Case Study
Passage with linked questions
Case Set 1
Case AnalysisPassage
A chemistry teacher demonstrates the law of conservation of mass to her Class 11 students. She takes a conical flask containing a solution of sodium sulphate and places a test tube with barium chloride solution inside the flask without mixing them. She weighs the entire setup on an analytical balance and records 120.52 g. She then tilts the flask so both solutions mix, causing a white precipitate of barium sulphate to form. After the reaction is complete, she weighs the flask again. The teacher explains that Antoine Lavoisier established this law in 1789 through careful combustion experiments, concluding that in all physical and chemical changes, there is no net change in mass. Students are asked to predict the final mass and explain the underlying principle.
Question 1: What mass will the balance show after the two solutions are mixed and the precipitate forms?
- The balance will show exactly 120.52 g after the reaction — the same as before mixing — because no material has entered or left the sealed flask system.
- The Law of Conservation of Mass states that matter can neither be created nor destroyed in any physical or chemical change; total mass of reactants equals total mass of products.
- Since the flask is closed, all reactants and products (including the precipitate) remain inside, so the total mass is conserved.
Question 2: Why did Lavoisier use closed systems in his combustion experiments to establish this law, and what would happen if the experiment above were done in an open flask?
- Lavoisier used closed systems to ensure all reactants and products — including gases — were accounted for; in open systems, gases like CO2 escape and appear as mass loss, giving a false impression that mass is not conserved.
- In an open flask, if gaseous products were released or if air contributed reactant mass, the measured mass before and after would differ, making it impossible to verify conservation without tracking every substance.
- The brilliance of Lavoisier's approach was exact measurement of masses of reactants and products in a controlled closed environment, which revealed that total mass is always conserved.
Question 3: A student argues that during the rusting of iron, the mass of the iron object increases, which seems to violate the Law of Conservation of Mass. Evaluate this argument using the law and explain what is actually happening.
- The student's observation is correct — the rusted iron object is heavier — but the conclusion is wrong; this does not violate the Law of Conservation of Mass because the system boundary has been incorrectly defined.
- Rusting is a chemical reaction between iron (Fe) and oxygen (O2) from the atmosphere to form iron oxide (Fe2O3); the oxygen atoms from air are incorporated into the rust, adding mass to the solid iron object.
- The total mass of the system (iron + oxygen consumed from air) remains constant; the apparent increase in mass of the iron object is exactly equal to the mass of oxygen absorbed from the surroundings, confirming conservation of mass.