Middle School Science Quiz: Atoms Rearranged In Reactions
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Atoms Rearranged In ReactionsQuestion 1 of 20

A particle model shows ammonia forming: N2+3H2→2NH3\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3. Before: one N≡N molecule and three H–H molecules. After: two molecules where each N is bonded to three H atoms. How many hydrogen atoms are present before the reaction, and how many are present after the reaction?

Before: 3 H atoms; After: 6 H atoms
Before: 6 H atoms; After: 6 H atoms
Before: 2 H atoms; After: 6 H atoms
Before: 6 H atoms; After: 3 H atoms
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Middle School Science Quiz

Middle School Science Quiz: Atoms Rearranged In Reactions

Practice Atoms Rearranged In Reactions in Middle School Science with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Atoms Rearranged In Reactions, giving you a quick way to practice the rules, question types, and explanations that matter most for Middle School Science.

How to use this quiz

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

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Question 1

A particle model shows ammonia forming: N2+3H2→2NH3\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3. Before: one N≡N molecule and three H–H molecules. After: two molecules where each N is bonded to three H atoms. How many hydrogen atoms are present before the reaction, and how many are present after the reaction?

  1. Before: 3 H atoms; After: 6 H atoms
  2. Before: 6 H atoms; After: 6 H atoms (correct answer)
  3. Before: 2 H atoms; After: 6 H atoms
  4. Before: 6 H atoms; After: 3 H atoms
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (N₂ and H₂ reactants become NH₃ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 1 N₂ molecule (containing 2 nitrogen atoms) and 3 H₂ molecules (each containing 2 hydrogen atoms, giving 6 total hydrogen atoms: 3×2). After the reaction, the products are 2 NH₃ molecules, each containing 1 nitrogen atom and 3 hydrogen atoms, giving 2 total nitrogen atoms (2×1) and 6 total hydrogen atoms (2×3). Comparing the counts: 2 nitrogen before equals 2 nitrogen after, and 6 hydrogen before equals 6 hydrogen after—this demonstrates that atoms are conserved. The atoms rearranged (nitrogen atoms that were bonded N≡N are now bonded to H, hydrogen atoms that were bonded H-H are now bonded to N), but no nitrogen or hydrogen atoms were created or destroyed. Choice B is correct because it correctly states that there are 6 H atoms before and 6 H atoms after. Choice A incorrectly claims there are only 3 H atoms before the reaction, when actually counting the atoms shows 3 H₂ molecules × 2 H atoms per molecule = 6 H atoms total in the reactants. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 6 H in reactants, must be 6 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 2

In this reaction model: Before: 2 H₂ molecules (H–H, H–H) and 1 O₂ molecule (O=O). After: 2 H₂O molecules (H–O–H and H–O–H). What happens to the hydrogen atoms during the reaction?

  1. They are destroyed when the H–H bonds break.
  2. They stay bonded to the same hydrogen atom, but the molecule is renamed water.
  3. They rearrange: hydrogen atoms that were bonded to hydrogen become bonded to oxygen in water, but the hydrogen atoms themselves are still the same atoms. (correct answer)
  4. They change into oxygen atoms to match the oxygen in water.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂ and O₂ reactants become H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For 2H₂ + O₂ → 2H₂O: Before the reaction, the reactants are [2 H₂ molecules (containing 4 total hydrogen atoms) and 1 O₂ molecule (containing 2 oxygen atoms)]. After the reaction, the products are [2 H₂O molecules, each containing 2 hydrogen and 1 oxygen, totaling 4 hydrogen and 2 oxygen atoms]. Comparing the counts: 4 hydrogen before equals 4 hydrogen after, and 2 oxygen before equals 2 oxygen after—this demonstrates that atoms are conserved. During the reaction, bonds broke: specific bonds like H–H and O=O in reactants and new bonds formed: specific bonds like H–O in products, but throughout this bond breaking and forming, every single atom remained (no atom vanished) and no new atoms appeared (no atoms created from nothing)—the atoms simply changed their bonding partners, which is what we mean by "rearranged." Choice C is correct because it accurately explains that atoms rearrange by changing bonding partners while staying present, specifically noting hydrogen atoms from H–H become bonded to oxygen in water but remain the same atoms. Choice A incorrectly claims atoms are destroyed when the H–H bonds break, when actually the Law of Conservation of Matter states atoms cannot be created or destroyed in chemical reactions—all atoms present after were present before, just rearranged. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 3

A particle model shows water decomposing:

Reactants (before):

  • 2 water molecules: H–O–H and H–O–H

Products (after):

  • 2 hydrogen molecules: H–H and H–H
  • 1 oxygen molecule: O=O

What evidence from the model shows that no atoms are created or destroyed?

  1. There are 4 H atoms and 2 O atoms before, and there are 4 H atoms and 2 O atoms after. (correct answer)
  2. There are fewer molecules after, so atoms must have been destroyed.
  3. The oxygen atoms disappear because oxygen gas forms.
  4. New hydrogen atoms are created because H₂ appears in the products.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂O reactants become H₂ and O₂ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 2 H₂O molecules (4 H and 2 O); after, 2 H₂ (4 H) and 1 O₂ (2 O)—comparing the counts: 4 H before equals 4 H after, and 2 O before equals 2 O after, demonstrating that atoms are conserved; the atoms rearranged (H-O-H bonds break, H-H and O=O bonds form), but no atoms were created or destroyed. Choice A is correct because it correctly states that atoms are conserved with equal counts of 4 H and 2 O before and after. Choice B incorrectly claims atoms are destroyed because there are fewer molecules after, when actually the Law of Conservation of Matter states atoms cannot be created or destroyed in chemical reactions—all atoms present after were present before, just rearranged, and molecule count can change without affecting atom conservation. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 4

A particle diagram shows: Before: 2 water molecules (H–O–H, H–O–H). After: 2 hydrogen molecules (H–H, H–H) and 1 oxygen molecule (O=O). Which statement best describes the bond changes shown?

  1. H–O bonds break in water, and new H–H and O=O bonds form in the products. (correct answer)
  2. H–H bonds break and H–O bonds form, making water from hydrogen and oxygen.
  3. No bonds change; only the spacing between molecules changes.
  4. Oxygen atoms break apart into smaller atoms, which then join to hydrogen.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂O reactants become H₂ and O₂ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For 2H₂O → 2H₂ + O₂: Before the reaction, the reactants are [2 H₂O molecules (4 H, 2 O)]. After the reaction, the products are [2 H₂ (4 H) and 1 O₂ (2 O)]. Comparing the counts: 4 H before equals 4 H after, 2 O before equals 2 O after—this demonstrates that atoms are conserved. During the reaction, bonds broke: specific bonds like H–O in water and new bonds formed: specific bonds like H–H and O=O in products, but throughout this bond breaking and forming, every single atom remained (no atom vanished) and no new atoms appeared (no atoms created from nothing)—the atoms simply changed their bonding partners, which is what we mean by "rearranged." Choice A is correct because it accurately describes the bond changes: H–O bonds break, and new H–H and O=O bonds form. Choice C denies rearrangement, claiming no bonds change, when the model clearly shows different bonding patterns: in reactants H–O–H but in products H–H and O=O, demonstrating atoms reconnected differently. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 5

A particle model shows methane burning:

Reactants (before):

  • 1 methane molecule: CH₄ (C bonded to 4 H)
  • 2 oxygen molecules: O₂ and O₂ (each is O=O)

Products (after):

  • 1 carbon dioxide molecule: CO₂ (O=C=O)
  • 2 water molecules: H₂O and H₂O (each is H–O–H)

How many oxygen (O) atoms are present before the reaction and after the reaction?

  1. Before: 2 O atoms; After: 4 O atoms
  2. Before: 4 O atoms; After: 4 O atoms (correct answer)
  3. Before: 4 O atoms; After: 3 O atoms
  4. Before: 2 O atoms; After: 2 O atoms
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (CH₄ and O₂ reactants become CO₂ and H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 1 CH₄ molecule (containing 1 C and 4 H) and 2 O₂ molecules (containing 4 total O atoms); after the reaction, the products are 1 CO₂ molecule (1 C and 2 O) and 2 H₂O molecules (4 H and 2 O), giving total 1 C, 4 H, 4 O before and after—comparing the counts for oxygen specifically: 4 O before equals 4 O after, demonstrating that oxygen atoms are conserved; the atoms rearranged (C-H bonds and O=O bonds break, C=O and H-O bonds form), but no atoms were created or destroyed. Choice B is correct because it accurately states there are 4 O atoms before and 4 O atoms after, properly verifying conservation by counting oxygen atoms. Choice A makes a counting error, stating 2 O atoms before but 4 O after, when carefully counting the atoms in the model shows 4 O before (2 O₂ means 4 O) and 4 O after (2 in CO₂ + 2 in 2 H₂O). To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 6

A particle model shows methane burning:

Before (reactants):

  • CH₄ (C–H bonds)
  • 2 O₂ molecules (O=O bonds)

After (products):

  • CO₂ (O=C=O)
  • 2 H₂O (H–O–H)

Which statement best explains what happens to the atoms during the reaction?

  1. Some oxygen atoms are created to provide enough atoms to form both CO₂ and H₂O.
  2. Atoms are conserved, but they rearrange: C and H separate from each other and form new bonds with O atoms to make CO₂ and H₂O. (correct answer)
  3. The methane molecule stays intact, and oxygen molecules simply attach to it without breaking any bonds.
  4. Hydrogen atoms change into oxygen atoms so water can form.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (CH₄ and O₂ reactants become CO₂ and H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are CH₄ (C–H bonds) and 2 O₂ (O=O bonds); after, CO₂ (O=C=O) and 2 H₂O (H–O–H)—atoms are conserved (1 C, 4 H, 4 O before and after), but they rearrange: C and H separate from each other (C–H bonds break), O=O bonds break, and new C=O and H–O bonds form with O atoms, demonstrating rearrangement without creation or destruction. Choice B is correct because it accurately explains that atoms are conserved but rearrange by changing bonding partners while staying present. Choice A incorrectly claims some oxygen atoms are created, when actually the Law of Conservation of Matter states atoms cannot be created or destroyed in chemical reactions—all atoms present after were present before, just rearranged. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 7

A particle model shows ammonia forming: N2+3H2→2NH3\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3. Which claim is correct about conservation of matter in this reaction?

  1. Matter is not conserved because there are 4 molecules after the reaction but only 4 molecules before.
  2. Matter is conserved because there are 2 N atoms and 6 H atoms before, and 2 N atoms and 6 H atoms after. (correct answer)
  3. Matter is conserved because nitrogen atoms are destroyed and replaced by hydrogen atoms.
  4. Matter is not conserved because the N≡N bond is too strong to break, so new nitrogen atoms must appear.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (N₂ and H₂ reactants become NH₃ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 1 N₂ molecule (containing 2 nitrogen atoms) and 3 H₂ molecules (containing 6 total hydrogen atoms: 2 in each molecule). After the reaction, the products are 2 NH₃ molecules, each containing 1 nitrogen atom and 3 hydrogen atoms, giving 2 total nitrogen atoms (2×1) and 6 total hydrogen atoms (2×3). Comparing the counts: 2 nitrogen before equals 2 nitrogen after, and 6 hydrogen before equals 6 hydrogen after—this demonstrates that atoms are conserved. The atoms rearranged (nitrogen atoms that were bonded N≡N are now bonded to H, hydrogen atoms that were bonded H-H are now bonded to N), but no nitrogen or hydrogen atoms were created or destroyed. Choice B is correct because it correctly states that matter is conserved with 2 N atoms and 6 H atoms before and after. Choice A incorrectly claims matter is not conserved because there are 4 molecules after but only 4 molecules before, when actually counting shows 4 molecules before (1 N₂ + 3 H₂) and 2 molecules after (2 NH₃)—but molecule count doesn't determine conservation, atom count does. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 6 H in reactants, must be 6 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 8

A model shows methane burning: Before: 1 methane molecule (CH₄) and 2 oxygen molecules (O=O and O=O). After: 1 carbon dioxide molecule (O=C=O) and 2 water molecules (H–O–H and H–O–H). How many oxygen atoms are shown before and after the reaction?

  1. Before: 2 O atoms; After: 4 O atoms
  2. Before: 4 O atoms; After: 4 O atoms (correct answer)
  3. Before: 3 O atoms; After: 4 O atoms
  4. Before: 4 O atoms; After: 2 O atoms
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (CH₄ and O₂ reactants become CO₂ and H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For CH₄ + 2O₂ → CO₂ + 2H₂O: Before the reaction, the reactants are [1 CH₄ molecule (containing 1 carbon and 4 hydrogen atoms) and 2 O₂ molecules (each containing 2 oxygen atoms, totaling 4 oxygen atoms)]. After the reaction, the products are [1 CO₂ molecule (containing 1 carbon and 2 oxygen atoms) and 2 H₂O molecules (each containing 2 hydrogen and 1 oxygen, totaling 4 hydrogen and 2 oxygen), giving overall 1 carbon, 4 hydrogen, and 4 oxygen atoms]. Comparing the counts: 1 carbon before equals 1 carbon after, 4 hydrogen before equals 4 hydrogen after, and 4 oxygen before equals 4 oxygen after—this demonstrates that atoms are conserved. The atoms rearranged (carbon atoms that were bonded to H are now bonded to O, oxygen atoms from O=O are now in CO₂ and H₂O), but no atoms were created or destroyed. Choice B is correct because it properly verifies conservation by counting atoms of oxygen correctly as 4 before and 4 after. Choice A incorrectly claims before: 2 O atoms; after: 4 O atoms, when carefully counting shows 4 O before (2 O₂ means 4 O atoms) and 4 O after (2 in CO₂ + 2 in 2H₂O). To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 9

A particle model shows: 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}. In the reactants, hydrogen atoms are bonded to hydrogen atoms (H–H) and oxygen atoms are bonded to oxygen atoms (O=O). In the products, each oxygen is bonded to two hydrogens (H–O–H). Which statement best explains how the atoms are rearranged?

  1. Hydrogen atoms become oxygen atoms, so the atoms change type during the reaction.
  2. The same atoms are present, but H–H and O=O bonds break and new H–O bonds form. (correct answer)
  3. No bonds break; the H–H bonds simply stretch into H–O bonds.
  4. New atoms are created to make the water molecules because water has three atoms per molecule.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂ and O₂ reactants become H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Counting atoms in the model before the reaction gives 6 total atoms: 4 hydrogen atoms (2 in each H₂ molecule) and 2 oxygen atoms (in the O₂ molecule), and counting after gives the same 6 total atoms: 4 hydrogen atoms (2 in each H₂O molecule) and 2 oxygen atoms (1 in each H₂O molecule)—the fact that these counts match, atom by atom, demonstrates the Law of Conservation of Matter. During the reaction, bonds broke: H-H bonds in hydrogen molecules and O=O bonds in oxygen molecules, and new bonds formed: H-O bonds in water molecules, but throughout this bond breaking and forming, every single atom remained (no atom vanished) and no new atoms appeared (no atoms created from nothing)—the atoms simply changed their bonding partners, which is what we mean by "rearranged." Choice B is correct because it accurately explains that the same atoms are present but H-H and O=O bonds break and new H-O bonds form. Choice A incorrectly claims hydrogen atoms become oxygen atoms, when actually atoms stay the same element—hydrogen atoms remain hydrogen atoms and oxygen atoms remain oxygen atoms, they just change which atoms they're bonded to. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 10

A particle model shows this reaction:

Reactants (before): 2 hydrogen molecules and 1 oxygen molecule

  • H₂: H–H and H–H (two separate molecules)
  • O₂: O=O (one molecule)

Products (after): 2 water molecules

  • H₂O: H–O–H and H–O–H (two separate molecules)

Based on the before-and-after model, which statement best shows that atoms are conserved (not created or destroyed) in this reaction?

  1. Atoms are not conserved because the reactants are gases and the product is liquid water.
  2. Atoms are conserved because there are 4 H atoms and 2 O atoms before the reaction, and there are 4 H atoms and 2 O atoms after the reaction. (correct answer)
  3. Atoms are conserved because the number of molecules stays the same (3 molecules before and 3 molecules after).
  4. Atoms are not conserved because the H atoms turn into O atoms when water forms.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂ and O₂ reactants become H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 2 H₂ molecules (containing 4 total hydrogen atoms) and 1 O₂ molecule (containing 2 oxygen atoms); after the reaction, the products are 2 H₂O molecules, each containing 2 hydrogen atoms and 1 oxygen atom, giving 4 total hydrogen atoms and 2 total oxygen atoms—comparing the counts: 4 hydrogen before equals 4 hydrogen after, and 2 oxygen before equals 2 oxygen after, demonstrating that atoms are conserved; the atoms rearranged (hydrogen atoms that were bonded H-H are now bonded H-O-H, oxygen atoms that were bonded O=O are now bonded to H), but no hydrogen or oxygen atoms were created or destroyed. Choice B is correct because it accurately explains that atoms are conserved with equal counts of 4 H and 2 O before and after the reaction. Choice A incorrectly claims atoms are not conserved because the reactants are gases and the product is liquid water, when actually the Law of Conservation of Matter states atoms cannot be created or destroyed in chemical reactions regardless of the state of matter—all atoms present after were present before, just rearranged. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 11

A particle model shows methane burning: Before: CH₄ (one C bonded to 4 H) + 2O₂ (O=O and O=O). After: CO₂ (O=C=O) + 2H₂O (H–O–H and H–O–H). Which statement correctly describes what is conserved in the model?

  1. The number of each type of atom (C, H, and O) stays the same before and after, even though the molecules change. (correct answer)
  2. The number of molecules stays the same before and after, so matter is conserved.
  3. Carbon atoms disappear, and new oxygen atoms appear to replace them.
  4. Only hydrogen atoms are conserved; oxygen atoms are created during burning.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (CH₄ and O₂ reactants become CO₂ and H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For CH₄ + 2O₂ → CO₂ + 2H₂O: Before the reaction, the reactants are [1 CH₄ (1 C, 4 H) and 2 O₂ (4 O)]. After the reaction, the products are [1 CO₂ (1 C, 2 O) and 2 H₂O (4 H, 2 O)], totaling 1 C, 4 H, 4 O. Comparing the counts: 1 C before equals 1 C after, 4 H before equals 4 H after, 4 O before equals 4 O after—this demonstrates that atoms are conserved. The atoms rearranged (C-H bonds in methane become C=O in CO₂, etc.), but no atoms were created or destroyed. Choice A is correct because it correctly states that atoms are conserved with equal counts of each type (C, H, O) before and after, even though molecules change. Choice B incorrectly claims the number of molecules stays the same, when the model shows 3 molecules before (CH₄ + 2O₂) and 3 after, but conservation is about atoms, not molecules—molecules do change. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 12

A particle model shows this reaction:

Reactants (before): 2H₂ + O₂ Products (after): 2H₂O

A student claims, "The reaction makes new atoms because there are water molecules after the reaction." Which response is best, using evidence from the model?

  1. The student is correct because molecules are the same thing as atoms.
  2. The student is correct because there are more atoms in each water molecule than in each hydrogen molecule.
  3. The student is incorrect because the total number of H atoms (4) and O atoms (2) is the same before and after; the atoms are just rearranged into different molecules. (correct answer)
  4. The student is incorrect because chemical reactions can destroy atoms, but this one does not.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂ and O₂ reactants become H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 2 H₂ (4 H) and O₂ (2 O); after, 2 H₂O (4 H and 2 O)—the total number of H atoms (4) and O atoms (2) is the same before and after, with the atoms just rearranged into different molecules (from H–H and O=O to H–O–H), demonstrating conservation despite new substances forming. Choice C is correct because it properly verifies conservation by counting atoms of each element and recognizes that atoms are rearranged, not created. Choice A confuses molecules with atoms, suggesting molecules are the same thing as atoms, when actually atoms are conserved but molecules change—molecules are groups of atoms, and while molecules are not conserved (H₂ and O₂ become H₂O), the atoms within them are. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 13

A particle model shows water decomposing:

Reactants (before): 2H₂O (each molecule is H–O–H) Products (after): 2H₂ (H–H) and O₂ (O=O)

Which statement correctly describes the bond changes?

  1. H–H bonds break and H–O bonds form.
  2. H–O bonds break, and H–H and O=O bonds form. (correct answer)
  3. O=O bonds break, and only H–O bonds form.
  4. No bonds change; the reaction only changes the spacing between atoms.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂O reactants become H₂ and O₂ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 2 H₂O (each H–O–H); after, 2 H₂ (H–H) and O₂ (O=O)—during the reaction, H–O bonds break, and H–H and O=O bonds form, but throughout this bond breaking and forming, every single atom remained and no new atoms appeared—the atoms simply changed their bonding partners, which is what we mean by 'rearranged.' Choice B is correct because it accurately describes the bond changes where H–O bonds break and H–H and O=O bonds form, while atoms are conserved. Choice D denies rearrangement, claiming no bonds change and the reaction only changes spacing, when the model clearly shows different bonding patterns: in reactants H–O, but in products H–H and O=O, demonstrating atoms reconnected differently. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 14

A particle model shows the synthesis of ammonia:

Before (reactants): 1 N₂ molecule and 3 H₂ molecules After (products): 2 NH₃ molecules

How many total atoms are shown before the reaction and after the reaction?

  1. Before: 8 atoms; After: 8 atoms (correct answer)
  2. Before: 6 atoms; After: 8 atoms
  3. Before: 8 atoms; After: 6 atoms
  4. Before: 7 atoms; After: 7 atoms
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (N₂ and H₂ reactants become NH₃ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 1 N₂ (2 N atoms) and 3 H₂ (6 H atoms), totaling 8 atoms; after, 2 NH₃ (each with 1 N and 3 H, so 2 N and 6 H), totaling 8 atoms—the fact that these total counts match demonstrates the Law of Conservation of Matter; during the reaction, N≡N and H–H bonds break and N–H bonds form, but every single atom remained and no new atoms appeared—the atoms simply changed their bonding partners. Choice A is correct because it properly verifies conservation by counting total atoms as 8 before and 8 after. Choice B makes a counting error, stating 6 atoms before but 8 after, when carefully counting the atoms in the model shows 8 before (2 N + 6 H) and 8 after (2 N + 6 H). To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 15

A particle model shows this reaction:

Reactants (before): 2H₂ + O₂

  • Two H₂ molecules (H–H, H–H)
  • One O₂ molecule (O=O)

Products (after): 2H₂O

  • Two H₂O molecules (H–O–H, H–O–H)

Which statement best describes how the atoms are rearranged?

  1. The same H atoms that were bonded to other H atoms become bonded to O atoms, forming H–O bonds in water. (correct answer)
  2. Hydrogen atoms are destroyed and replaced by oxygen atoms to make water.
  3. The atoms stay bonded to the same partners; only the molecules move closer together.
  4. New atoms appear in the products because water has 3 atoms per molecule.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂ and O₂ reactants become H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Before the reaction, the reactants are 2 H₂ (4 H) and 1 O₂ (2 O); after, 2 H₂O (4 H and 2 O)—counts match; the atoms rearranged, with the same H atoms that were bonded to other H atoms (H–H) becoming bonded to O atoms (forming H–O–H), but no atoms were created or destroyed. Choice A is correct because it accurately describes how atoms are rearranged by changing bonding partners from H–H and O=O to H–O bonds while staying present. Choice B incorrectly claims atoms are destroyed and replaced, when actually the Law of Conservation of Matter states atoms cannot be created or destroyed in chemical reactions—all atoms present after were present before, just rearranged. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules, (2) count atoms of each element in all product molecules, (3) compare the counts—they should match exactly, (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect. The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 16

A particle diagram represents the synthesis of ammonia: Before: 1 nitrogen molecule (N≡N) and 3 hydrogen molecules (H–H, H–H, H–H). After: 2 ammonia molecules (each is N bonded to 3 H). Which bonds break and which bonds form according to the model?

  1. N–H bonds break, and N≡N bonds form.
  2. No bonds break; the atoms just move closer together.
  3. The N≡N bond and the H–H bonds break, and new N–H bonds form in ammonia. (correct answer)
  4. Hydrogen atoms change into nitrogen atoms so N–H bonds can form.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (N₂ and H₂ reactants become NH₃ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For N₂ + 3H₂ → 2NH₃: Before the reaction, the reactants are [1 N₂ molecule (containing 2 nitrogen atoms) and 3 H₂ molecules (each containing 2 hydrogen atoms, totaling 6 hydrogen atoms)]. After the reaction, the products are [2 NH₃ molecules, each containing 1 nitrogen and 3 hydrogen atoms, giving 2 nitrogen and 6 hydrogen atoms]. Comparing the counts: 2 nitrogen before equals 2 nitrogen after, and 6 hydrogen before equals 6 hydrogen after—this demonstrates that atoms are conserved. During the reaction, bonds broke: specific bonds like N≡N and H–H in reactants and new bonds formed: specific bonds like N–H in products, but throughout this bond breaking and forming, every single atom remained (no atom vanished) and no new atoms appeared (no atoms created from nothing)—the atoms simply changed their bonding partners, which is what we mean by "rearranged." Choice C is correct because it accurately explains that atoms rearrange by changing bonding partners while staying present, specifically noting the N≡N and H–H bonds break and new N–H bonds form. Choice D incorrectly claims hydrogen atoms change into nitrogen atoms, when actually the Law of Conservation of Matter states atoms cannot be created, destroyed, or changed into different elements in chemical reactions—all atoms present after were present before, just rearranged. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 17

A particle model for methane burning shows: Before: CH₄ + 2O₂. After: CO₂ + 2H₂O. Which comparison of carbon and hydrogen atoms before vs. after is correct?

  1. Before: 1 C and 4 H; After: 1 C and 4 H (correct answer)
  2. Before: 1 C and 4 H; After: 2 C and 4 H
  3. Before: 1 C and 2 H; After: 1 C and 4 H
  4. Before: 1 C and 4 H; After: 1 C and 2 H
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (CH₄ and O₂ reactants become CO₂ and H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For CH₄ + 2O₂ → CO₂ + 2H₂O: Before the reaction, the reactants are [CH₄ (1 C, 4 H) + 2O₂ (4 O)]. After the reaction, the products are [CO₂ (1 C, 2 O) + 2H₂O (4 H, 2 O), totaling 1 C, 4 H, 4 O]. Comparing the counts: 1 C before equals 1 C after, 4 H before equals 4 H after—this demonstrates that atoms are conserved (focusing on C and H as asked). The atoms rearranged, but counts match for C and H. Choice A is correct because it correctly states that atoms are conserved with equal counts of C (1) and H (4) before and after. Choice D makes a counting error, stating before: 1 C and 4 H; after: 1 C and 2 H, when carefully counting shows 4 H after (2 H₂O has 4 H). To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 18

A particle model shows this reaction: Before (reactants): 2 hydrogen molecules (H–H, H–H) and 1 oxygen molecule (O=O). After (products): 2 water molecules (H–O–H and H–O–H). Based on the model, which statement best shows that atoms are conserved (not created or destroyed) during the reaction?

  1. Atoms are conserved because the molecules stay the same; hydrogen and oxygen molecules are still present after the reaction.
  2. Atoms are conserved because there are 4 H atoms and 2 O atoms before the reaction and also 4 H atoms and 2 O atoms after the reaction, even though the bonds change. (correct answer)
  3. Atoms are conserved because 2 new oxygen atoms are created when water forms.
  4. Atoms are conserved because hydrogen atoms turn into oxygen atoms to make water.
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂ and O₂ reactants become H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For H₂ + O₂ → H₂O: Before the reaction, the reactants are [2 H₂ molecules (containing 4 total hydrogen atoms: 2 in each molecule) and 1 O₂ molecule (containing 2 oxygen atoms)]. After the reaction, the products are [2 H₂O molecules, each containing 2 hydrogen atoms and 1 oxygen atom, giving 4 total hydrogen atoms (2×2) and 2 total oxygen atoms (2×1)]. Comparing the counts: 4 hydrogen before equals 4 hydrogen after, and 2 oxygen before equals 2 oxygen after—this demonstrates that atoms are conserved. The atoms rearranged (hydrogen atoms that were bonded H-H are now bonded H-O, oxygen atoms that were bonded O=O are now bonded to H), but no hydrogen or oxygen atoms were created or destroyed. Choice B is correct because it accurately explains that atoms are conserved with equal counts before and after (4 H and 2 O on both sides), even though the bonds change. Choice A is incorrect because it claims molecules stay the same, when actually the molecules change from H₂ and O₂ to H₂O, while atoms are conserved but rearranged. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 19

A model shows decomposition of water: Before: 2 water molecules (H–O–H and H–O–H). After: 2 hydrogen molecules (H–H, H–H) and 1 oxygen molecule (O=O). Which count correctly compares atoms before vs. after?

  1. Before: 4 H and 2 O; After: 4 H and 2 O (correct answer)
  2. Before: 2 H and 2 O; After: 4 H and 2 O
  3. Before: 4 H and 2 O; After: 2 H and 4 O
  4. Before: 4 H and 2 O; After: 6 H and 2 O
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂O reactants become H₂ and O₂ products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. For 2H₂O → 2H₂ + O₂: Before the reaction, the reactants are [2 H₂O molecules, each containing 2 hydrogen and 1 oxygen, totaling 4 hydrogen and 2 oxygen atoms]. After the reaction, the products are [2 H₂ molecules (each containing 2 hydrogen, totaling 4 hydrogen) and 1 O₂ molecule (containing 2 oxygen atoms)]. Comparing the counts: 4 hydrogen before equals 4 hydrogen after, and 2 oxygen before equals 2 oxygen after—this demonstrates that atoms are conserved. The atoms rearranged (hydrogen atoms that were bonded H-O are now bonded H-H, oxygen atoms that were bonded to H are now bonded O=O), but no hydrogen or oxygen atoms were created or destroyed. Choice A is correct because it correctly states that atoms are conserved with equal counts before and after (4 H and 2 O on both sides). Choice D makes a counting error, stating before: 4 H and 2 O; after: 6 H and 2 O, when carefully counting the atoms in the model shows correct counts that match as 4 H and 2 O after (2 H₂ has 4 H, O₂ has 2 O). To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.

Question 20

A particle model shows: 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}. Which quantity stays the same from reactants to products (showing conservation of matter)?

  1. The number of each type of atom (H atoms and O atoms). (correct answer)
  2. The number of molecules (it must always stay the same).
  3. The types of bonds (H–H and O=O must stay present).
  4. The identity of each molecule (hydrogen molecules stay hydrogen molecules).
Explanation: This question tests understanding of the Law of Conservation of Matter: in chemical reactions, atoms are rearranged into new molecules but no atoms are created or destroyed. During a chemical reaction, the atoms present in the reactant molecules disconnect from their current bonding partners (bonds break) and reconnect with different atoms to form product molecules (new bonds form)—the key principle is that while the molecules change (H₂ and O₂ reactants become H₂O products), the atoms themselves are conserved: every atom in the products was present in the reactants, and every atom from the reactants appears in the products, meaning you can count atoms of each element and get the same number before and after the reaction. Counting atoms in the model before the reaction gives 6 total atoms: 4 hydrogen atoms (2 in each H₂ molecule) and 2 oxygen atoms (in the O₂ molecule), and counting after gives the same 6 total atoms: 4 hydrogen atoms (2 in each H₂O molecule) and 2 oxygen atoms (1 in each H₂O molecule)—the fact that these counts match, atom by atom, demonstrates the Law of Conservation of Matter. During the reaction, bonds broke: H-H bonds in hydrogen molecules and O=O bonds in oxygen molecules, and new bonds formed: H-O bonds in water molecules, but throughout this bond breaking and forming, every single atom remained (no atom vanished) and no new atoms appeared (no atoms created from nothing)—the atoms simply changed their bonding partners, which is what we mean by "rearranged." Choice A is correct because it properly identifies that the number of each type of atom (H atoms and O atoms) stays the same. Choice B incorrectly claims the number of molecules must always stay the same, when actually the reaction shows 3 reactant molecules (2 H₂ + 1 O₂) becoming 2 product molecules (2 H₂O)—conservation applies to atoms, not molecules. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O atoms, all C atoms, etc.), (2) count atoms of each element in all product molecules (same procedure), (3) compare the counts—they should match exactly (if 4 H in reactants, must be 4 H in products), (4) if counts match for every element, atoms are conserved; if they don't match, either you miscounted or the model is incorrect (unbalanced representation). The reason atoms are conserved is fundamental: atoms are the basic building blocks of matter and cannot be created, destroyed, or changed into different elements during chemical reactions (that requires nuclear reactions, not chemical reactions)—so in all chemical reactions, whether burning, rusting, cooking, or any other reaction, the atoms present at the start must all appear at the end, though they'll be bonded differently, in different molecules, creating substances with different properties.