Middle School Science Quiz: Mass Stays The Same
20 questions · exam conditions
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Mass Stays The SameQuestion 1 of 20

A student wants to test whether mass is conserved during a reaction that produces a gas (like CO2_2). Which setup is best for observing conservation of mass using before-and-after mass measurements?

Mix the reactants in an open beaker so any gas can escape, then measure the beaker's mass before and after.
Mix the reactants in a container with a loose lid so pressure does not build up, then measure mass before and after.
Mix the reactants in a sealed container (or with a balloon attached to capture gas) and measure the mass of the entire system before and after.
Measure only the mass of the gas produced, because that is the only part that changes.
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Middle School Science Quiz

Middle School Science Quiz: Mass Stays The Same

Practice Mass Stays The Same 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 Mass Stays The Same, giving you a quick way to practice the rules, question types, and explanations that matter most for Middle School Science.

How to use this quiz

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.

All questions

Question 1

A student wants to test whether mass is conserved during a reaction that produces a gas (like CO2_2). Which setup is best for observing conservation of mass using before-and-after mass measurements?

  1. Mix the reactants in an open beaker so any gas can escape, then measure the beaker's mass before and after.
  2. Mix the reactants in a container with a loose lid so pressure does not build up, then measure mass before and after.
  3. Mix the reactants in a sealed container (or with a balloon attached to capture gas) and measure the mass of the entire system before and after. (correct answer)
  4. Measure only the mass of the gas produced, because that is the only part that changes.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. To properly test mass conservation when a gas like CO₂ is produced, the critical requirement is a closed system that captures all products including gases—if CO₂ escapes into the air, measuring only what remains in the container would show apparent mass loss even though the total mass (container + escaped gas) is actually conserved. A sealed container or one with a balloon attached creates this closed system by trapping all gases produced, allowing accurate measurement of the total mass before and after reaction. Without this closed system, escaping gas makes it impossible to verify mass conservation through simple before-and-after weighing. Choice C is correct because it properly identifies that a sealed container (or one with a balloon to capture gas) creates the closed system needed to measure all products including gases, allowing verification of mass conservation through total system mass measurements. Choice A fails because an open beaker allows gas to escape unmeasured into the air, making the final mass measurement incomplete and showing false mass loss. Choice B's loose lid still allows gas escape under pressure, creating the same problem as an open container. Choice D incorrectly suggests measuring only the gas mass, missing that conservation requires measuring the total mass of all reactants before and all products after, not just one component.

Question 2

In a closed system, a student measures the mass of a sealed container and its contents before and after a chemical reaction. The mass is 175.0 g before and 175.0 g after (±0.1 g). Which equation correctly represents what the measurements show?

  1. mass of reactants<mass of products\text{mass of reactants} < \text{mass of products} because new substances formed.
  2. mass of reactants=mass of products\text{mass of reactants} = \text{mass of products} because the same atoms are present in a closed system. (correct answer)
  3. mass of reactants>mass of products\text{mass of reactants} > \text{mass of products} because gases always reduce mass.
  4. mass of reactants=0\text{mass of reactants} = 0 because atoms rearrange into energy.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 175.0 g, and after the reaction in the sealed container, the total mass is still 175.0 g (±0.1 g). The closed system is critical here—the container was sealed so nothing could escape or enter, meaning all products are captured and measured. This equality (175.0 g = 175.0 g) proves that mass was conserved, and the atomic explanation is that the same atoms present in reactants are present in products, just rearranged. Choice B is correct because it correctly explains that mass of reactants = mass of products because the same atoms are present in a closed system. Choice C is wrong because it claims mass of reactants > mass of products because gases always reduce mass, when actually gases have mass and, if captured in a closed system, contribute to the total staying the same. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 3

A student measures a sealed (closed) container before and after a reaction that produces a noticeable temperature change. The mass is 75.3 g before and 75.3 g after.

Which statement best interprets this evidence?

  1. The equal masses show that even though energy changed (temperature changed), the total mass stayed the same because the atoms were conserved in the closed system. (correct answer)
  2. The equal masses show that no chemical reaction happened, because chemical reactions always change mass.
  3. The equal masses show that gases have no mass, so producing gas would not affect total mass.
  4. The equal masses show that the balance must be broken, because reactions always create or destroy atoms.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show the sealed container has mass 75.3 g before and 75.3 g after a reaction that caused a noticeable temperature change. This demonstrates a crucial point: while the reaction clearly occurred (evidenced by the temperature change indicating energy was released or absorbed), the mass remained constant because atoms were conserved—the same atoms present initially are present finally, just bonded differently. The sealed container ensured no atoms entered or left, allowing accurate verification that mass conservation is independent of energy changes. Choice A is correct because it properly recognizes that equal masses demonstrate mass conservation even when energy changes occur (temperature change), and correctly explains this through atom conservation in the closed system—mass and energy are separately conserved in chemical reactions. Choice B incorrectly claims no reaction occurred, contradicting the evidence of temperature change which proves a reaction did happen. Choice C makes the false claim that gases have no mass, when actually all matter including gases has mass. Choice D incorrectly suggests reactions create or destroy atoms and that equal measurements indicate a broken balance, when actually the equal measurements confirm the fundamental law that atoms (and therefore mass) are conserved.

Question 4

A student measures the mass of a sealed zip-lock bag containing two liquids separated in small cups. Mass before mixing: 42.7 g. The student mixes the liquids inside the sealed bag; bubbles form (gas is produced). Mass after the reaction finishes: 42.7 g.

Which claim is best supported by these measurements?

  1. The total mass stayed the same because the atoms in the reactants were rearranged into products, and none of the matter left the sealed bag. (correct answer)
  2. The total mass stayed the same because the reaction created new atoms to replace the lost ones.
  3. The total mass stayed the same because bubbles have no mass.
  4. The total mass stayed the same because the liquids did not really react.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass of the sealed zip-lock bag is 42.7 g, and after the reaction in the sealed container (with bubbles/gas formed), the total mass is still 42.7 g. The closed system is critical here—the bag was sealed before the reaction so no gases could escape and no air could enter, meaning all the products (including any gases formed) are captured and measured. This equality (42.7 g = 42.7 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (with total atomic mass summing to 42.7 g) are present finally (still summing to 42.7 g), just bonded into different molecules. Choice A is correct because it properly explains that mass stayed the same due to atom rearrangement with no matter leaving the sealed system. Choice B incorrectly claims reactions create new atoms, violating the fundamental principle that atoms are neither created nor destroyed in chemical reactions. Choice C wrongly states bubbles have no mass, when actually gas bubbles contain atoms with mass. Choice D dismisses the reaction as not real despite the evidence of bubble formation showing chemical change.

Question 5

A particle model shows a reaction in a closed system. Before: 2 atoms of X (mass 5 u each) and 3 atoms of Y (mass 2 u each). After: the same atoms are rearranged into a new substance.

What is the total mass before and after, and what does that show?

  1. Before: 2(5)+3(2)=162(5)+3(2)=16 u; After: 16 u. This shows mass is conserved because the same atoms are present, just rearranged. (correct answer)
  2. Before: 2(5)+3(2)=102(5)+3(2)=10 u; After: 16 u. This shows mass increases when bonds form.
  3. Before: 16 u; After: 14 u. This shows mass decreases because atoms combine.
  4. Before: 16 u; After: cannot be determined because particle models cannot show mass conservation.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The particle model shows 2 atoms of X (5 u each) and 3 atoms of Y (2 u each) before the reaction, giving total mass: 2(5) + 3(2) = 10 + 6 = 16 u. After the reaction, the same 2 X atoms and 3 Y atoms are present (just rearranged into different molecules), so the total mass remains: 2(5) + 3(2) = 16 u. You can verify by counting: every atom from the reactants appears in the products (none created, none destroyed), so the sum of atomic masses before equals the sum after, which is why when we measure with a balance, we get the same reading. Choice A is correct because it properly calculates the total mass before as 2(5) + 3(2) = 16 u and after as 16 u, correctly concluding that this demonstrates mass conservation because the same atoms are present, just rearranged into new molecular arrangements. Choice B makes an arithmetic error, calculating 2(5) + 3(2) = 10 u instead of 16 u, and incorrectly claims mass increases when bonds form. Choice C claims mass decreases to 14 u, which is mathematically impossible given the same atoms are present. Choice D incorrectly states that particle models cannot show mass conservation, when actually counting atoms and their masses in particle models is an excellent way to verify conservation.

Question 6

In a sealed flask (closed system), 10.0 g of solution A is mixed with 15.0 g of solution B. A precipitate forms (a new solid appears). The mass of the sealed flask + contents is 128.6 g before mixing and 128.5 g after mixing (balance uncertainty ±0.1 g). What is the best conclusion?

  1. Mass is conserved within measurement uncertainty because the total mass did not change in the closed system. (correct answer)
  2. Mass was destroyed because a solid formed, so the mass must decrease.
  3. Mass increased because new matter was created during the reaction.
  4. The system must have been open because the mass stayed about the same.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 128.6 g (including flask + 10.0 g A + 15.0 g B), and after the reaction in the sealed container, the total mass is 128.5 g (products combined). The closed system is critical here—the flask was sealed before the reaction so no gases could escape and no air could enter, meaning all the products (including the precipitate) are captured and measured. This near-equality (128.6 g ≈ 128.5 g within ±0.1 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially are present finally, just bonded into different molecules like the solid precipitate. Choice A is correct because it properly calculates total mass before and after showing they're equal within uncertainty, verifying conservation. Choice B is wrong because it claims mass was destroyed because a solid formed, so the mass must decrease, when actually the formation of a precipitate doesn't destroy mass—it's just atoms rearranging, and total mass stays the same. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 7

A student seals a zip-top bag (closed system) containing 30.0 g of vinegar and 3.0 g of baking soda in a small cup inside the bag. The total mass of the sealed bag and everything in it is 210.6 g before mixing. After mixing, the bag inflates with gas, and the total mass is 210.6 g (±0.1 g). Which claim is best supported by the data?

  1. The reaction created extra matter, but it was too small for the balance to detect.
  2. The mass stayed the same because the same atoms were present before and after, just rearranged into new substances, and the closed bag kept all products. (correct answer)
  3. The mass stayed the same because gases have zero mass.
  4. The mass stayed the same only because no chemical reaction occurred.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 210.6 g, and after the reaction in the sealed bag, the total mass is still 210.6 g (±0.1 g). The closed system is critical here—the bag was sealed before the reaction so no gases could escape and no air could enter, meaning all the products (including the gas that inflated the bag) are captured and measured. This equality (210.6 g = 210.6 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (in vinegar and baking soda) are present finally (in products), just bonded into different molecules. Choice B is correct because it correctly explains that mass is conserved because the same atoms were present before and after, just rearranged into new substances, and the closed bag kept all products. Choice C is wrong because it claims the mass stayed the same because gases have zero mass, when actually gases do have mass (from their atoms like CO₂), and that's why capturing them keeps the total mass constant. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 8

A student puts 50.0 g of vinegar in a flask and 5.0 g of baking soda in a balloon. The balloon is carefully attached to the top of the flask so the system is closed (no gas can escape). The mass of the entire system (flask + vinegar + balloon + baking soda) is 155.2 g before mixing. The student tips the baking soda into the vinegar; the balloon inflates with CO2_2 gas. After the reaction stops, the mass of the entire closed system is 155.2 g (±0.1 g). Which statement best uses this evidence?

  1. Mass decreased because the CO2_2 gas has no mass once it enters the balloon.
  2. The equal before-and-after masses show that in a closed system, atoms are conserved and only rearranged during the reaction. (correct answer)
  3. Mass stayed the same only because the reaction was reversible.
  4. The data cannot support any conclusion because measurements always stay the same in any experiment.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 155.2 g, and after the reaction in the sealed container, the total mass is still 155.2 g (±0.1 g). The closed system is critical here—the balloon and flask were sealed before the reaction so no gases could escape and no air could enter, meaning all the products (including any gases formed like CO₂) are captured and measured. This equality (155.2 g = 155.2 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (with total atomic mass summing to 155.2 g) are present finally (still summing to 155.2 g), just bonded into different molecules. Choice B is correct because it accurately identifies the closed system's role in allowing verification by capturing all products and correctly explains that mass is conserved because atoms are conserved and only rearranged. Choice A is wrong because it claims mass decreased because the CO₂ gas has no mass once it enters the balloon, when actually gases do have mass (from their atoms) and were included in the measurement, so total mass stayed the same. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 9

A student reacts two chemicals that produce a gas. In an open beaker, the mass goes from 120.0 g before to 118.7 g after. In a closed system (the same reaction in a sealed container that captures the gas), the mass goes from 120.0 g before to 120.0 g after (±0.1 g). Why does the open beaker show a mass decrease?

  1. Atoms disappeared during the reaction in the open beaker but not in the sealed container.
  2. Gas products escaped from the open beaker, so not all products were included in the final mass measurement. (correct answer)
  3. The law of conservation of mass only applies to closed systems because atoms change into energy in open systems.
  4. Mass always decreases in chemical reactions that make bubbles, even in closed systems.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. In this closed system (sealed container), mass stays constant at 120.0 g before and after because all products are retained—if the system were open (beaker unsealed), gases produced in the reaction would escape into the surrounding air, and measuring only the remaining solids/liquids would show apparent mass loss (to 118.7 g after), but the total mass including the escaped gases would still equal the original 120.0 g, demonstrating that conservation holds even when measurement is complicated by gas escape. The closed system is critical here—the container captured the gas, allowing full measurement, while the open beaker did not, leading to the apparent decrease. This shows that mass is always conserved, but open systems can make it seem otherwise if products leave the measured area. Choice B is correct because it accurately identifies the closed system's role in allowing verification by capturing all products and explains the apparent loss in the open system due to escaped gases. Choice A is wrong because it claims atoms disappeared during the reaction in the open beaker but not in the sealed container, when actually no atoms disappear in either case—mass conservation is universal, and the difference is just whether all atoms are weighed after. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 10

A reaction is carried out in a sealed container (closed system). The measured mass is 133.2 g before the reaction and 133.2 g after the reaction. Which statement best explains why these measurements support conservation of mass?

  1. Because the container is closed, the same atoms remain in the system; reactions rearrange atoms, so total mass stays the same. (correct answer)
  2. Because the container is closed, atoms can leave the system as energy, so the mass stays the same.
  3. Because the masses match exactly, the reaction must have stopped before any products formed.
  4. Because the masses match, it proves molecules are conserved and never change into different molecules.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 133.2 g, and after the reaction in the sealed container, the total mass is still 133.2 g. The closed system is critical here—the container was sealed so no matter could escape or enter, meaning all the products are captured and measured. This equality (133.2 g = 133.2 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially are present finally, just bonded into different molecules. Choice A is correct because it correctly explains that mass is conserved because atoms are conserved (same atoms = same mass) in the closed system. Choice B is wrong because it confuses mass conservation with energy, claiming atoms leave as energy, but actually atoms stay and mass is separate from energy in chemical reactions. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 11

A student claims, "Mass is always conserved, so it doesn't matter whether the container is open or closed when we measure mass." Which response best corrects the student using evidence about reactions that produce gases?

  1. The student is correct; gases escaping do not affect measured mass.
  2. In an open container, gas products can escape (or gases from air can enter), so the measured mass of the container can change even though total mass is conserved overall. (correct answer)
  3. Mass is conserved only in open containers because air helps keep the mass the same.
  4. Mass is not conserved in any reaction because atoms disappear when bonds break.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. In this closed system, mass stays constant because all products are retained—if the system were open (container unsealed), gases produced in the reaction would escape into the surrounding air (or air gases could enter), and measuring only the remaining contents would show apparent mass change, but the total mass including the escaped gases would still equal the original, demonstrating that conservation holds even when measurement is complicated by gas escape. Choice B is correct because it accurately identifies the closed system's role in allowing verification by capturing all products, and explains apparent changes in open systems. Choice A is wrong because it claims gases escaping do not affect measured mass, when actually in open systems, escaping gases (which have mass) cause the measured mass to decrease. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 12

In a sealed flask (closed system), a student mixes two clear solutions. The flask + contents has a mass of 210.7 g before mixing. After mixing, a solid forms (a precipitate) and the flask + contents has a mass of 210.8 g. The balance has an uncertainty of ±0.1 g. What is the best conclusion?

  1. Mass clearly increased because 210.8 g is larger than 210.7 g, so mass is not conserved.
  2. Mass is conserved within the measurement uncertainty because the 0.1 g difference could be due to the balance. (correct answer)
  3. Mass is conserved only if a gas forms, not if a solid forms.
  4. Mass cannot be conserved in a sealed flask because nothing can enter.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 210.7 g, and after the reaction in the sealed container, the total mass is 210.8 g, with the 0.1 g difference within ±0.1 g uncertainty. The closed system is critical here—the flask was sealed so no matter could escape or enter, meaning all the products (including the precipitate) are captured and measured. This near-equality proves that mass was conserved, and the atomic explanation is that the same atoms present initially are present finally, just bonded into different molecules. Choice B is correct because it correctly interprets the constant mass (within uncertainty) as evidence that atoms were conserved. Choice A is wrong because it dismisses the equal measurements as showing mass increased, when actually the 0.1 g difference reflects measurement uncertainty, not a real change. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 13

A student puts vinegar in a flask and baking soda in a balloon. The balloon is carefully stretched over the flask opening so the system is closed (no gas can escape). The mass of the entire system (flask + vinegar + balloon + baking soda) is 186.4 g before mixing. The student tips the balloon so the baking soda falls in, and the balloon inflates with CO2_2. After the reaction stops, the mass of the entire closed system is 186.4 g (±0.1 g). Which statement best uses this evidence to support conservation of mass?

  1. Mass is not conserved because a gas formed, so the total mass must increase.
  2. The equal before-and-after masses show the same atoms were present; they were rearranged into new substances while all products stayed in the closed system. (correct answer)
  3. The mass stayed the same only because the reaction was reversible.
  4. The mass stayed the same because the balance always reads the same number, even when mass changes.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 186.4 g, and after the reaction in the sealed container, the total mass is still 186.4 g (±0.1 g). The closed system is critical here—the balloon and flask were sealed so no gases could escape and no air could enter, meaning all the products (including CO₂ gas) are captured and measured. This equality (186.4 g = 186.4 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (with total atomic mass summing to 186.4 g) are present finally (still summing to 186.4 g), just bonded into different molecules. Choice B is correct because it correctly explains that mass is conserved because atoms are conserved and mass comes from atoms. Choice A is wrong because it claims mass is not conserved when gas forms, but actually gas has mass and is included in the closed system measurement, so total mass stays the same. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 14

In a sealed flask (closed system), 12.0 g of solution A is mixed with 8.0 g of solution B. The mass of the sealed flask + contents is 152.6 g before mixing and 152.6 g after a visible color change occurs. Which calculation best verifies conservation of mass for the substances inside the flask?

  1. Total before =12.0 g+8.0 g=20.0 g=12.0\,\text{g}+8.0\,\text{g}=20.0\,\text{g} and total after =20.0 g=20.0\,\text{g}, so mass is conserved (within measurement uncertainty). (correct answer)
  2. Total before =12.0 g−8.0 g=4.0 g=12.0\,\text{g}-8.0\,\text{g}=4.0\,\text{g} and total after =20.0 g=20.0\,\text{g}, so mass is not conserved.
  3. Total before =152.6 g=152.6\,\text{g} and total after =150.6 g=150.6\,\text{g}, so mass decreased by 2.0 g.
  4. You cannot verify conservation of mass unless the flask is open to the air.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass of substances is 12.0 g + 8.0 g = 20.0 g (with flask at 152.6 g total), and after the reaction in the sealed container, the total mass is still 152.6 g, implying substances remain 20.0 g. The closed system is critical here—the flask was sealed before the reaction so no gases could escape and no air could enter, meaning all the products are captured and measured. This equality (20.0 g = 20.0 g for substances) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (with total atomic mass summing to 20.0 g) are present finally (still summing to 20.0 g), just bonded into different molecules. Choice A is correct because it properly calculates total mass before and after showing they're equal, verifying conservation. Choice B is wrong because it makes an arithmetic error calculating total mass, claiming 12.0 g - 8.0 g = 4.0 g when actually it's 12.0 g + 8.0 g = 20.0 g, leading to false conclusion that mass changed. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 15

A reaction that produces a gas is done two ways:

• Setup 1: Reactants are mixed in an open beaker on a balance. The mass reading drops from 120.0 g to 118.8 g. • Setup 2: The same amounts of reactants are mixed in a closed system with a balloon attached to capture the gas. The mass stays 120.0 g before and after (±0.1 g).

Why does the closed system help you observe conservation of mass?

  1. Because sealing the system captures gas products so no matter leaves, making the before-and-after mass comparison valid. (correct answer)
  2. Because closed systems make reactions stop sooner, which keeps mass the same.
  3. Because mass is only conserved when a balloon is used.
  4. Because open systems always gain mass from the air, so the mass should increase, not decrease.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. In this closed vs open system comparison: In the closed system (with balloon), mass stays constant at 120.0 g before and after because all products are retained—the balloon captures the gas produced. In the open system (open beaker), gases produced in the reaction escape into the surrounding air, and measuring only the remaining solids/liquids shows apparent mass loss (120.0 g to 118.8 g), but the total mass including the escaped gases (1.2 g) would still equal the original 120.0 g, demonstrating that conservation holds even when measurement is complicated by gas escape. Choice A is correct because it accurately identifies the closed system's role in allowing verification by capturing all products—sealing prevents gas escape so all matter stays in the system for measurement. Choice B incorrectly claims closed systems make reactions stop sooner, which has nothing to do with mass conservation. Choice C wrongly limits conservation to balloon use specifically, when any closed system works. Choice D misunderstands open systems, claiming they gain mass from air when actually this reaction shows mass loss due to gas escape.

Question 16

A particle model represents a reaction in a closed system. Each atom has a labeled mass. Before: there are 2 X atoms (3 u each) and 2 Y atoms (5 u each). After: the same atoms are bonded differently to form products.

What does this model show about total mass?

  1. Total mass decreases because bonds break during the reaction.
  2. Total mass increases because new molecules form.
  3. Total mass stays the same: 2(3 u)+2(5 u)=16 u2(3\,u)+2(5\,u)=16\,u before and 16 u16\,u after because the same atoms are present, just rearranged. (correct answer)
  4. Total mass cannot be determined from a particle model because atoms have no mass.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The particle model shows 2 X atoms (3 u each) and 2 Y atoms (5 u each) before the reaction, giving total mass: 2(3 u) + 2(5 u) = 6 u + 10 u = 16 u. After the reaction, the same atoms (2 X and 2 Y) are present but arranged into different molecules, so the total mass is still 16 u. You can verify by counting: every atom from the reactants appears in the products (none created, none destroyed), so the sum of atomic masses before equals the sum after, which is why when we measure with a balance, we get the same reading. Choice C is correct because it properly calculates total mass before and after showing they're equal (16 u = 16 u), verifying conservation and correctly explains that mass is conserved because atoms are conserved. Choice A incorrectly claims mass decreases when bonds break, missing that breaking bonds doesn't destroy atoms—the same atoms with the same masses remain. Choice B wrongly states mass increases when new molecules form, not understanding that new molecules are made from existing atoms, not new atoms. Choice D makes the absurd claim that atoms have no mass, when atomic mass is a fundamental property of atoms.

Question 17

In a closed system, a student puts 50.0 g of vinegar in a flask and 5.0 g of baking soda in a balloon attached to the flask. The balloon is attached and sealed so no gas can escape. The mass of the entire system (flask + vinegar + balloon + baking soda) is 155.2 g before mixing. The student tips the baking soda into the vinegar; a reaction occurs and the balloon inflates with CO2_2. After the reaction stops, the mass of the entire system is 155.2 g.

Which statement best uses this evidence to support conservation of mass?

  1. Mass was not conserved because a gas formed, so the mass should have increased.
  2. Mass was conserved because the total mass before and after the reaction is the same in a closed system, meaning the same atoms are still present but rearranged. (correct answer)
  3. Mass was conserved only because the reaction is reversible.
  4. Mass stayed the same only because the balance is not sensitive enough to detect any real mass change.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 155.2 g (flask + vinegar + balloon + baking soda), and after the reaction in the sealed container, the total mass is still 155.2 g. The closed system is critical here—the balloon was sealed to the flask before the reaction so no CO₂ gas could escape and no air could enter, meaning all the products (including the CO₂ gas that inflated the balloon) are captured and measured. This equality (155.2 g = 155.2 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (with total atomic mass summing to 155.2 g) are present finally (still summing to 155.2 g), just bonded into different molecules. Choice B is correct because it properly identifies that the equal masses before and after verify conservation and correctly explains that mass is conserved because atoms are conserved and mass comes from atoms. Choice A incorrectly claims mass should increase when gas forms, missing that gas was already present as atoms in the reactants—the CO₂ atoms came from the carbon in baking soda and oxygen in both reactants, so forming gas doesn't create new mass. Choice C wrongly limits conservation to reversible reactions when actually all chemical reactions conserve mass. Choice D dismisses the equal measurements as balance insensitivity, when actually they reflect the fundamental law that mass is always conserved.

Question 18

A reaction is performed in a closed system (sealed bag). Masses are recorded:

Reactant A: 12.4 g Reactant B: 7.6 g Total mass before reaction (bag + contents): 45.0 g Total mass after reaction (bag + contents): 45.0 g

Which conclusion is best supported by these measurements?

  1. Mass was conserved in the closed system; the atoms in the reactants were rearranged into products without changing total mass. (correct answer)
  2. Mass increased because the reaction released energy.
  3. Mass decreased because some matter must have disappeared during the reaction.
  4. The data prove that molecules are conserved and cannot change during reactions.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, Reactant A (12.4 g) + Reactant B (7.6 g) = 20.0 g of reactants, and the total system mass (bag + contents) is 45.0 g. After the reaction in the sealed bag, the total system mass remains 45.0 g, proving that the 20.0 g of reactants became 20.0 g of products. The sealed bag ensured a closed system where no atoms could enter or leave, meaning all atoms from A and B are still present, just rearranged into new product molecules. Choice A is correct because it properly concludes that mass was conserved in the closed system and correctly explains that atoms in the reactants were simply rearranged into products without changing the total mass—this is the fundamental principle of mass conservation. Choice B incorrectly claims mass increased due to energy release, confusing energy changes with mass changes—chemical reactions can release energy without changing mass. Choice C incorrectly suggests mass decreased and matter disappeared, which violates conservation laws—atoms cannot be destroyed in chemical reactions. Choice D misinterprets the data as proving molecule conservation when actually molecules change in reactions; only atoms (and therefore mass) are conserved.

Question 19

A student wants to test whether mass stays the same during a reaction that produces a gas. Which setup best makes a closed system so the student can measure mass before and after fairly?

  1. Mix the reactants in an open beaker so the gas can bubble out easily.
  2. Mix the reactants in a flask with a balloon attached to capture the gas, then measure the mass of the whole setup before and after. (correct answer)
  3. Mix the reactants on a paper towel so you can see the reaction clearly.
  4. Heat the reactants strongly in an open container so the reaction finishes faster.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. In this scenario, to verify mass conservation for a gas-producing reaction, a closed system like a flask with an attached balloon captures the gas, allowing measurement of the entire system before and after without loss. Choice B is correct because it accurately identifies the closed system's role in allowing verification by capturing all products, including gases, for fair mass comparison. Choice A is wrong because it claims closed system is unnecessary by suggesting an open beaker, when actually gas escape in open system would make mass seem to change even though it's truly conserved. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.

Question 20

In a sealed flask (closed system), a student mixes two clear solutions and a solid forms (a precipitate). The mass of the sealed flask + contents is 212.6 g before mixing and 212.6 g after mixing (±0.1 g). What is the best explanation for why the mass stayed the same?

  1. The same atoms were present before and after; they just rearranged into new substances, and the sealed flask kept all matter inside. (correct answer)
  2. Some atoms disappeared, but the balance cannot detect missing atoms.
  3. Mass stays the same only during physical changes, not chemical reactions.
  4. The mass stayed the same because energy was converted into mass to replace what was lost.
Explanation: This question tests understanding that mass remains constant in a closed system during chemical reactions, and this can be verified through measurements and explained using atom conservation. When a chemical reaction occurs in a closed system (sealed container where nothing can enter or leave), careful mass measurements before and after always show that total mass is conserved: mass_before = mass_after, typically within ±0.1 g measurement uncertainty. This measurable mass conservation happens because atoms are conserved—since each atom has a specific mass (hydrogen ≈ 1 u, carbon ≈ 12 u, oxygen ≈ 16 u) and chemical reactions only rearrange atoms into new molecules without creating or destroying atoms, the total mass (which is just the sum of all the atomic masses) must remain the same even though the substances and their properties change. The measurements show mass conservation clearly: before the reaction, the total mass is 212.6 g, and after the reaction in the sealed flask, the total mass is still 212.6 g (including the precipitate). The closed system is critical here—the flask was sealed before the reaction so no matter could escape or enter, meaning all the products (including the solid precipitate) are captured and measured. This equality (212.6 g = 212.6 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (with total atomic mass summing to 212.6 g) are present finally (still summing to 212.6 g), just bonded into different molecules. Choice A is correct because it correctly explains that mass is conserved because atoms are conserved and mass comes from atoms, and accurately identifies the closed system's role in allowing verification by capturing all products. Choice B is wrong because it claims some atoms disappeared, when actually all atoms are conserved (none disappear) and the balance shows the same mass because the same atoms are still there, just rearranged. Experimental verification of mass conservation: (1) measure total mass of reactants using balance (write down value), (2) ensure system is closed (seal container before reaction, use balloon to capture gases, or perform in sealed bag), (3) allow reaction to occur (mix, heat, wait for completion), (4) measure total mass of system after reaction (same balance, careful measurement), (5) compare: if mass_before ≈ mass_after (within ±0.1 g for typical balance), mass is conserved. The atomic understanding explains why this always works: chemical reactions are just atoms rearranging—bonds break between some atoms, new bonds form between others, creating different molecules, but throughout this process, every single atom remains (you can account for each one in the products), and since mass is a property of atoms (each atom contributes its atomic mass to the total), having the same atoms means having the same mass, whether measured in atomic mass units at the microscopic level or grams on a balance at the macroscopic level—this is one of the most important connections between what we can see (balance readings) and what we can't see (atoms), showing that careful measurement and atomic theory agree perfectly.