Middle School Science Quiz: Check Atom Conservation

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Check Atom ConservationQuestion 1 of 20

Particle model for decomposing hydrogen peroxide (colors: H = white, O = red).

Reactants (before): 2 molecules H₂O₂

H₂O₂: (H–O–O–H) and (H–O–O–H)

Products (after): 2 molecules H₂O and 1 molecule O₂

H₂O: (H–O–H) and (H–O–H)
O₂: (O–O)

Complete the conservation check by choosing the correct counts for H and O before and after.

H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Conserved

H: 2 before, 4 after ✗; O: 4 before, 4 after ✓ → Not conserved

H: 4 before, 2 after ✗; O: 4 before, 2 after ✗ → Not conserved

H: 4 before, 4 after ✓; O: 2 before, 4 after ✗ → Not conserved

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Middle School Science Quiz

Middle School Science Quiz: Check Atom Conservation

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

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

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

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Question 1

Particle model for decomposing hydrogen peroxide (colors: H = white, O = red).

Reactants (before): 2 molecules H₂O₂

H₂O₂: (H–O–O–H) and (H–O–O–H)

Products (after): 2 molecules H₂O and 1 molecule O₂

H₂O: (H–O–H) and (H–O–H)
O₂: (O–O)

Complete the conservation check by choosing the correct counts for H and O before and after.

H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Conserved (correct answer)
H: 2 before, 4 after ✗; O: 4 before, 4 after ✓ → Not conserved
H: 4 before, 2 after ✗; O: 4 before, 2 after ✗ → Not conserved
H: 4 before, 4 after ✓; O: 2 before, 4 after ✗ → Not conserved

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 2 H₂O₂ → 2 H₂O + O₂, counting systematically: Hydrogen atoms in reactants: 2 H₂O₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms, Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 2 H₂O₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms, Oxygen atoms in products: 2 H₂O molecules, each has 1 O atom (2×1 = 2) plus 1 O₂ with 2 O atoms, total is 2 + 2 = 4 O atoms—these match (4 = 4) so oxygen is conserved. Since both hydrogen and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 4 H before and 4 H after, and 4 O before and 4 O after, properly verifying that all elements are conserved by checking each independently. Choice C makes a counting error, stating 2 H after when careful counting shows 4: 2 H₂O molecules × 2 H per molecule = 4 H total, and similarly errs on oxygen. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂O₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂O means 2 molecules, each with 2 H and 1 O = 4 H and 2 O total, not just 2 H and 1 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 H₂O₂ is 2 molecules but 8 atoms: 4 H + 4 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 2

A particle model shows the reaction $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$ . Count each type of atom in the reactants and products. Are atoms conserved (same number of each element before and after)?

H: 2 before, 4 after ✗; O: 2 before, 2 after ✓ → Not conserved
H: 4 before, 4 after ✓; O: 2 before, 2 after ✓ → Conserved (correct answer)
H: 4 before, 2 after ✗; O: 2 before, 2 after ✓ → Not conserved
Total atoms: 6 before, 6 after ✓ → Conserved (no need to check each element)

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction 2H₂ + O₂ → 2H₂O, counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms; Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these counts match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms; Oxygen atoms in products: 2 H₂O molecules, each has 1 O atom, total is 2×1 = 2 O atoms—these also match (2 = 2) so oxygen is conserved. Since both hydrogen and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice B is correct because it accurately counts atoms showing 4 H before and 4 H after, and 2 O before and 2 O after, properly verifying that all elements are conserved by checking each independently. Choice A makes a counting error, stating 2 hydrogen atoms before when careful counting shows 4: 2 molecules × 2 H per molecule = 4 H total; Choice C also miscounts hydrogen as 2 after when it's actually 4; Choice D claims conservation based only on total atoms (6 before and after) without checking each element separately, missing that you could have wrong distributions that still total 6. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved → model correct. Common mistakes to avoid: forgetting coefficients (2H₂ means 2 molecules, each with 2 H atoms = 4 H total, not just 2 H), counting molecules instead of atoms, or stopping after checking one element—careful systematic counting ensures you catch any conservation violations in reaction models.

Question 3

A student draws this particle model for forming carbon dioxide (color key: C = black, O = red).

Before: 1 molecule C (single C atom) + 1 molecule O₂ (O–O) After: 2 molecules CO₂ (O–C–O and O–C–O)

Is the model correct? If not, what is wrong?

Correct; carbon and oxygen are both conserved
Not correct; it creates extra carbon atoms (C: 1 before, 2 after) (correct answer)
Not correct; it destroys oxygen atoms (O: 2 before, 1 after)
Not correct; oxygen atoms change into carbon atoms

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 1 C + 1 O₂ and after: 2 CO₂, counting systematically: Carbon atoms in reactants: 1 C atom (single), total 1 C; Carbon atoms in products: 2 CO₂ each has 1 C, total 2×1=2 C—these do not match (1 ≠ 2) so carbon is not conserved; Oxygen atoms in reactants: 1 O₂ has 2 O, total 2 O; Oxygen atoms in products: 2 CO₂ each has 2 O, total 2×2=4 O—these do not match (2 ≠ 4) so oxygen is not conserved; Since both carbon and oxygen show unequal before-after counts, this model violates conservation and must be incorrect. Choice B is correct because it correctly identifies the model as not correct due to creating extra carbon atoms (1 before vs 2 after), based on systematic counting. Choice D is wrong because it claims oxygen atoms change into carbon atoms, but actually the model shows creation of extra atoms of both types, not transformation—conservation means no creation or destruction, not changing one element into another. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present, add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 CO₂ means 2 molecules, each with 1 C and 2 O = 2 C and 4 O total), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + O counts instead of checking separately), (d) counting molecules instead of atoms (2 CO₂ is 2 molecules but 6 atoms: 2 C + 4 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 4

Particle model (color key: H = white, O = red). Before: 2 molecules of H₂ (H–H, H–H) plus 1 molecule of O₂ (O–O). After: 2 molecules of H₂O (H–O–H, H–O–H). Count H and O atoms in reactants and products and decide whether atoms are conserved.

H: 2 before, 4 after ✗; O: 2 before, 2 after ✓ → Not conserved
H: 4 before, 4 after ✓; O: 2 before, 2 after ✓ → Conserved (correct answer)
H: 4 before, 2 after ✗; O: 2 before, 2 after ✓ → Not conserved
Total atoms: 6 before, 6 after, so it must be conserved (no need to check each element)

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 2 H₂ + 1 O₂ and after: 2 H₂O, counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms; Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these counts match (4 = 4) so hydrogen is conserved; Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms; Oxygen atoms in products: 2 H₂O molecules, each has 1 O atom, total is 2×1 = 2 O atoms—these also match (2 = 2) so oxygen is conserved; Since both hydrogen and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice B is correct because it accurately counts atoms showing 4 H before and 4 H after, 2 O before and 2 O after, and properly verifies that all elements are conserved by checking each independently. Choice D is wrong because it verifies only total atoms without checking each element separately, missing that you could have 6 total atoms before and after but wrong distribution: 6 H and 0 O before vs 0 H and 6 O after would violate conservation even though totals match. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂ means 2 molecules, each with 2 H atoms = 4 H total, not just 2 H), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 H₂O is 2 molecules but 6 atoms: 4 H + 2 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 5

Counting-table check (color key: Mg = gray, O = red). A particle model shows:

Before: 2 atoms Mg + 1 molecule O₂ (O–O) After: 2 units of MgO (Mg–O and Mg–O)

Which completed table is correct?

Element | Reactants | Products | Conserved? Mg | ? | ? | ? O | ? | ? | ?

Mg: 2 before, 2 after ✓; O: 2 before, 2 after ✓ (correct answer)
Mg: 1 before, 2 after ✗; O: 2 before, 2 after ✓
Mg: 2 before, 1 after ✗; O: 2 before, 1 after ✗
Mg: 2 before, 2 after ✓; O: 1 before, 2 after ✗

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 2 Mg + 1 O₂ and after: 2 MgO, counting systematically: Magnesium atoms in reactants: 2 Mg atoms, total 2 Mg; Magnesium atoms in products: 2 MgO each has 1 Mg, total 2×1=2 Mg—these match (2=2) so Mg is conserved; Oxygen atoms in reactants: 1 O₂ has 2 O, total 2 O; Oxygen atoms in products: 2 MgO each has 1 O, total 2×1=2 O—these match (2=2) so oxygen is conserved; Since both Mg and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 Mg before and 2 Mg after, 2 O before and 2 O after, and properly verifies that all elements are conserved by checking each independently with ✓. Choice B makes a counting error, stating 1 Mg before when careful counting shows 2 (from 2 Mg atoms), confusing the lack of coefficient with count. Systematic atom counting procedure: (1) identify all element types present in the reaction (Mg, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 Mg means 2 atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 MgO means 2 units, each with 1 Mg and 1 O = 2 Mg and 2 O total), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding Mg + O counts instead of checking separately), (d) counting molecules instead of atoms (2 MgO is 2 units but 4 atoms: 2 Mg + 2 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 6

A particle model represents: Before: 1 molecule C₂H₄ (2 C and 4 H) + 3 molecules O₂ (O–O, O–O, O–O). After: 2 molecules CO₂ + 2 molecules H₂O. (Color key: C = black, H = white, O = red.)

Are atoms conserved? Count each element before and after.

C: 2 before, 2 after ✓; H: 4 before, 4 after ✓; O: 6 before, 6 after ✓ → Conserved (correct answer)
C: 2 before, 1 after ✗; H: 4 before, 4 after ✓; O: 6 before, 6 after ✓ → Not conserved
C: 2 before, 2 after ✓; H: 4 before, 2 after ✗; O: 6 before, 6 after ✓ → Not conserved
C: 2 before, 2 after ✓; H: 4 before, 4 after ✓; O: 3 before, 6 after ✗ → Not conserved

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 1 C₂H₄ + 3 O₂ and after: 2 CO₂ + 2 H₂O, counting systematically: Carbon atoms in reactants: 1 C₂H₄ has 2 C atoms, total 2 C; Carbon atoms in products: 2 CO₂ each has 1 C, total 2×1=2 C—match (2=2); Hydrogen atoms in reactants: 1 C₂H₄ has 4 H, total 4 H; Hydrogen atoms in products: 2 H₂O each has 2 H, total 2×2=4 H—match (4=4); Oxygen atoms in reactants: 3 O₂ each has 2 O, total 3×2=6 O; Oxygen atoms in products: 2 CO₂ each has 2 O (2×2=4) plus 2 H₂O each has 1 O (2×1=2), total 4+2=6 O—match (6=6); Since carbon, hydrogen, and oxygen all show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 C before and 2 C after, 4 H before and 4 H after, 6 O before and 6 O after, and properly verifies that all elements are conserved by checking each independently. Choice D makes a counting error, claiming 3 O before when careful counting shows 6 (3 O₂ × 2 = 6), missing the coefficient and counting molecules instead of atoms. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (3 O₂ has 3×2 = 6 O atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (3 O₂ means 3 molecules, each with 2 O = 6 O total, not just 3 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 CO₂ + 2 H₂O is 4 molecules but 12 atoms: 2 C + 4 H + 6 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 7

A student draws this particle model for making water (color key: H = white, O = red).

Before: 2 molecules of H₂ (H–H, H–H) + 1 molecule of O₂ (O–O) After: 1 molecule of H₂O (H–O–H)

Is this model correct? If not, which element is NOT conserved?

Correct; H and O are both conserved
Not correct; hydrogen is not conserved (H: 4 before, 2 after)
Not correct; oxygen is not conserved (O: 2 before, 1 after)
Not correct; both hydrogen and oxygen are not conserved (H: 4 before, 2 after; O: 2 before, 1 after) (correct answer)

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 2 H₂ + 1 O₂ and after: 1 H₂O, counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms; Hydrogen atoms in products: 1 H₂O molecule has 2 H atoms, total is 2 H atoms—these counts do not match (4 ≠ 2) so hydrogen is not conserved; Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms; Oxygen atoms in products: 1 H₂O molecule has 1 O atom, total is 1 O atom—these do not match (2 ≠ 1) so oxygen is not conserved; Since both hydrogen and oxygen show unequal before-after counts, this model violates conservation and must be incorrect. Choice D is correct because it correctly identifies the model as unbalanced based on systematic counting, properly verifying that both elements are not conserved with specific mismatched counts. Choice A is wrong because it claims conservation when the model actually shows unequal counts for both elements, missing that all elements must be conserved, not just some. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂ means 2 molecules, each with 2 H atoms = 4 H total, not just 2 H), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (1 H₂O is 1 molecule but 3 atoms: 2 H + 1 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 8

Complete the conservation table for this particle model (color key: C = black, O = red).

Before: 1 molecule CO (C–O) + 1 molecule O₂ (O–O) After: 1 molecule CO₂ (O–C–O)

Which row correctly shows the oxygen (O) atom count before vs. after?

O: 2 before, 2 after ✓
O: 3 before, 2 after ✗ (correct answer)
O: 2 before, 3 after ✗
O: 1 before, 2 after ✗

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 1 CO + 1 O₂ and after: 1 CO₂, counting systematically: Oxygen atoms in reactants: 1 CO molecule has 1 O atom plus 1 O₂ molecule has 2 O atoms, total is 1+2=3 O atoms; Oxygen atoms in products: 1 CO₂ molecule has 2 O atoms, total is 2 O atoms—these counts do not match (3 ≠ 2) so oxygen is not conserved; However, for completeness, carbon: 1 before (from CO), 1 after (from CO₂)—matches, but since oxygen mismatches, the model is incorrect. Choice B is correct because it accurately counts oxygen atoms as 3 before and 2 after, properly calculates counts accounting for all molecules, and correctly identifies the mismatch with ✗. Choice A makes a counting error, stating 2 O before when careful counting shows 3 (1 from CO + 2 from O₂), missing the O in CO. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present, add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting to count atoms from all molecules (missing the O in CO, counting only O₂ as 2 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + O counts instead of checking separately), (d) counting molecules instead of atoms (2 reactants but 4 atoms: C, O, O, O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 9

Particle model for burning methane (color key: C = black, H = white, O = red).

Before: 1 molecule CH₄ (C with 4 H attached) + 2 molecules O₂ (O–O and O–O) After: 1 molecule CO₂ (O–C–O) + 2 molecules H₂O (H–O–H and H–O–H)

Which counting table correctly verifies whether atoms are conserved?

C: 1 before, 1 after ✓; H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Conserved (correct answer)
C: 1 before, 2 after ✗; H: 4 before, 4 after ✓; O: 2 before, 4 after ✗ → Not conserved
C: 1 before, 1 after ✓; H: 2 before, 4 after ✗; O: 4 before, 4 after ✓ → Not conserved
C: 1 before, 1 after ✓; H: 4 before, 2 after ✗; O: 4 before, 2 after ✗ → Not conserved

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 1 CH₄ + 2 O₂ and after: 1 CO₂ + 2 H₂O, counting systematically: Carbon atoms in reactants: 1 CH₄ molecule has 1 C atom, total is 1 C atom; Carbon atoms in products: 1 CO₂ molecule has 1 C atom, total is 1 C atom—these match (1 = 1) so carbon is conserved; Hydrogen atoms in reactants: 1 CH₄ has 4 H atoms, total is 4 H atoms; Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these match (4 = 4); Oxygen atoms in reactants: 2 O₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms; Oxygen atoms in products: 1 CO₂ has 2 O atoms plus 2 H₂O each has 1 O atom (2×1=2), total is 2 + 2 = 4 O atoms—these match (4 = 4); Since carbon, hydrogen, and oxygen all show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 1 C before and 1 C after, 4 H before and 4 H after, 4 O before and 4 O after, and properly verifies that all elements are conserved by checking each independently. Choice B makes a counting error, stating 2 C after when careful counting shows only 1 (from 1 CO₂), and claiming 2 O before when it's 4 (from 2 O₂), while also incorrectly showing O as 4 after but marking it as mismatched. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 O₂ has 2×2 = 4 O atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂O means 2 molecules, each with 2 H and 1 O = 4 H and 2 O total, not just 2 H and 1 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 H₂O is 2 molecules but 6 atoms: 4 H + 2 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 10

Unbalanced (incorrect) particle model for forming water (colors: H = white, O = red).

Reactants (before): 2 molecules of H₂ and 1 molecule of O₂

H₂: (H–H) and (H–H)
O₂: (O–O)

Products (after): 1 molecule of H₂O

H₂O: (H–O–H)

Is this particle model correct? If not, which element is NOT conserved?

Correct; H and O are both conserved.
Incorrect; hydrogen is not conserved (4 H before, 2 H after).
Incorrect; oxygen is not conserved (2 O before, 1 O after).
Incorrect; both hydrogen and oxygen are not conserved (H: 4→2, O: 2→1). (correct answer)

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 2 H₂ + O₂ → H₂O, counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms, Hydrogen atoms in products: 1 H₂O molecule has 2 H atoms, total is 2 H atoms—these do not match (4 ≠ 2) so hydrogen is not conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms, Oxygen atoms in products: 1 H₂O molecule has 1 O atom, total is 1 O atom—these do not match (2 ≠ 1) so oxygen is not conserved. Since both hydrogen and oxygen show unequal before-after counts, this model violates conservation and must be incorrect. Choice D is correct because it correctly identifies the model as unbalanced based on systematic counting, properly calculating counts accounting for coefficients and showing both H: 4→2 and O: 2→1 are not conserved. Choice A is wrong because it claims conservation when the model actually shows unequal counts for both elements, missing that all elements must be conserved, not just some. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂ means 2 molecules, each with 2 H atoms = 4 H total, not just 2 H), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (1 H₂O is 1 molecule but 3 atoms: 2 H + 1 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 11

Particle model for forming water (colors: H = white, O = red).

Reactants (before): 2 molecules of H₂ and 1 molecule of O₂

H₂: (H–H) and (H–H)
O₂: (O–O)

Products (after): 2 molecules of H₂O

H₂O: (H–O–H) and (H–O–H)

Count H and O atoms before and after. Are atoms conserved in this model?

H: 2 before, 4 after ✗; O: 2 before, 2 after ✓ → Not conserved
H: 4 before, 4 after ✓; O: 2 before, 2 after ✓ → Conserved (correct answer)
H: 4 before, 2 after ✗; O: 2 before, 2 after ✓ → Not conserved
Total atoms: 6 before and 6 after, so conserved (no need to check each element)

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 2 H₂ + O₂ → 2 H₂O, counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms, Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these counts match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms, Oxygen atoms in products: 2 H₂O molecules, each has 1 O atom, total is 2×1 = 2 O atoms—these also match (2 = 2) so oxygen is conserved. Since both hydrogen and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice B is correct because it accurately counts atoms showing 4 H before and 4 H after, and 2 O before and 2 O after, properly verifying that all elements are conserved by checking each independently. Choice D is wrong because it verifies only total atoms without checking each element separately, missing that you could have 6 total atoms before and after but wrong distribution: for example, 6 H and 0 O before vs 0 H and 6 O after would violate conservation even though totals match. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂ means 2 molecules, each with 2 H atoms = 4 H total, not just 2 H), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 H₂O is 2 molecules but 6 atoms: 4 H + 2 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 12

Particle model for burning methane (colors: C = black, H = white, O = red).

Reactants (before): 1 molecule CH₄ and 2 molecules O₂

CH₄: C with 4 H attached (C + 4H)
O₂: (O–O) and (O–O)

Products (after): 1 molecule CO₂ and 2 molecules H₂O

CO₂: (O–C–O)
H₂O: (H–O–H) and (H–O–H)

Which counting table correctly verifies whether atoms are conserved?

C: 1 before, 1 after ✓; H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Conserved (correct answer)
C: 1 before, 2 after ✗; H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Not conserved
C: 1 before, 1 after ✓; H: 4 before, 2 after ✗; O: 2 before, 4 after ✗ → Not conserved
C: 1 before, 1 after ✓; H: 4 before, 8 after ✗; O: 4 before, 2 after ✗ → Not conserved

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction CH₄ + 2 O₂ → CO₂ + 2 H₂O, counting systematically: Carbon atoms in reactants: 1 CH₄ molecule has 1 C atom, total is 1 C atom, Carbon atoms in products: 1 CO₂ molecule has 1 C atom, total is 1 C atom—these match (1 = 1) so carbon is conserved. Hydrogen atoms in reactants: 1 CH₄ molecule has 4 H atoms, total is 4 H atoms, Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 2 O₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms, Oxygen atoms in products: 1 CO₂ has 2 O atoms plus 2 H₂O each with 1 O atom (2×1 = 2), total is 2 + 2 = 4 O atoms—these match (4 = 4) so oxygen is conserved. Since carbon, hydrogen, and oxygen all show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 1 C before and 1 C after, 4 H before and 4 H after, and 4 O before and 4 O after, properly verifying that all elements are conserved by checking each independently. Choice B makes a counting error, stating 2 C atoms after when careful counting shows only 1: 1 CO₂ molecule has 1 C atom. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 O₂ has 2×2 = 4 O atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂O means 2 molecules, each with 2 H and 1 O = 4 H and 2 O total, not just 2 H and 1 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + H + O counts instead of checking separately), (d) counting molecules instead of atoms (CH₄ is 1 molecule but 5 atoms: 1 C + 4 H), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 13

Particle model for burning hydrogen sulfide (colors: H = white, S = yellow, O = red).

Reactants (before): 2 molecules H₂S and 3 molecules O₂

H₂S: (H–S–H) and (H–S–H)
O₂: (O–O), (O–O), (O–O)

Products (after): 2 molecules SO₂ and 2 molecules H₂O

SO₂: (O–S–O) and (O–S–O)
H₂O: (H–O–H) and (H–O–H)

Which option correctly verifies conservation by counting each element?

H: 4 before, 4 after ✓; S: 2 before, 2 after ✓; O: 6 before, 6 after ✓ → Conserved (correct answer)
H: 4 before, 2 after ✗; S: 2 before, 2 after ✓; O: 6 before, 6 after ✓ → Not conserved
H: 2 before, 4 after ✗; S: 2 before, 1 after ✗; O: 6 before, 6 after ✓ → Not conserved
Total atoms are the same before and after, so it must be conserved (no need to separate elements).

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 2 H₂S + 3 O₂ → 2 SO₂ + 2 H₂O, counting systematically: Hydrogen atoms in reactants: 2 H₂S molecules, each has 2 H atoms, total is 2×2 = 4 H atoms, Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these match (4 = 4) so hydrogen is conserved. Sulfur atoms in reactants: 2 H₂S molecules, each has 1 S atom, total is 2×1 = 2 S atoms, Sulfur atoms in products: 2 SO₂ molecules, each has 1 S atom, total is 2×1 = 2 S atoms—these match (2 = 2) so sulfur is conserved. Oxygen atoms in reactants: 3 O₂ molecules, each has 2 O atoms, total is 3×2 = 6 O atoms, Oxygen atoms in products: 2 SO₂ each with 2 O (2×2 = 4) plus 2 H₂O each with 1 O (2×1 = 2), total is 4 + 2 = 6 O atoms—these match (6 = 6) so oxygen is conserved. Since hydrogen, sulfur, and oxygen all show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 4 H before and 4 H after, 2 S before and 2 S after, and 6 O before and 6 O after, properly verifying that all elements are conserved by checking each independently. Choice D is wrong because it verifies only total atoms without checking each element separately, missing that you could have the same total atoms before and after but wrong distribution: for example, 4 H, 0 S, 8 O before vs 0 H, 4 S, 8 O after would violate conservation even though totals match. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, S, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂S has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (3 O₂ means 3 molecules, each with 2 O atoms = 6 O total, not just 2 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + S + O counts instead of checking separately), (d) counting molecules instead of atoms (2 SO₂ is 2 molecules but 6 atoms: 2 S + 4 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 14

Particle model for rusting iron (simplified) (colors: Fe = gray, O = red).

Reactants (before): 4 atoms Fe and 3 molecules O₂

Fe: Fe Fe Fe Fe
O₂: (O–O), (O–O), (O–O)

Products (after): 2 formula units Fe₂O₃

Fe₂O₃: (Fe Fe O O O) and (Fe Fe O O O)

Which counting is correct, and are atoms conserved?

Fe: 4 before, 4 after ✓; O: 6 before, 6 after ✓ → Conserved (correct answer)
Fe: 4 before, 2 after ✗; O: 6 before, 6 after ✓ → Not conserved
Fe: 2 before, 4 after ✗; O: 6 before, 3 after ✗ → Not conserved
Fe: 4 before, 4 after ✓; O: 3 before, 6 after ✗ → Not conserved

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 4 Fe + 3 O₂ → 2 Fe₂O₃, counting systematically: Iron atoms in reactants: 4 Fe atoms, total is 4 Fe atoms, Iron atoms in products: 2 Fe₂O₃, each has 2 Fe atoms, total is 2×2 = 4 Fe atoms—these match (4 = 4) so iron is conserved. Oxygen atoms in reactants: 3 O₂ molecules, each has 2 O atoms, total is 3×2 = 6 O atoms, Oxygen atoms in products: 2 Fe₂O₃, each has 3 O atoms, total is 2×3 = 6 O atoms—these match (6 = 6) so oxygen is conserved. Since both iron and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 4 Fe before and 4 Fe after, and 6 O before and 6 O after, properly verifying that all elements are conserved by checking each independently. Choice C makes a counting error, stating 2 Fe before when careful counting shows 4: 4 Fe atoms total, and errs on oxygen as 3 after per unit but forgets to multiply by 2 units for 6 O total. Systematic atom counting procedure: (1) identify all element types present in the reaction (Fe, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (3 O₂ has 3×2 = 6 O atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 Fe₂O₃ means 2 units, each with 2 Fe and 3 O = 4 Fe and 6 O total, not just 2 Fe and 3 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding Fe + O counts instead of checking separately), (d) counting molecules instead of atoms (2 Fe₂O₃ is 2 units but 10 atoms: 4 Fe + 6 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 15

Particle model for making ammonia (colors: N = blue, H = white).

Reactants (before): 1 molecule N₂ and 3 molecules H₂

N₂: (N–N)
H₂: (H–H), (H–H), (H–H)

Products (after): 2 molecules NH₃

NH₃: (N with 3 H attached) and (N with 3 H attached)

Count atoms of each element. Are atoms conserved?

N: 2 before, 2 after ✓; H: 6 before, 6 after ✓ → Conserved (correct answer)
N: 2 before, 1 after ✗; H: 6 before, 6 after ✓ → Not conserved
N: 2 before, 2 after ✓; H: 3 before, 6 after ✗ → Not conserved
N: 1 before, 2 after ✗; H: 6 before, 3 after ✗ → Not conserved

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction N₂ + 3 H₂ → 2 NH₃, counting systematically: Nitrogen atoms in reactants: 1 N₂ molecule has 2 N atoms, total is 2 N atoms, Nitrogen atoms in products: 2 NH₃ molecules, each has 1 N atom, total is 2×1 = 2 N atoms—these match (2 = 2) so nitrogen is conserved. Hydrogen atoms in reactants: 3 H₂ molecules, each has 2 H atoms, total is 3×2 = 6 H atoms, Hydrogen atoms in products: 2 NH₃ molecules, each has 3 H atoms, total is 2×3 = 6 H atoms—these match (6 = 6) so hydrogen is conserved. Since both nitrogen and hydrogen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 N before and 2 N after, and 6 H before and 6 H after, properly verifying that all elements are conserved by checking each independently. Choice C makes a counting error, stating 3 H before when careful counting shows 6: 3 H₂ molecules × 2 H per molecule = 6 H total. Systematic atom counting procedure: (1) identify all element types present in the reaction (N, H), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (3 H₂ has 3×2 = 6 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (3 H₂ means 3 molecules, each with 2 H atoms = 6 H total, not just 2 H), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding N + H counts instead of checking separately), (d) counting molecules instead of atoms (2 NH₃ is 2 molecules but 8 atoms: 2 N + 6 H), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 16

Particle model for burning carbon monoxide (colors: C = black, O = red).

Reactants (before): 2 molecules CO and 1 molecule O₂

CO: (C–O) and (C–O)
O₂: (O–O)

Products (after): 2 molecules CO₂

CO₂: (O–C–O) and (O–C–O)

Which statement correctly checks conservation by counting each element?

C: 2 before, 2 after ✓; O: 4 before, 4 after ✓ → Conserved (correct answer)
C: 1 before, 2 after ✗; O: 4 before, 4 after ✓ → Not conserved
C: 2 before, 2 after ✓; O: 2 before, 4 after ✗ → Not conserved
There are 3 molecules before and 2 molecules after, so atoms are not conserved.

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 2 CO + O₂ → 2 CO₂, counting systematically: Carbon atoms in reactants: 2 CO molecules, each has 1 C atom, total is 2×1 = 2 C atoms, Carbon atoms in products: 2 CO₂ molecules, each has 1 C atom, total is 2×1 = 2 C atoms—these match (2 = 2) so carbon is conserved. Oxygen atoms in reactants: 2 CO molecules, each has 1 O atom (2×1 = 2) plus 1 O₂ molecule with 2 O atoms, total is 2 + 2 = 4 O atoms, Oxygen atoms in products: 2 CO₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms—these match (4 = 4) so oxygen is conserved. Since both carbon and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 C before and 2 C after, and 4 O before and 4 O after, properly verifying that all elements are conserved by checking each independently. Choice D is wrong because it counts molecules instead of atoms: states 3 before and 2 after (counting CO and O₂ molecules) when the question asks about atoms, which are 2 C + 4 O before and 2 C + 4 O after. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 CO has 2×1 = 2 C atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 CO means 2 molecules, each with 1 C and 1 O = 2 C and 2 O total, not just 1 C and 1 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + O counts instead of checking separately), (d) counting molecules instead of atoms (2 CO₂ is 2 molecules but 6 atoms: 2 C + 4 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 17

Unbalanced (incorrect) particle model for forming carbon dioxide (colors: C = black, O = red).

Reactants (before): 1 molecule C and 1 molecule O₂

C: (C)
O₂: (O–O)

Products (after): 2 molecules CO₂

CO₂: (O–C–O) and (O–C–O)

Which option correctly identifies what is wrong by counting atoms?

The model is correct: C and O counts match on both sides.
Carbon is not conserved: 1 C before, 2 C after.
Oxygen is not conserved: 2 O before, 4 O after.
Both carbon and oxygen are not conserved (C: 1→2, O: 2→4). (correct answer)

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction C + O₂ → 2 CO₂, counting systematically: Carbon atoms in reactants: 1 C atom, total is 1 C atom, Carbon atoms in products: 2 CO₂ molecules, each has 1 C atom, total is 2×1 = 2 C atoms—these do not match (1 ≠ 2) so carbon is not conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms, Oxygen atoms in products: 2 CO₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms—these do not match (2 ≠ 4) so oxygen is not conserved. Since both carbon and oxygen show unequal before-after counts, this model violates conservation and must be incorrect. Choice D is correct because it correctly identifies the model as unbalanced based on systematic counting, showing both C: 1→2 and O: 2→4 are not conserved. Choice A is wrong because it claims conservation when the model actually shows unequal counts for both elements, missing that all elements must be conserved, not just some. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (1 O₂ has 1×2 = 2 O atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 CO₂ means 2 molecules, each with 1 C and 2 O = 2 C and 4 O total, not just 1 C and 2 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + O counts instead of checking separately), (d) counting molecules instead of atoms (2 CO₂ is 2 molecules but 6 atoms: 2 C + 4 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 18

Multiple particle models are shown (color key: H = white, O = red).

Model 1: Before: H₂ + O₂. After: H₂O. Model 2: Before: 2H₂ + O₂. After: 2H₂O. Model 3: Before: 2H₂ + O₂. After: H₂O₂ + H₂.

Which model(s) correctly conserve BOTH H and O atoms?

Model 1 only
Model 2 only
Models 1 and 2 only
Models 2 and 3 only (correct answer)

Explanation: This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For these models, counting systematically: Model 1 (H₂ + O₂ → H₂O): H: 2 before (1 H₂ ×2), 2 after (1 H₂O ×2)—match; O: 2 before (1 O₂ ×2), 1 after (1 H₂O ×1)—mismatch (2≠1), so not conserved; Model 2 (2 H₂ + O₂ → 2 H₂O): H: 4 before (2 H₂ ×2), 4 after (2 H₂O ×2)—match; O: 2 before, 2 after (2 H₂O ×1)—match, conserved; Model 3 (2 H₂ + O₂ → H₂O₂ + H₂): H: 4 before, after H₂O₂ (2 H) + H₂ (2 H)=4—match; O: 2 before, H₂O₂ (2 O)—match, conserved; Thus, models 2 and 3 correctly conserve both H and O. Choice D is correct because it correctly identifies models 2 and 3 as balanced based on systematic counting, properly verifying that all elements are conserved in those by checking each independently. Choice A is wrong because it claims only model 1 conserves atoms, but model 1 has unequal O counts (2 before vs 1 after), missing that all elements must be conserved. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂O means 2 molecules, each with 2 H = 4 H total), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (H₂O₂ + H₂ is 2 molecules but 6 atoms: 4 H + 2 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.

Question 19

A particle model represents $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ . Count atoms for each element before and after. Are atoms conserved?

Ca: 1 before, 1 after ✓; C: 1 before, 1 after ✓; O: 3 before, 3 after ✓ → Atoms are conserved. (correct answer)
Ca: 1 before, 1 after ✓; C: 1 before, 2 after ✗; O: 3 before, 3 after ✓ → Atoms are not conserved.
Ca: 1 before, 0 after ✗; C: 1 before, 1 after ✓; O: 3 before, 3 after ✓ → Atoms are not conserved.
Ca: 1 before, 1 after ✓; C: 1 before, 1 after ✓; O: 2 before, 3 after ✗ → Atoms are not conserved.

Question 20

A student claims the particle model for $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$ conserves atoms. Which set of counts correctly checks conservation?

C: 3 before, 3 after ✓; H: 8 before, 8 after ✓; O: 10 before, 10 after ✓ → Atoms are conserved. (correct answer)
C: 3 before, 3 after ✓; H: 8 before, 4 after ✗; O: 10 before, 10 after ✓ → Atoms are not conserved.
C: 3 before, 6 after ✗; H: 8 before, 8 after ✓; O: 10 before, 10 after ✓ → Atoms are not conserved.
C: 3 before, 3 after ✓; H: 8 before, 8 after ✓; O: 5 before, 10 after ✗ → Atoms are not conserved.