- A particle model shows methane burning: CH4+2O2→CO2+2H2O. Based on this model, which set of bond changes must happen during the reaction (while the atoms themselves are conserved)?
Answer: C–H and O=O bonds break; C=O and O–H bonds form.
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. Counting atoms in the model before the reaction gives 9 total atoms: 1 carbon atom (in CH₄), 4 hydrogen atoms (in CH₄), and 4 oxygen atoms (2 in each O₂ molecule), and counting after gives the same 9 total atoms: 1 carbon atom (in CO₂), 4 hydrogen atoms (2 in each H₂O molecule), and 4 oxygen atoms (2 in CO₂ plus 1 in each H₂O)—the fact that these counts match, atom by atom, demonstrates the Law of Conservation of Matter. During the reaction, bonds broke: C-H bonds in methane and O=O bonds in oxygen molecules, and new bonds formed: C=O bonds in carbon dioxide and O-H 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 accurately explains that C-H and O=O bonds break while C=O and O-H bonds form. Choice B incorrectly claims C=O and O-H bonds break, when actually these bonds are formed during the reaction (they exist in the products CO₂ and H₂O, not in the reactants CH₄ and O₂). 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.
- A model shows ammonia synthesis: Before: N₂ (N≡N) + 3H₂ (H–H, H–H, H–H). After: 2NH₃ (two molecules, each N bonded to 3 H). How many total atoms are shown before the reaction and after the reaction?
Answer: Before: 8 atoms; After: 8 atoms
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 (2 N) + 3 H₂ (6 H), totaling 8 atoms]. After the reaction, the products are [2 NH₃ (each 1 N + 3 H, totaling 2 N + 6 H), or 8 atoms]. Comparing the counts: 2 N before equals 2 N after, 6 H before equals 6 H after, and total 8 atoms match—this demonstrates that atoms are conserved. The atoms rearranged, but the total number remains the same. Choice A is correct because it accurately verifies conservation by counting total atoms as 8 before and 8 after. Choice B makes a counting error, stating before: 5 atoms; after: 8 atoms, when carefully counting shows 8 before (2 N + 6 H) and 8 after. 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.
- A model of 2H2+O2→2H2O shows that H–H bonds and the O=O bond are present before the reaction, but only H–O bonds are present after. What does this change in bonding show?
Answer: Bonds broke and new bonds formed, meaning the same atoms rearranged into different molecules.
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. The model shows that in the reaction 2H₂ + O₂ → 2H₂O, the types of bonds change completely: before the reaction there are H-H bonds (in H₂ molecules) and O=O bonds (in O₂ molecules), but after the reaction there are only H-O bonds (in H₂O molecules)—this change in bonding patterns is the key evidence that atoms have rearranged. During the reaction, the H-H bonds in hydrogen molecules must break to free the hydrogen atoms, and the O=O bond in the oxygen molecule must break to free the oxygen atoms. These freed atoms then form new H-O bonds to create water molecules. The atoms themselves don't change (hydrogen stays hydrogen, oxygen stays oxygen), but their bonding partners change completely: hydrogen atoms that were bonded to other hydrogen atoms are now bonded to oxygen atoms instead. Choice C is correct because it accurately explains that bonds broke and new bonds formed, meaning the same atoms rearranged into different molecules. Choice A incorrectly claims atoms were created because new types of bonds appeared, when actually the same atoms just formed different bonds—no atoms were created. Choice B incorrectly claims atoms changed identity (H became O), which is impossible in chemical reactions—atoms keep their element identity and only change bonding partners. Choice D incorrectly claims no chemical reaction happened because atoms must keep the same bonding partners, when actually chemical reactions are defined by atoms changing bonding partners while being conserved. The complete change from H-H and O=O bonds to only H-O bonds proves that a chemical reaction occurred: same atoms (4 H and 2 O), different arrangement.
- 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?
Answer: 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.
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.
- A particle model shows this reaction: 2H2+O2→2H2O. Before the reaction there are 2 hydrogen molecules (each is H–H) and 1 oxygen molecule (O=O). After the reaction there are 2 water molecules (each is H–O–H). Which statement best shows that atoms are conserved (not created or destroyed) in this reaction?
Answer: Atoms are conserved because there are 4 H atoms and 2 O atoms before the reaction, and 4 H atoms and 2 O atoms after the reaction.
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: 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 correctly states that atoms are conserved with equal counts before and after. Choice A incorrectly claims atoms are not conserved because there are more molecules, when actually the Law of Conservation of Matter states atoms cannot be created or destroyed in chemical reactions—the number of molecules can change (3 reactant molecules become 2 product molecules here), but the number of atoms of each element must stay the same. 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.
- A model shows the reaction 2H2+O2→2H2O. Before the reaction there are 2 hydrogen molecules (H–H, H–H) and 1 oxygen molecule (O=O). After the reaction there are 2 water molecules (H–O–H and H–O–H). Which statement best shows that atoms are conserved in this reaction?
Answer: 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.
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 the reaction 2H₂ + O₂ → 2H₂O, the atoms present in the reactant molecules disconnect from their current bonding partners (H-H bonds and O=O bonds break) and reconnect with different atoms to form product molecules (new H-O 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. 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. Choice B is correct because it accurately states that atoms are conserved with equal counts before and after: 4 H atoms and 2 O atoms in both reactants and products. Choice A incorrectly claims atoms are not conserved due to phase change (gas to liquid), when actually conservation of atoms is independent of physical state—atoms don't disappear just because molecules condense from gas to liquid. Choice C makes the error of counting molecules (3 total) instead of atoms, confusing molecular conservation with atomic conservation—molecules change but atoms are conserved. Choice D incorrectly claims new H atoms are created, violating the fundamental law that atoms cannot be created or destroyed in chemical reactions.
- 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?
Answer: Before: 4 H and 2 O; After: 4 H and 2 O
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.
- For the reaction CH4+2O2→CO2+2H2O, which set of bonds is most likely broken in the reactants and which set is formed in the products?
Answer: Broken: C–H and O=O bonds; Formed: C=O bonds (in CO2) and H–O bonds (in H2O)
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 the combustion reaction CH₄ + 2O₂ → CO₂ + 2H₂O, bonds in the reactant molecules must break and new bonds must form in the product molecules—understanding which bonds break and form helps visualize how atoms rearrange while being conserved. In the reactants: CH₄ (methane) has four C-H bonds connecting the central carbon to four hydrogen atoms, and 2O₂ means two oxygen molecules, each with an O=O double bond. For the reaction to proceed, these existing bonds must break to free the atoms for rearrangement. In the products: CO₂ has two C=O double bonds (carbon double-bonded to each oxygen), and 2H₂O has H-O bonds (each water molecule has two H-O bonds). These are completely different bonds than in the reactants, showing that atoms have rearranged. Choice B is correct because it accurately identifies that C-H bonds (in methane) and O=O bonds (in oxygen gas) are broken, while C=O bonds (in carbon dioxide) and H-O bonds (in water) are formed. Choice A incorrectly suggests C=O bonds are broken, but there are no C=O bonds in the reactants—these bonds are formed, not broken. Choice C incorrectly suggests H-O bonds are broken, but there are no H-O bonds in the reactants—water is a product, not a reactant. Choice D incorrectly claims no bonds break and atoms keep the same bonding partners, when clearly the bonding pattern changes completely from C-H and O=O bonds to C=O and H-O bonds. This bond breaking and forming is how atoms rearrange while maintaining conservation: 1 C, 4 H, and 4 O atoms are present both before and after, just connected differently. - For the reaction 2H2O→2H2+O2, which bonds must break and which bonds must form (while keeping the same atoms)?
Answer: H–O bonds break; H–H bonds and O=O bonds form.
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 the reaction 2H₂O → 2H₂ + O₂, 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)—this is the decomposition of water, where water molecules break apart and atoms rearrange into hydrogen and oxygen gas molecules. In the reactants, 2H₂O means 2 water molecules, each with H-O-H structure, so there are H-O bonds connecting hydrogen to oxygen. In the products, we have 2H₂ (with H-H bonds) and O₂ (with O=O double bond). For this reaction to occur, the H-O bonds in water must break to free the hydrogen and oxygen atoms, then new H-H bonds must form to create H₂ molecules and new O=O bonds must form to create O₂ molecules. Choice A is correct because it accurately states that H-O bonds break (in the water molecules) and H-H bonds and O=O bonds form (in the product molecules). Choice B incorrectly states that O=O bonds stay the same, but there are no O=O bonds in the reactants (water has H-O bonds, not O=O bonds)—the O=O bonds must form during the reaction. Choice C is incomplete because it doesn't mention that H-H bonds must also form, not just the O=O bonds. Choice D incorrectly claims no bonds break or form, when clearly the bonding pattern changes completely from H-O-H in water to H-H in hydrogen gas and O=O in oxygen gas. To verify atom conservation: 2H₂O contains 4 H atoms and 2 O atoms total, while 2H₂ + O₂ also contains 4 H atoms and 2 O atoms total—same atoms, different bonding arrangement.
- A particle model shows ammonia forming: N2+3H2→2NH3. 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?
Answer: Before: 6 H atoms; After: 6 H atoms
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.
- In the reaction N2+3H2→2NH3, what happens to the atoms during the reaction?
Answer: The same 2 N atoms and 6 H atoms are present before and after, but they rearrange so N–H bonds form in NH3.
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 the reaction N₂ + 3H₂ → 2NH₃, the atoms present in the reactant molecules disconnect from their current bonding partners (N≡N triple bonds and H-H bonds break) and reconnect with different atoms to form product molecules (new N-H bonds form)—the key principle is that while the molecules change (N₂ and H₂ reactants become NH₃ products), the atoms themselves are conserved. Counting atoms in the reactants: N₂ contains 2 nitrogen atoms, and 3H₂ means 3 hydrogen molecules, each containing 2 hydrogen atoms, giving 3×2 = 6 total hydrogen atoms. Counting atoms in the products: 2NH₃ means 2 ammonia molecules, each containing 1 nitrogen atom and 3 hydrogen atoms, giving 2×1 = 2 total nitrogen atoms and 2×3 = 6 total hydrogen atoms. Comparing the counts: 2 nitrogen before equals 2 nitrogen after, and 6 hydrogen before equals 6 hydrogen after—the atoms rearranged (nitrogen atoms that were bonded N≡N are now bonded to hydrogen, hydrogen atoms that were bonded H-H are now bonded to nitrogen), but no nitrogen or hydrogen atoms were created or destroyed. Choice D is correct because it accurately explains that the same 2 N atoms and 6 H atoms are present before and after, but they rearrange so N-H bonds form in NH₃. Choice A incorrectly claims some nitrogen atoms disappear and new hydrogen atoms are created, violating the Law of Conservation of Matter—atoms cannot be created or destroyed in chemical reactions. Choice B incorrectly states that atoms stay bonded the same way and molecules just stick together, when actually bonds must break (N≡N and H-H) and new bonds must form (N-H) to create ammonia. Choice C incorrectly claims nitrogen atoms change into hydrogen atoms, which is impossible in chemical reactions—atoms retain their identity (element type) and only change bonding partners. - 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?
Answer: The number of each type of atom (C, H, and O) stays the same before and after, even though the molecules change.
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.
- 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?
Answer: The same H atoms that were bonded to other H atoms become bonded to O atoms, forming H–O bonds in water.
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.
- A particle model shows methane burning: CH4+2O2→CO2+2H2O. Which statement correctly describes how atoms are rearranged?
Answer: Hydrogen atoms that were bonded to carbon in CH4 end up bonded to oxygen in H2O, and carbon ends up bonded to oxygen in CO2.
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 the combustion reaction CH₄ + 2O₂ → CO₂ + 2H₂O, the atoms present in the reactant molecules disconnect from their current bonding partners (C-H bonds in methane and O=O bonds in oxygen break) and reconnect with different atoms to form product molecules (new C=O bonds in carbon dioxide and H-O bonds in water form)—the key principle is that while the molecules change completely, the atoms themselves are conserved. In methane (CH₄), the carbon atom is bonded to four hydrogen atoms (C-H bonds). During the reaction, these C-H bonds break, and the carbon atom forms new bonds with oxygen atoms to create CO₂ (with C=O double bonds). Similarly, the hydrogen atoms that were bonded to carbon in CH₄ form new bonds with oxygen atoms to create H₂O molecules (H-O bonds). The oxygen atoms from O₂ molecules (originally in O=O bonds) end up bonded to both carbon (in CO₂) and hydrogen (in H₂O). Choice C is correct because it accurately describes how atoms are rearranged: hydrogen atoms that were bonded to carbon in CH₄ end up bonded to oxygen in H₂O, and carbon ends up bonded to oxygen in CO₂. Choice A incorrectly claims the carbon atom is destroyed and new oxygen atoms appear, violating conservation—the carbon atom from CH₄ is still present in CO₂. Choice B incorrectly states the same molecules are present before and after, when clearly CH₄ and O₂ are completely different molecules than CO₂ and H₂O. Choice D incorrectly claims oxygen atoms change into hydrogen atoms, which is impossible—atoms keep their identity (element type) in chemical reactions. Counting verification: Reactants have 1 C, 4 H, and 4 O atoms; Products have 1 C (in CO₂), 4 H (in 2H₂O), and 4 O atoms (2 in CO₂, 2 in 2H₂O)—perfect conservation. - A particle model shows: 2H2+O2→2H2O. Which quantity stays the same from reactants to products (showing conservation of matter)?
Answer: The number of each type of atom (H atoms and O atoms).
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.
- A particle model shows ammonia forming: N2+3H2→2NH3. Which claim is correct about conservation of matter in this reaction?
Answer: Matter is conserved because there are 2 N atoms and 6 H atoms before, and 2 N atoms and 6 H atoms after.
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.
- A particle model shows decomposition of water:
Reactants (before): 2 water molecules
- H–O–H H–O–H
Products (after): 2 hydrogen molecules and 1 oxygen molecule
- H–H H–H O=O
Which statement correctly describes what happens to the atoms during the reaction?
Answer: The same H and O atoms are present before and after, but they are rearranged into different molecules.
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 (each with 2 H and 1 O, so 4 H and 2 O total); after the reaction, the products are 2 H₂ molecules (4 H total) and 1 O₂ (2 O)—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 B is correct because it correctly states that atoms are conserved with equal counts before and after, just rearranged into different molecules. Choice A incorrectly claims some hydrogen atoms are destroyed because there are fewer H atoms in products, 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 (counting shows 4 H on both sides); Choice C wrongly suggests oxygen atoms change into hydrogen, but atoms stay the same identity, only bonding changes. To verify atom conservation in chemical reactions: (1) count atoms of each element in all reactant molecules (count all H atoms, all O 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.
- A particle model shows methane burning: CH4+2O2→CO2+2H2O. Which statement correctly compares the total atoms before and after the reaction?
Answer: The total number of atoms is the same before and after (1 C, 4 H, and 4 O).
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 carbon atom and 4 hydrogen atoms) and 2 O₂ molecules (containing 4 total oxygen atoms: 2 in each molecule), giving 9 total atoms. After the reaction, the products are 1 CO₂ molecule (containing 1 carbon atom and 2 oxygen atoms) and 2 H₂O molecules (each containing 2 hydrogen atoms and 1 oxygen atom), giving 1 carbon atom, 4 hydrogen atoms (2×2), and 4 oxygen atoms (2 + 2×1) for 9 total 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 C-H are now bonded C=O, hydrogen atoms that were bonded to C are now bonded to O, oxygen atoms that were bonded O=O are now bonded to C or H), but no atoms were created or destroyed. Choice C is correct because it correctly states the total number of atoms is the same before and after with 1 C, 4 H, and 4 O. Choice A incorrectly claims there are more total atoms after the reaction because new bonds formed, when actually the Law of Conservation of Matter states atoms cannot be created or destroyed in chemical reactions—bond formation doesn't create new atoms, it just connects existing atoms 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.
- 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?
Answer: H–O bonds break, and H–H and O=O bonds form.
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.
- A particle model shows methane burning:
Reactants (before): 1 methane molecule and 2 oxygen molecules
- CH4 (a C atom bonded to 4 H atoms)
- O=O O=O
Products (after): 1 carbon dioxide molecule and 2 water molecules
- O=C=O
- H–O–H H–O–H
How many oxygen atoms are present before and after the reaction?
Answer: Before: 4 O atoms; After: 4 O atoms
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 (each with 2 O, so 4 O total), giving 4 O atoms overall; after the reaction, the products are 1 CO₂ (1 C and 2 O) and 2 H₂O (each with 2 H and 1 O, so 4 H and 2 O), giving 4 O atoms total (2 from CO₂ + 2 from H₂O)—comparing the counts: 4 oxygen before equals 4 oxygen after—this demonstrates that atoms are conserved; during the reaction, bonds broke (C-H in CH₄, O=O in O₂) and new bonds formed (C=O in CO₂, H-O in H₂O), 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 properly verifies conservation by counting atoms of oxygen, showing equal numbers (4 O) before and after. Choice A makes a counting error, stating 2 O before but 4 O after, when carefully counting the atoms in the model shows 4 O before (2 O₂ molecules × 2 O each) and 4 O after (2 in CO₂ + 1 each in 2 H₂O); Choice C incorrectly claims 4 O before but 3 O after, which is a miscount since products have 4 O total. 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 O in reactants, must be 4 O 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.
