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: before the reaction, the closed system mass is 200.0 g, and after the reaction (with gas produced and captured), the mass is 199.9 g, a difference of only 0.1 g. Since the balance uncertainty is ±0.1 g, this means the measurements are 200.0 ± 0.1 g and 199.9 ± 0.1 g, which overlap (both could actually be 199.9 g or 200.0 g). This equality within experimental error proves that mass was conserved—the tiny 0.1 g difference is not a real mass change but simply measurement uncertainty inherent in the balance. Choice C is correct because it properly recognizes that the 0.1 g difference is within the balance uncertainty (±0.1 g), meaning the masses are effectively the same within experimental error, confirming mass conservation in the closed system. Choice A incorrectly treats any measured difference as proof mass isn't conserved, failing to account for measurement uncertainty that exists in all real experiments. Choice B incorrectly claims the system must have been open because gases were produced, missing that closed systems can contain gases—that's the whole point of using sealed containers to verify mass conservation when gases form. Choice D incorrectly suggests mass conservation depends on the phase of products formed, when actually mass is conserved regardless of whether products are solids, liquids, or gases.

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.

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.

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 142.7 g, and after the reaction in the sealed setup, the total mass is still 142.7 g (including the CO₂ in the balloon). The closed system is critical here—the balloon was attached before the reaction so no gases could escape and no air could enter, meaning all the products are captured and measured. This equality (142.7 g = 142.7 g) proves that mass was conserved, and the atomic explanation is that the same atoms present initially (with total atomic mass summing to 142.7 g) are present finally (still summing to 142.7 g), just bonded into different molecules like CO₂. Choice A is correct because it correctly explains that mass is conserved because atoms are conserved (same atoms = same mass) and they were rearranged into new substances. Choice C is wrong because it confuses mass conservation with energy conservation, or thinks mass converts to energy in chemical reactions (not true—mass and energy are separately conserved 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.

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 bag), mass stays constant at 38.4 g before and after because all products are retained—if the system were open (cup unsealed), gases produced in the reaction would escape into the surrounding air, and measuring only the remaining solids/liquids would show apparent mass loss (perhaps only a smaller value g after), but the total mass including the escaped gases would still equal the original 38.4 g, demonstrating that conservation holds even when measurement is complicated by gas escape. Choice A is correct because it accurately identifies that in an open system, the measured mass would likely decrease due to gas escape, even though true mass is conserved. Choice B is wrong because it claims closed system is unnecessary, 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.

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 98.2 g (±0.1 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 (decrease by 0.6 g), but the total mass including the escaped gases would still equal the original 98.2 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. Choice D is wrong because it claims closed system is unnecessary, 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.

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 6 atoms before the reaction (2 C atoms at 12 u each, 4 H atoms at 1 u each) with total mass 28 u, and the same 6 atoms after the reaction (the same 2 C, 4 H atoms), just arranged into different molecules, with the same total mass. 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 B is correct because it correctly explains that mass is conserved because atoms are conserved (same atoms = same mass). Choice A is wrong because it claims new atoms are created, but actually reactions only rearrange existing atoms, 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.

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 3 atoms before the reaction (1 O at 16 u, 2 H at 1 u each) with total mass 18 u, and the same 3 atoms after the reaction (the same 1 O, 2 H atoms), just arranged into different molecules, with the same total mass. 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 B is correct because it properly calculates total mass before and after showing they're equal (18 u), verifying conservation. Choice D is wrong because it confuses mass conservation with energy, thinking mass depends on energy released, but actually mass and energy are separately conserved 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.

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.

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.