combine in fixed whole-number counts, and each atom contributes a fixed mass, the mass ratio of elements in a compound is always the same.
Modern caveat: We now know that atoms can be split (in nuclear reactions) and that atoms of an element may differ in mass (isotopes). Dalton's theory still works perfectly for ordinary chemistry, where only chemical changes are involved.
Quick Recap
| Concept | Statement | Discoverer |
|---|---|---|
| Conservation of Mass | Mass before reaction = mass after reaction. | Lavoisier (1789) |
| Constant Proportions | A pure compound contains its elements in fixed mass ratio. | Proust (1799) |
| Atomic Theory | Matter is made of indivisible atoms that combine in whole-number ratios. | Dalton (1808) |
Competency-Based Questions
A student takes 6.3 g of iron filings and 4.0 g of sulphur powder, mixes them in a closed test tube, and heats the mixture strongly. A black solid forms. The closed tube weighs the same before and after heating. The black solid contains iron and sulphur in the mass ratio 7 : 4.
Q1. The fact that the closed tube has the same mass before and after heating illustrates which law? L1
(b) The total mass is unchanged because atoms only rearrange — none are created or destroyed.
Q2. Out of the 6.3 g iron and 4.0 g sulphur taken, find which one is in excess and the mass of the unreacted residue. L3
Required ratio Fe : S \(= 7 : 4\). For 4.0 g S, iron required \(= 4 \times \dfrac{7}{4} = 7.0\) g. But only 6.3 g iron is available, so iron is the limiting reagent. Sulphur consumed \(= 6.3 \times \dfrac{4}{7} = 3.6\) g. Unreacted sulphur \(= 4.0 - 3.6 = 0.4\) g.
Q3. State, in your own words, what the law of constant proportions tells us about a sample of pure iron sulphide collected from any source. L2
A pure sample of iron sulphide will always contain iron and sulphur in the same fixed mass ratio (7 : 4) regardless of where it was collected from or how it was made — that is the essence of constant composition.
Q4. Fill in the blank: According to Dalton's theory, atoms of different elements differ in their __________ and __________. L1
Mass and chemical properties.
Q5. State whether True or False with reason: "If 5 g of sodium reacts with 8 g of chlorine to give 13 g of sodium chloride, the law of conservation of mass is being violated." L4
False. Total mass of reactants = 5 + 8 = 13 g, which equals the mass of the product. So the law is obeyed, not violated.
Assertion–Reason Questions
Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.
A: The total mass of products in a chemical reaction equals the total mass of reactants.
R: Atoms are neither created nor destroyed during a chemical reaction.
(A) Both statements are true and R is exactly why A holds — Dalton's fifth postulate is the molecular reason behind the law of conservation of mass.
A: A 9 g sample of pure water from a river and a 9 g sample of pure water made in a lab will both contain 1 g of hydrogen and 8 g of oxygen.
R: The law of constant proportions states that a compound has a fixed mass composition irrespective of source.
(A) Both statements are true and R correctly explains A. Water always has H : O \(=1:8\) by mass.
A: When a candle burns in open air, the wax visibly disappears, so mass is destroyed.
R: If the burning is carried out in a sealed jar, the total mass remains constant.
(D) Assertion is false — wax does not vanish; it converts to CO2 and water vapour that escape into the air. Reason is true: in a sealed jar, every product is trapped and the balance reading does not change.
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