Law of Conservation of Mass – Long Answer Questions
Medium Level (Application & Explanation)
Q1. In the activity mixing Copper sulphate (CuSO4) and Sodium carbonate (Na2CO3), explain the observations and why the total mass remains the same before and after the reaction.
Answer:
When solutions of CuSO4 and Na2CO3 are mixed, a precipitate of copper carbonate (CuCO3) forms and sodium sulphate (Na2SO4) remains in solution. The reaction is:
CuSO4 + Na2CO3 → CuCO3(s) + Na2SO4
At the atomic level, atoms are only rearranged; none are created or destroyed. The atoms present in the reactants reorganize to give the products.
If the flask is sealed with a cork, no substance (solid, liquid, or gas) escapes. Thus, the total mass measured before and after mixing remains the same, demonstrating the Law of Conservation of Mass.
The visual formation of a precipitate does not mean loss of mass; it is just a change in physical form of some atoms.
Q2. Why is it important to seal the reaction flask with a cork during this type of experiment? What could happen if it is not sealed?
Answer:
A cork or stopper keeps the reaction system closed, preventing gases or vapours formed during reaction from escaping. This ensures the mass of the system is conserved and measurable.
If the flask is not sealed, any gas produced (for example, if a reaction releases CO2 or other gases) can escape into the surroundings, causing the measured mass to decrease. This would give the wrong conclusion that mass is lost.
Even if no gas is produced, an open setup can let drops spill, or evaporation of water may occur with temperature change, altering mass.
Therefore, to verify the law accurately, the system must be sealed and handled carefully to avoid loss by spillage or evaporation.
Q3. Use a simple stoichiometric example to show by calculation how mass is conserved when 0.10 mol of CuSO4 reacts with 0.10 mol of Na2CO3.
Answer:
Reaction: CuSO4 + Na2CO3 → CuCO3 + Na2SO4. If 0.10 mol of each react: moles of products formed will be 0.10 mol CuCO3 and 0.10 mol Na2SO4.
Total mass of reactants = 0.10×159.6 + 0.10×106.0 = 26.56 g.
Total mass of products = 0.10×123.6 + 0.10×142.0 = 26.56 g.
The numbers match (within rounding), proving mass is conserved. This shows how stoichiometry and molar masses demonstrate the Law of Conservation of Mass.
Q4. List and explain at least five laboratory precautions you must follow to correctly demonstrate the Law of Conservation of Mass during the activity.
Answer:
Use a sealed container (cork or stopper): Prevents escape of gases and loss of mass by evaporation.
Weigh accurately on a clean scale: Ensure tare and zero the balance to avoid systematic errors.
Avoid spillage: Handle solutions carefully when transferring to prevent loss of material.
Allow temperature to stabilize: Hot solutions cause convection and buoyancy errors; cool before final weighing to avoid apparent mass changes.
Dry outer surfaces: Wipe any droplets from outside the vessel before weighing to avoid extra mass reading.
Following these steps helps ensure the measured mass change reflects only chemical rearrangement, not experimental loss.
Q5. Distinguish between physical change and chemical change with
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to mass conservation. Give two examples of each and comment on mass before and after.
Answer:
Physical changes alter the state or appearance but not the chemical identity. Examples: (1) Melting ice to water (H2O), (2) Dissolving salt (NaCl) in water. In both cases, when the system is closed, the mass remains the same because atoms remain unchanged.
Chemical changes produce new substances via rearrangement of atoms. Examples: (1) Formation of copper carbonate precipitate from CuSO4 + Na2CO3, (2) Burning paper producing ash and gases. In a closed system, total mass is conserved; in an open system, escaping gases can make measured mass appear to change, though atoms are still conserved overall.
The key is that atoms are neither created nor destroyed in both types; conservation holds if the whole system is accounted for.
High Complexity (Analytical & Scenario-Based)
Q6. Two balloons A (full of gas) and B (empty) are sealed separately. You puncture balloon A so the gas flows into B through a tube but the combined mass of both balloons measured before and after remains the same. Explain why, mentioning what would happen if a leak occurred.
Answer:
When gas flows from balloon A to B through a closed tube, the total number of gas molecules in the closed two-balloon system does not change. Therefore the combined mass measured before and after remains constant. The gas distribution changes but total mass is conserved.
If there is a leak during transfer, some gas molecules escape into the room, so the combined mass of the two balloons will decrease. That apparent loss is not a violation of the law; it is due to the system not being closed.
The conservation holds only for a closed system; any exchange with surroundings (leak) causes measured mass to change.
Q7. While cooking, an egg is boiled and some steam is released. A student weighs the egg before and after boiling and finds a small decrease in mass. Analyse why this happens and how the Law of Conservation of Mass still holds.
Answer:
During boiling, water inside the egg turns into steam and some steam escapes. The student’s measured mass decreases because mass leaves the egg as vapour.
Inside the egg, chemical changes like protein denaturation happen, but those changes do not destroy atoms. If the system were closed so steam could not escape, the total mass would remain constant.
Thus the observed decrease is due to loss of material to the surroundings, not disappearance of mass. The Law of Conservation of Mass remains valid if all products (steam, gases, ash, residue) are included in the mass balance.
Q8. A piece of magnesium ribbon is burned in a closed container and its mass increases after reaction. Explain the observation in terms of the Law of Conservation of Mass, and why the mass of the metal alone changes.
Answer:
Burning magnesium is a chemical reaction with oxygen: 2 Mg + O2 → 2 MgO. Inside a closed container, oxygen from the air combines with magnesium to form magnesium oxide. The total mass of the sealed container does not change.
The mass of the magnesium metal alone increases because it has chemically combined with oxygen atoms from the air; magnesium oxide contains both Mg and O, so the solid weighs more than the original metal.
If the container were open, oxygen taken from the air would still add to the mass of the product, but measuring only the metal piece before and after would show apparent mass gain — conservation is maintained for the whole closed system, not necessarily for one component.
Q9. Design an experiment using Barium chloride (BaCl2) and Sodium sulphate (Na2SO4) to prove the Law of Conservation of Mass. Include expected mass calculation for 0.050 mol of each reagent.
Answer:
Reaction: BaCl2 + Na2SO4 → BaSO4(s) + 2 NaCl. Use sealed flask with cork and weigh flask + reagents before mixing. Mix by tilting and weigh again.
For 0.050 mol each: reactants mass = 0.050×208.23 + 0.050×142.04 = 17.51 g. Products mass = 0.050×233.39 + 0.100×58.44 = 17.51 g.
Expected: no change in total mass before and after mixing. Any measured difference indicates experimental error or loss/gain to surroundings.
Q10. A student burns paper in an open beaker and notes a large decrease in mass. She concludes mass is not conserved in chemical reactions. Critically analyse this conclusion and propose two ways she could modify the experiment to correctly test mass conservation.
Answer:
The student’s conclusion is incorrect because mass is conserved, but the experiment was not closed. Burning produces gases (CO2, H2O vapour, smoke) that escape, so the beaker’s measured mass decreases. The missing mass is in the gaseous products dispersed into air.
To correctly test the law, she could: (1) Use a closed container (like a sealed crucible with a vent to a gas-collecting trap) to prevent gas escape and weigh the entire apparatus before and after. (2) Collect and weigh the gases produced (for example, by passing them into a gas-collecting syringe or absorbing CO2 in a reagent) so that all reaction products are included in the mass balance.
With these changes, the total mass remains constant, confirming the law.
