Middle School Science Quiz: Explain Conservation Of Mass

20 questions · exam conditions

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Explain Conservation Of MassQuestion 1 of 20

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?

Mass was destroyed during the reaction and turned into energy.

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.

Only reversible reactions conserve mass; burning is not reversible.

Atoms disappeared, but only oxygen atoms, so mass conservation still holds.

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Middle School Science Quiz

Middle School Science Quiz: Explain Conservation Of Mass

Practice Explain Conservation Of Mass in Middle School Science with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Explain Conservation Of Mass, giving you a quick way to practice the rules, question types, and explanations that matter most for Middle School Science.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

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?

Mass was destroyed during the reaction and turned into energy.
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. (correct answer)
Only reversible reactions conserve mass; burning is not reversible.
Atoms disappeared, but only oxygen atoms, so mass conservation still holds.

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.

Question 2

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?

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. (correct answer)
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.

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.

Question 3

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?

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. (correct answer)
When ice melts, it absorbs heat and turns that energy into extra mass that replaces lost mass.
Mass conservation only applies to chemical reactions, so this result does not relate to conservation of mass.
The mass stays the same because liquids always have the same mass as solids, no matter what substance it is.

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.

Question 4

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 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. (correct answer)
Sealing the container turns gases into solids, which always increases mass to match the starting mass.
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.

Question 5

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?

The reaction created new mass, but the balance could not detect the increase.
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. (correct answer)
Mass is conserved only when a gas is produced, not in other reactions.
Molecules are conserved, so the exact same molecules must be present before and after the reaction.

Question 6

A student investigates conservation of mass using a closed system. They put 50 g of vinegar in a flask and 5 g of baking soda in a balloon attached to the flask. The balloon is tipped so the baking soda falls into the vinegar. The CO $_2$ gas produced stays inside the balloon. The entire setup is placed on a balance.

Mass data (g):

Before reaction (flask + vinegar + balloon + baking soda): 55 g
After reaction (flask + liquid + balloon filled with CO $_2$ ): 55 g

Which statement best explains why the total mass stayed the same?

Mass stayed the same because the atoms were rearranged into new substances, but no atoms entered or left the sealed setup. (correct answer)
Mass stayed the same because chemical reactions always destroy the same amount of mass that they create.
Mass stayed the same because the balloon did not let liquids move, so gases do not affect mass.
Mass stayed the same because only reversible reactions conserve mass.

Question 7

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?

The iron atoms and oxygen atoms were destroyed and replaced by rust molecules with the same mass.
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. (correct answer)
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.

Question 8

In a closed-system experiment, a student measures a total mass of 55 g before a baking soda + vinegar reaction and 55 g after. Another student claims, "The mass stayed the same because the balloon is stretchy and makes extra space."

Which response is best?

Correct; creating extra space increases mass so the totals balance out.
Incorrect; mass stayed the same because the system was closed and the same atoms were present before and after, just rearranged. (correct answer)
Correct; mass conservation depends on the container's volume, not on the substances.
Incorrect; mass stayed the same because gases do not have 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 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 (with balloon)—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, 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, accurately identifying that closed system is necessary to measure conservation, not the balloon's stretchiness. Choice A incorrectly claims mass is created or destroyed during the reaction, violating the Law of Conservation of Mass, and confuses volume with mass conservation. 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.

Question 9

Steel wool rusts when it reacts with oxygen to form iron oxide. A student seals steel wool in a jar so nothing can enter or leave. The jar (including the air inside) has a mass of 100 g before rusting and 100 g after rusting.

Why does the total mass remain the same even though the steel wool looks different after rusting?

Because atoms are conserved; the same atoms are still present, just rearranged into new substances. (correct answer)
Because rusting destroys some iron atoms, reducing mass, but the balance cannot detect it.
Because chemical reactions always make mass disappear, but sealing the jar prevents it.
Because molecules must be conserved in chemical reactions, so mass cannot change.

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 of the jar including steel wool and air is 100 g, and after the reaction, the total mass including rust and remaining air 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 jar)—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, 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 explains that mass is conserved because atoms are conserved and atoms have mass, properly connecting atom conservation to mass conservation. Choice D 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, and doesn't connect atom conservation to mass conservation, missing the link: same atoms → same total atomic mass → same measured 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.

Question 10

A student investigates mass conservation using a closed system. They place 5 g of baking soda in a balloon and 50 g of vinegar in a flask. The balloon is stretched over the flask opening so the CO $_2$ gas stays in the balloon. The entire setup is massed before and after the reaction.

Mass data (g):

Before: baking soda 5 g, vinegar 50 g, total 55 g
After: products in flask + CO $_2$ in balloon, total 55 g

Is mass conserved in this reaction? Use the data to support your answer.

No. The vinegar turns into gas, so the total mass must decrease.
Yes. The total mass is 55 g before and 55 g after, so $\text{mass}_{\text{before}}=\text{mass}_{\text{after}}$ . (correct answer)
No. The total mass after must be 50 g because baking soda is used up.
Yes, but only because energy was converted into mass during the reaction.

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 of products including CO₂ in the balloon 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 with balloon)—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, 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, accurately identifying that closed system is necessary to measure conservation. Choice A incorrectly claims mass is created or destroyed during the reaction, violating the Law of Conservation of Mass, and 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.

Question 11

A student melts an ice cube in a sealed plastic bag (closed system). The ice has a mass of 20 g before melting. After it melts into liquid water, the mass is still 20 g.

Which evidence best supports the claim that mass is conserved during this change?

The shape changed from solid to liquid, so mass must be conserved.
The mass before (20 g) equals the mass after (20 g) in a closed system. (correct answer)
The water molecules were destroyed and replaced with new ones of the same mass.
The temperature stayed constant, which proves mass conservation.

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 change, measuring the mass carefully shows conservation: before melting, the mass of ice is 20 g, and after melting, the mass of liquid water 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 bag)—if the container were open and water could evaporate, we might measure less mass after, but the total mass including evaporated water would still equal the original mass, we just wouldn't capture it all in our measurement; the conservation holds because the physical change rearranged molecules 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, accurately identifying that closed system is necessary to measure conservation. Choice C incorrectly claims mass is created or destroyed during the reaction, violating the Law of Conservation of Mass, and suggests atoms can be destroyed but mass stays same (impossible—if atoms destroyed, mass would decrease). 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.

Question 12

A student tests mass conservation with a sealed container.

Mass data (g):

Before reaction: container + reactants = 100 g
After reaction: container + products (including gases) = 100 g

What is the best conclusion from these measurements?

Mass was conserved because the total mass of the closed system stayed the same before and after the reaction. (correct answer)
Mass was not conserved because chemical reactions always change mass.
Mass was conserved only because the products were solids; gases would break the law of conservation of mass.
Mass was conserved because the balance cancels out any mass changes during chemical 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. 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 container + reactants = 100 g, and after the reaction, the total mass is container + products (including gases) = 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 A is correct because it correctly calculates total mass before and after showing they're equal and properly explains that mass is conserved because atoms are conserved and atoms have mass. Choice C incorrectly claims mass was conserved only because the products were solids; gases would break the law of conservation of mass, 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.

Question 13

A student claims: "When a reaction makes a gas, mass is not conserved because the gas disappears." They test a gas-producing reaction in a closed system (balloon on flask).

Mass data (g):

Before: 55 g
After (balloon inflated with CO $_2$ ): 55 g

Which response best corrects the student's claim using the data?

The claim is correct; the mass should have dropped below 55 g, so the balance must be broken.
The claim is incorrect; the gas does not disappear, and because it is trapped in the closed system, the measured total mass stays 55 g. (correct answer)
The claim is correct because only solids and liquids count toward mass in chemistry.
The claim is incorrect because chemical reactions create mass to replace what turns into gas.

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 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 (balloon on flask)—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 and properly explains that mass is conserved because atoms are conserved and atoms have mass. Choice C incorrectly claims only solids and liquids count toward mass in chemistry, but gases also have mass and 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.

Question 14

A baking soda and vinegar reaction is done in two ways.

Closed system: baking soda in a balloon and vinegar in a flask. The balloon traps CO $_2$ .

Total mass before: 55 g
Total mass after: 55 g

If the same reaction were done in an open system (no balloon, gas escapes), what would most likely happen to the measured mass on the balance, and why?

The measured mass would likely decrease because CO $_2$ gas escapes and is no longer on the balance, even though total mass (including the escaped gas) is still conserved. (correct answer)
The measured mass would stay exactly the same because mass conservation never requires a closed system.
The measured mass would increase because gas escaping makes the remaining materials heavier.
The measured mass would become zero because gases have no 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 in the closed system, 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 with balloon)—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 accurately identifies that in an open system, gases can escape making mass seem to change even though total mass (including escaped gases) is conserved. Choice B 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.

Question 15

A candle is burned in two different setups.

Setup 1 (open system): A candle is lit in open air. The wax decreases from 20 g to 1 g of solid wax left.

Setup 2 (closed system): The same type of candle is lit inside a sealed container that is weighed.

Before: sealed container + candle + oxygen inside = 100 g
After: sealed container + gases (CO $_2$ and water vapor) + remaining candle = 100 g

Why does the candle seem to "lose mass" in the open system but not in the closed system?

In the open system, some mass is destroyed by fire; in the closed system, fire cannot destroy mass.
In the open system, gases produced can escape and are not included in the measured mass; in the closed system, all products (including gases) are trapped and counted. (correct answer)
In the open system, oxygen has no mass, so it cannot affect the measurement.
In the closed system, the balance automatically forces the masses to be equal 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 in the closed system, the total mass is sealed container + candle + oxygen inside = 100 g, and after the reaction, the total mass is sealed container + gases (CO₂ and water vapor) + remaining candle = 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 by trapping all products including gases. Choice A incorrectly claims mass is destroyed by fire; in the closed system, fire cannot destroy mass, 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.

Question 16

Two trials of the same closed-system reaction are performed (balloon traps gas). The entire apparatus is weighed each time.

Trial data (g):

Trial 1: before 55 g, after 55 g
Trial 2: before 55 g, after 55 g

Which statement is best supported by these results?

The results show mass conservation because the total mass is equal before and after in both trials. (correct answer)
The results show mass is conserved only in Trial 1, not Trial 2.
The results show mass increases in chemical reactions, but the increase is too small to see.
The results show mass conservation is true only when no gas is produced.

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 in Trial 1 and 2, the total mass is 55 g, and after the reaction in both trials, 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 (balloon traps gas)—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 correctly calculates total mass before and after showing they're equal in both trials. Choice D incorrectly claims the results show mass conservation is true only when no gas is produced, but gas is produced here and mass is still 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.

Question 17

A student investigates mass conservation using a closed system. They put 5 g of baking soda in a balloon and 50 g of vinegar in a flask. The balloon is stretched over the flask opening so the gas stays trapped. The entire setup is placed on a balance.

Mass data (g):

Before reaction: baking soda 5 g + vinegar 50 g = 55 g total
After reaction (balloon inflated with CO $_2$ ): total mass = 55 g

Do the measurements show conservation of mass? Use the data to support your answer.

No; mass is not conserved because a gas was produced, so mass must decrease.
Yes; the total mass stayed 55 g before and after, showing mass was conserved in the closed system. (correct answer)
No; the mass should be 50 g after because the baking soda is used up.
Yes; mass is conserved only because vinegar turns into energy, replacing the missing 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 baking soda 5 g + vinegar 50 g = 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 with balloon)—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 and accurately identifies that closed system is necessary to measure conservation. Choice A incorrectly claims mass is not conserved because a gas was produced, so mass must decrease, 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.

Question 18

A student performs two trials of the baking soda + vinegar reaction in a closed system (CO $_2$ trapped in a balloon).

Trial 1:

Before: 55 g
After: 55 g

Trial 2:

Before: 55 g
After: 55 g

Which claim is best supported by these repeated results?

Mass conservation depends on luck; sometimes mass is conserved and sometimes it is not.
Mass is conserved in this closed-system reaction because the total mass is the same before and after in both trials. (correct answer)
Mass increases each time because gases are lighter than liquids.
Mass is only conserved in Trial 1 because Trial 2 repeats the same numbers.

Question 19

A candle burns in a sealed jar (closed system). The student measures the mass of the sealed jar and contents.

Mass data:

Before burning: 100 g
After burning (candle partly used; CO $_2$ and water vapor trapped in jar): 100 g

Which evidence best supports the claim that mass was conserved?

The candle got shorter, so the missing wax must have turned into energy.
The jar looked foggy inside, so mass must have increased.
The total measured mass was 100 g both before and after the reaction in a sealed jar. (correct answer)
The flame went out, so the reaction stopped and mass was destroyed.

Question 20

A student burns a candle in open air (open system) and measures only the candle before and after.

Mass data:

Candle before: 20 g
Candle after: 1 g

Which statement best explains why this does not disprove conservation of mass?

Conservation of mass is false for burning reactions, so the candle's mass is destroyed.
The missing mass became heat and light, which have mass in ordinary chemical reactions.
Most of the wax turned into gases (CO $_2$ and water vapor) that escaped into the air, so the measurement did not include all products. (correct answer)
The balance can only measure solids, so it always shows mass loss during reactions.