Explain Conservation of Mass
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Middle School Physical Science › Explain Conservation of Mass
A candle is burned in an open system (in open air). The student measures only the candle and its holder.
Measurements:
- Before burning: 20 g
- After burning: 1 g (wax residue)
Which statement best explains why it seems like mass was lost?
Only reversible reactions conserve mass; burning is not reversible.
Atoms disappeared, but only oxygen atoms, so mass conservation still holds.
The missing mass left as gases (like CO$_2$ and water vapor) that were not captured or measured, so the measurement did not include all products.
Mass was destroyed during the reaction and turned into energy.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows apparent non-conservation: before the reaction, the total mass is 20 g, and after the reaction, the measured mass is 1 g—but this is because the system is open and gases could escape, so we might measure less mass after (if gases left), but the total mass including escaped gases would still equal the original mass, we just wouldn't capture it all in our measurement; the conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice B is correct because it properly explains that mass is conserved because atoms are conserved and atoms have mass, but in open systems, measurements may not capture all mass. Choice A incorrectly claims mass is created or destroyed during the reaction, violating the Law of Conservation of Mass. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance); common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning; the atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.
A student compares a closed-system and open-system setup for the reaction of baking soda and vinegar.
Closed system (balloon attached): total mass before = 55 g, total mass after = 55 g.
Open system (no balloon; gas escapes): mass of flask + liquid before = 55 g, mass after = 51 g.
Which statement best describes what these results show?
The closed system must be wrong because producing gas should always increase total mass.
The reaction creates 4 g of mass in the closed system and destroys 4 g in the open system.
Mass is conserved only in closed systems; in open systems the law of conservation of mass does not apply.
The open system appears to lose mass because CO$_2$ gas escapes, but total mass would still be conserved if the escaped gas were included.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows conservation in closed system: before the reaction, the total mass is 55 g, and after the reaction, the total mass is 55 g—the fact that these are equal (within measurement uncertainty of perhaps ±0.1 g) confirms mass is conserved; this measurement is only accurate because the system is closed (sealed container)—if the container were open and gases could escape, we might measure less mass after (if gases left) or more mass after (if gases from air entered), but the total mass including escaped/added gases would still equal the original mass, we just wouldn't capture it all in our measurement; the conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice B is correct because it properly explains that mass is conserved because atoms are conserved and atoms have mass, but in open systems, measurements may not capture all mass. Choice A incorrectly claims mass is created or destroyed during the reaction, violating the Law of Conservation of Mass. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance); common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning; the atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.
An ice cube melts in a cup that is covered so no water can spill out. The ice cube has a mass of 20 g before melting.
After melting, the liquid water has a mass of 20 g.
Which statement best explains why the mass is the same even though the appearance changed?
The mass stays the same because liquids always have the same mass as solids, no matter what substance it is.
Mass conservation only applies to chemical reactions, so this result does not relate to conservation of mass.
When ice melts, it absorbs heat and turns that energy into extra mass that replaces lost mass.
Melting is a physical change, so the same water molecules (and atoms) are still present; they are just arranged differently, so the mass stays 20 g.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows conservation: before the reaction, the total mass is 20 g, and after the reaction, the total mass is 20 g—the fact that these are equal (within measurement uncertainty of perhaps ±0.1 g) confirms mass is conserved; this measurement is only accurate because the system is closed (sealed container)—if the container were open and gases could escape, we might measure less mass after (if gases left) or more mass after (if gases from air entered), but the total mass including escaped/added gases would still equal the original mass, we just wouldn't capture it all in our measurement; the conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice A is correct because it properly connects atom conservation to mass conservation. Choice C incorrectly claims mass is created or destroyed during the reaction, violating the Law of Conservation of Mass. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance); common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning; the atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.
In a closed system, a student reacts baking soda and vinegar with a balloon capturing the CO$_2$. The masses are measured on a balance.
Data:
- Before: baking soda = 5 g, vinegar = 50 g, total = 55 g
- After: total mass of flask + balloon + contents = 55 g
Is mass conserved? Use the data to support your answer.
No; mass is not conserved because gas was produced, so the total mass must increase.
Yes; mass is conserved only in reactions that do not produce gases, so this result must be a coincidence.
No; the mass after should be 50 g because the baking soda was used up.
Yes; the total mass before is $5 + 50 = 55$ g and the total mass after is 55 g, so $\text{mass}{\text{before}} = \text{mass}{\text{after}}$.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows conservation: before the reaction, the total mass is 5 g baking soda + 50 g vinegar = 55 g, and after the reaction, the total mass is 55 g—the fact that these are equal (within measurement uncertainty of perhaps ±0.1 g) confirms mass is conserved; this measurement is only accurate because the system is closed (sealed container)—if the container were open and gases could escape, we might measure less mass after (if gases left) or more mass after (if gases from air entered), but the total mass including escaped/added gases would still equal the original mass, we just wouldn't capture it all in our measurement; the conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice B is correct because it correctly calculates total mass before and after showing they're equal. Choice A incorrectly claims mass is not conserved because gas was produced, so the total mass must increase. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance); common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning; the atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.
A candle is burned in a sealed container so that all products (including gases) stay inside. The mass of the container + contents is measured.
| measurement | mass (g) |
|---|---|
| before burning | 100 |
| after burning | 100 |
Why is sealing the container important for observing conservation of mass in this reaction?
Sealing the container turns gases into solids, which always increases mass to match the starting mass.
Sealing the container makes the reaction stop, so the mass cannot change.
Sealing the container captures all products (including CO$_2$ and water vapor), so nothing leaves or enters and the total mass can be compared fairly.
A closed system is not needed because conservation of mass can only be tested by measuring the candle wax alone.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows conservation: before the reaction, the total mass is 100 g, and after the reaction, the total mass is 100 g—the fact that these are equal (within measurement uncertainty of perhaps ±0.1 g) confirms mass is conserved; this measurement is only accurate because the system is closed (sealed container)—if the container were open and gases could escape, we might measure less mass after (if gases left) or more mass after (if gases from air entered), but the total mass including escaped/added gases would still equal the original mass, we just wouldn't capture it all in our measurement; the conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice B is correct because it accurately identifies that closed system is necessary to measure conservation. Choice D incorrectly dismisses the need for closed system, missing that in open system gases can escape making mass seem to change even though total mass (including escaped gases) is conserved. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance); common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning; the atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.
A student does the baking soda + vinegar reaction in an open flask (no balloon). The student measures only the flask and its liquid/solid contents.
Mass data (g):
- Before: flask + vinegar + baking soda = 55 g
- After: flask + remaining liquid/solid = 53 g (CO$_2$ gas escaped)
Which statement is the best explanation for the 2 g decrease in measured mass?
The gas that formed escaped the system, so it was not included in the after-measurement; if the escaped gas were included, total mass would be conserved.
The reaction destroyed 2 g of matter, so mass was not conserved.
The balance always reads lower after a reaction because the products are less dense.
The vinegar absorbed energy, which reduced its mass by 2 g.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—in the baking soda and vinegar reaction, atoms rearrange to form products including CO₂ gas, but no atoms are created or destroyed, so if we could measure all products, total mass would be unchanged. In this open system measurement, the apparent mass decrease is explained by escape of products: before the reaction, total mass is 55 g (flask + vinegar + baking soda), but after the reaction, measured mass is only 53 g because CO₂ gas escaped into the air and wasn't included in the measurement—the 2 g difference represents the mass of escaped CO₂. If we could capture and weigh the escaped CO₂, we'd find: mass of flask contents after (53 g) + mass of escaped CO₂ (2 g) = original total mass (55 g), confirming that mass was conserved but not all of it was measured. Choice B is correct because it accurately explains that the gas formed during the reaction escaped the open system and wasn't included in the after-measurement, but if the escaped gas were included, total mass would be conserved. Choice A incorrectly claims the reaction destroyed 2 g of matter, violating the fundamental law that matter cannot be created or destroyed in chemical reactions—the 2 g didn't disappear from existence, it just left the measurement system as gas. The key lesson: open systems allow matter exchange with surroundings, making it impossible to track all mass involved in a reaction, which is why closed systems are essential for demonstrating mass conservation experimentally.
In a sealed jar (closed system), steel wool rusts. The student records the following.
Mass data (g):
- Before: jar + contents = 100 g
- After: jar + contents = 100 g
Which statement correctly connects conservation of atoms to conservation of mass in this reaction?
Mass stays the same because chemical reactions create new matter to replace what is used up.
Mass stays the same because oxygen atoms disappear as the iron atoms turn into rust atoms.
Mass stays the same because atoms are conserved; the same atoms are present before and after, just rearranged into new molecules.
Mass stays the same because molecules are conserved; the exact same molecules exist before and after.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows conservation: before the reaction, the total mass is jar + contents = 100 g, and after the reaction, the total mass is jar + contents = 100 g—the fact that these are equal (within measurement uncertainty of perhaps ±0.1 g) confirms mass is conserved. This measurement is only accurate because the system is closed (sealed jar)—if the container were open and gases could escape, we might measure less mass after (if gases left) or more mass after (if gases from air entered), but the total mass including escaped/added gases would still equal the original mass, we just wouldn't capture it all in our measurement. The conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice B is correct because it properly connects atom conservation to mass conservation. Choice A incorrectly confuses mass conservation with molecule conservation, claiming molecules stay the same (they don't—molecules change in reactions), when it's atoms that are conserved. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance). Common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning. The atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.
A student mixes two solutions in a sealed plastic bag (a closed system) and measures mass.
Mass data (g):
- Reactant solution A: 10 g
- Reactant solution B: 5 g
- Total before mixing: 15 g
- Total after reaction (a solid forms inside the bag): 15 g
Which piece of evidence best demonstrates conservation of mass in this experiment?
The reactants had different masses, so mass cannot be conserved.
The bag is plastic, so mass measurements are not meaningful.
A solid formed, which proves mass must have increased.
The total mass before and after is the same (15 g), showing mass was conserved.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows conservation: before the reaction, the total mass is reactant solution A 10 g + reactant solution B 5 g = 15 g, and after the reaction, the total mass is 15 g—the fact that these are equal (within measurement uncertainty of perhaps ±0.1 g) confirms mass is conserved. This measurement is only accurate because the system is closed (sealed plastic bag)—if the container were open and gases could escape, we might measure less mass after (if gases left) or more mass after (if gases from air entered), but the total mass including escaped/added gases would still equal the original mass, we just wouldn't capture it all in our measurement. The conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice B is correct because it correctly calculates total mass before and after showing they're equal. Choice A incorrectly claims a solid formed, which proves mass must have increased, violating the Law of Conservation of Mass. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance). Common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning. The atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.
A student places 50 g of vinegar in a flask and 5 g of baking soda in a balloon attached to the flask. The balloon is lifted so the baking soda falls into the vinegar, and the system is kept closed because the balloon traps the CO$_2$ gas that forms. The total mass measured on a balance is 55 g before the reaction and 55 g after the reaction.
Which statement best explains why the total mass stayed the same?
Atoms are conserved in a chemical reaction, so the same atoms (and total mass) are present before and after; the balloon keeps the gas from escaping.
The reaction created new mass, but the balance could not detect the increase.
Molecules are conserved, so the exact same molecules must be present before and after the reaction.
Mass is conserved only when a gas is produced, not in other reactions.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. In this reaction, measuring the mass carefully shows conservation: before the reaction, the total mass is 50 g + 5 g = 55 g, and after the reaction, the total mass is still 55 g—the fact that these are equal confirms mass is conserved. This measurement is only accurate because the system is closed (balloon traps the CO₂)—if the container were open and gases could escape, we might measure less mass after, but the total mass including escaped gases would still equal the original mass. Choice B is correct because it properly explains that mass is conserved because atoms are conserved and correctly identifies that the balloon keeps the gas from escaping, allowing accurate measurement. Choice A incorrectly claims mass was created, violating the Law of Conservation of Mass; Choice C incorrectly suggests mass is only conserved when gas is produced (it's conserved in ALL reactions); Choice D confuses mass conservation with molecule conservation, claiming molecules stay the same when actually molecules change in reactions—it's atoms that are conserved. The atomic explanation: since each element has a characteristic atomic mass and chemical reactions conserve atoms, the total mass must also be conserved because mass is just the sum of all the atomic masses.
A student performs a sealed-jar rusting experiment and records the masses.
| item measured (sealed jar + contents) | before (g) | after (g) |
|---|---|---|
| total mass | 100 | 100 |
How does the idea that atoms are conserved explain these measurements?
No atoms entered or left the sealed jar; the same atoms that were present before are still present after, just bonded differently, so the total mass stays 100 g.
The iron atoms and oxygen atoms were destroyed and replaced by rust molecules with the same mass.
Only oxygen atoms have mass, so as long as oxygen stays in the jar, the total mass stays the same.
Mass stayed the same because the balance automatically corrects measurements to show conservation of mass.
Explanation
This question tests understanding of the Law of Conservation of Mass: the total mass of products in a chemical reaction equals the total mass of reactants because atoms are conserved. Mass is conserved in all chemical reactions because atoms are conserved and each atom has a specific mass—during a reaction, atoms rearrange into different molecules but no atoms are created (which would add mass) or destroyed (which would remove mass), so if the same atoms are present before and after (just bonded differently), the total mass must be the same. For example, if reactants contain 4 hydrogen atoms (each with mass ~1 amu) and 2 oxygen atoms (each with mass ~16 amu), the total mass is (4×1) + (2×16) = 36 amu, and the products containing those same 4 H and 2 O atoms also have total mass 36 amu, whether arranged as H₂ + O₂ or as H₂O—same atoms means same mass. In this reaction, measuring the mass carefully shows conservation: before the reaction, the total mass is 100 g, and after the reaction, the total mass is 100 g—the fact that these are equal (within measurement uncertainty of perhaps ±0.1 g) confirms mass is conserved; this measurement is only accurate because the system is closed (sealed container)—if the container were open and gases could escape, we might measure less mass after (if gases left) or more mass after (if gases from air entered), but the total mass including escaped/added gases would still equal the original mass, we just wouldn't capture it all in our measurement; the conservation holds because the chemical reaction rearranged atoms (broke bonds in reactants, formed new bonds in products) but didn't create or destroy any atoms, and since atoms carry the mass, conserving atoms means conserving mass. Choice B is correct because it properly connects atom conservation to mass conservation. Choice A incorrectly claims mass is created or destroyed during the reaction, violating the Law of Conservation of Mass. Verifying mass conservation experimentally: (1) measure total mass of all reactants before reaction using a balance, (2) allow reaction to occur in closed system (sealed container so gases can't escape), (3) measure total mass of all products after reaction (including any gases produced), (4) compare: mass_before should equal mass_after within measurement error (±0.1 g typical for classroom balance); common pitfall: burning something in open air and thinking mass is lost because ash weighs less than original material—this is misleading because the gases (CO₂ and H₂O vapor) that formed during burning escaped into the air and weren't measured, but if you could capture all the gases and measure them, you'd find mass_ash + mass_gases = mass_wood + mass_oxygen_consumed, confirming conservation even for burning; the atomic explanation: since each element has a characteristic atomic mass (H ≈ 1, C ≈ 12, O ≈ 16, etc.) and chemical reactions conserve atoms (same number of H atoms before and after, same number of O atoms, etc.), the total mass must also be conserved because mass is just the sum of all the atomic masses: if you have the same atoms, you have the same total atomic mass, period.