Conservation of Mass: 18 flashcards
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1. State the law of conservation of mass.
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In a closed system, matter is neither created nor destroyed during a chemical or physical change; the total mass before equals the total mass after.
2. In a chemical reaction, the total mass of reactants equals what?
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The total mass of the products.
3. During a reaction, what happens to the atoms of the reactants?
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They are rearranged and recombine into new products; no atoms are created or destroyed.
4. Why must a system be closed to observe conservation of mass accurately?
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Because in an open system, matter (such as gases) can escape or enter, causing an apparent mass change.
5. Why does burning a candle in a sealed container show no mass change?
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All products, including gases like carbon dioxide and water vapour, remain trapped inside, so total mass is conserved.
6. What usually causes an apparent loss of mass in an open system?
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A gas escaping from the system (for example, carbon dioxide from vinegar and baking soda).
7. How does conservation of mass relate to balancing chemical equations?
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Balancing ensures the same number of each type of atom appears on both sides, directly reflecting conservation of atoms and mass.
8. Who is credited with establishing the law of conservation of mass?
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Antoine Lavoisier, through experiments in the late 18th century.
9. Does conservation of mass apply to physical changes like melting and dissolving?
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Yes. The mass of ice equals the mass of melted water, and the mass of a salt solution equals the mass of salt plus water.
10. What is the difference between mass and weight?
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Mass is the amount of matter (measured in kilograms); weight is a force depending on gravity (measured in newtons).
11. Does conservation of mass hold for nuclear reactions?
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Not exactly; in nuclear reactions a small amount of mass converts to energy according to $E=mc^2$. For chemistry, mass conservation is an excellent approximation.
12. In the reaction $2H_2 + O_2 \rightarrow 2H_2O$, how many hydrogen and oxygen atoms appear on each side?
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4 hydrogen atoms and 2 oxygen atoms on each side.
13. Why is conservation of mass fundamental to stoichiometry?
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It allows chemists to calculate reactant and product quantities because no mass is lost in the reaction.
14. What is a closed system?
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A system that does not exchange matter with its surroundings.
15. Give an everyday example showing conservation of mass.
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Dissolving salt in water: mass of solution = mass of salt + mass of water.
16. What does a balanced chemical equation show about atoms?
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That the number of atoms of each element is the same on both sides of the equation.
17. In the combustion of methane $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, verify atom counts.
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1 C, 4 H, 4 O on both sides—the equation is balanced.
18. Why does the law of conservation of mass require capturing all products, including gases?
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Because escaping gases carry mass away; if not captured, the measured mass appears to decrease.
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