Law of Conservation of Mass – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Describe the given activity set-up and explain how it proves the Law of Conservation of Mass.
Answer:
- We prepare two 5% solutions of chemicals from groups X and Y.
- We keep solution Y in a conical flask, and solution X in an ignition tube.
- We hang the tube inside the flask and seal the mouth with a cork.
- We weigh the whole set-up before mixing the solutions.
- We tilt the flask to mix them, a precipitate may form, and then we weigh again.
- The mass before and mass after mixing remain the same.
- This shows that mass is conserved in a closed system, proving the Law of Conservation of Mass.
Q2. Why is a cork used to seal the flask in this experiment? Explain its importance for accurate results.
Answer:
- The cork makes the system closed.
- A closed system prevents loss of any gas, vapor, or splashes.
- Even if no gas is formed, water vapor can escape and change the mass.
- The cork also avoids spillage when we tilt and swirl the flask.
- It keeps all reactants and products inside during weighing.
- Because nothing escapes, the total mass stays the same.
- This ensures a fair test of the Law of Conservation of Mass.
Q3. Use the reaction between barium chloride and sodium sulphate to explain mass conservation.
Answer:
- Reaction: Barium chloride + Sodium sulphate → Barium sulphate (precipitate) + Sodium chloride.
- In symbols: BaCl2 + Na2SO4 → BaSO4(s) + 2NaCl.
- The number of each atom is the same on both sides.
- Atoms only rearrange to form new products.
- The precipitate formed, BaSO4, stays in the flask.
- No atoms are created or destroyed in the process.
- So the total mass of reactants equals the total mass of products.
Q4. What do you expect to observe when copper sulphate reacts with sodium carbonate? Explain why mass remains constant.
Answer:
- The blue copper sulphate solution reacts with sodium carbonate.
- A greenish solid, copper carbonate (CuCO3), forms as a precipitate.
- The solution becomes less blue because copper ions leave the solution.
- Sodium sulphate remains dissolved in the water.
- All matter stays inside the sealed flask due to the cork.
- The mass before and mass after mixing stays equal.
- This is because atoms only rearranged to form a precipitate, not lost.
Q5. State the Law of Conservation of Mass. Show it with one chemical and two physical change examples from daily life.
Answer:
- The law says: Mass cannot be created or destroyed in a chemical reaction.
- Chemical example: Lead nitrate + Sodium chloride → Lead chloride (precipitate) + Sodium nitrate; mass stays constant.
- Physical example 1: Freezing water into ice; mass does not change, only the state changes.
- Physical example 2: Gas moving from one balloon to another; total mass of both balloons is the same.
- In all cases, the same atoms are present before and after.
- They only rearrange or change state, so total mass is conserved.
- This is true when the system is closed and nothing escapes.
High Complexity (Analysis & Scenario-Based)
Q6. A student forgets to cork the flask. After mixing, the mass decreases slightly. Analyse why this happened and how to fix it.
Answer:
- Without a cork, the system is open.
- Some liquid may splash out during tilting and swirling.
- Water vapor may evaporate, especially if the room is warm.
- Tiny droplets may stick to the balance pan or outside the flask.
- These small losses reduce the measured mass.
- To fix it, always use a tight cork, handle gently, and weigh quickly.
- Keep the setup sealed to test mass conservation correctly.
Q7. Design a fair test to compare all three pairs (CuSO4–Na2CO3, BaCl2–Na2SO4, Pb(NO3)2–NaCl) and demonstrate the law.
Answer:
- Keep the same volume and concentration (5%) for all pairs.
- Use the same type of flask, ignition tube, and a tight cork.
- Weigh each setup before and after mixing, on the same balance.
- Observe formation of precipitate in each reaction.
- Record masses, colors, and states of products.
- Expect: No change in total mass for all three.
- Conclude: In each case, atoms rearrange, so mass is conserved.
Q8. In open air, a burning paper loses mass. Explain why the law still holds, and propose a closed-system demonstration.
Answer:
- In open air, paper seems to lose mass after burning.
- But its mass becomes gases like CO2 and water vapor, plus ash.
- These gases escape, so the measured mass drops.
- The law still holds because the mass of the products equals the mass of paper + oxygen used.
- Closed-system demo: Burn paper in a sealed container with a fixed lid and measure mass before and after.
- The total mass remains the same, as nothing escapes.
- This proves mass conservation in combustion.
Q9. You mix baking soda and vinegar in two ways: open beaker and sealed flask. Predict mass changes and justify.
Answer:
- In an open beaker, it fizzes and releases CO2 gas to air.
- The measured mass seems to decrease because gas escapes.
- In a sealed flask with a cork, CO2 stays inside.
- The mass before and after remains equal in the sealed case.
- The law is about closed systems where nothing leaves or enters.
- In both cases, atoms only rearrange.
- The difference in mass reading is due to gas loss, not a break of the law.
Q10. A student reports: initial mass 120.00 g; final mass 119.92 g after mixing Pb(NO3)2 and NaCl in a sealed flask. Does this violate the law? Analyse.
Answer:
- The law is a natural principle; it is not violated in real reactions.
- The small difference (0.08 g) may be due to instrument error or drafts.
- There could be a leaky cork, slight evaporation, or spillage on tilting.
- The balance may have limited precision, or the flask was wet outside.
- Recheck with a better seal, dry the outside, and use a shielded balance.
- Repeat readings and take an average.
- With proper control, the final mass will match the initial mass, showing mass conservation.
Q11. In the “two balloons” scenario, gas is moved from one balloon to another. Explain why total mass stays the same, and relate it to atoms.
Answer:
- The gas is not destroyed; it only moves.
- The total number of gas molecules remains the same.
- The mass of both balloons together stays constant.
- Only the distribution of gas changes between the balloons.
- This is like reactants turning into products; atoms rearrange but are not lost.
- So the total mass is conserved in the whole system.
- This supports the Law of Conservation of Mass clearly.
Q12. Cooking an egg gives steam and smell. If we collect everything in a sealed container, what will happen to total mass? Explain.
Answer:
- In open air, some steam and vapors leave, so mass seems to drop.
- In a sealed container, all gases, vapors, and liquids stay inside.
- The mass before and after cooking will be the same.
- The egg’s proteins change shape (a chemical/physical change), but atoms remain.
- The system is closed, so nothing is lost.
- Thus, the total mass is conserved during cooking.
- This shows the law works in daily life too.