Middle School Science Quiz: Molecules Vs Crystal Structures
20 questions · exam conditions
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Molecules Vs Crystal StructuresQuestion 1 of 20

Which substance is best described as an extended structure (continuous network) rather than a substance made of discrete molecules?

Oxygen gas (O2_2)
Water vapor (H2_2O)
Table salt (NaCl) crystal
Carbon dioxide gas (CO2_2)
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Middle School Science Quiz

Middle School Science Quiz: Molecules Vs Crystal Structures

Practice Molecules Vs Crystal Structures 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 Molecules Vs Crystal Structures, giving you a quick way to practice the rules, question types, and explanations that matter most for Middle School Science.

How to use this quiz

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

All questions

Question 1

Which substance is best described as an extended structure (continuous network) rather than a substance made of discrete molecules?

  1. Oxygen gas (O2_2)
  2. Water vapor (H2_2O)
  3. Table salt (NaCl) crystal (correct answer)
  4. Carbon dioxide gas (CO2_2)
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like O₂, H₂O, or CO₂), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Choice C is correct because it correctly categorizes the substance based on its structure type, identifying table salt (NaCl) crystal as an extended continuous network unlike the molecular gases. Choice A is incorrect because it treats oxygen gas as an extended structure, when it is actually made of discrete O₂ molecules. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 2

Which substance is best described as an extended crystal structure (a continuous repeating pattern with no separate molecules)?

  1. Oxygen gas, O2_2
  2. Carbon dioxide, CO2_2
  3. Table salt, NaCl (correct answer)
  4. Water vapor, H2_2O
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. For water vs salt: Water exists as individual H₂O molecules—you can see separate molecules in the liquid or gas, and each molecule has exactly 2 H atoms bonded to 1 O atom, forming discrete units that can move independently; Salt (NaCl), however, is an extended structure where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a continuous 3D pattern—there's no such thing as 'one NaCl molecule' because every sodium is surrounded by chlorides and every chloride by sodiums in a network extending throughout the crystal, so the formula NaCl just tells us the 1:1 ratio. Choice C is correct because it correctly categorizes table salt (NaCl) as an extended crystal structure based on its continuous ionic lattice. Choice A incorrectly claims oxygen gas (O₂), which is made of discrete O₂ molecules, is an extended structure. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 3

Which list contains only molecular substances (made of discrete molecules), not extended crystal structures?

  1. H2_2O, CO2_2, O2_2 (correct answer)
  2. NaCl, H2_2O, O2_2
  3. Diamond (C), CO2_2, NaCl
  4. NaCl, Cu (metal), SiO2_2 (quartz)
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. For water vs salt: Water exists as individual H₂O molecules—you can see separate molecules in the liquid or gas, and each molecule has exactly 2 H atoms bonded to 1 O atom, forming discrete units that can move independently; Salt (NaCl), however, is an extended structure where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a continuous 3D pattern—there's no such thing as 'one NaCl molecule' because every sodium is surrounded by chlorides and every chloride by sodiums in a network extending throughout the crystal, so the formula NaCl just tells us the 1:1 ratio; similarly, diamond (C) is an extended carbon network, while CO₂ and O₂ are molecular. Choice A is correct because it lists only molecular substances: H₂O, CO₂, O₂, all with discrete molecules. Choice D incorrectly includes extended structures like NaCl, Cu (metal lattice), and SiO₂ (quartz network) as if they were molecular. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 4

A diagram shows sugar as many separate groups of atoms (each group is one C12_{12}H22_{22}O11_{11} unit) with space between groups. Another diagram shows a metal as identical atoms arranged in a repeating pattern that continues across the whole solid.

Which choice correctly matches each substance to its structure type?

  1. Sugar: extended crystal lattice; Metal: separate molecules.
  2. Sugar: separate molecules; Metal: extended structure (continuous lattice). (correct answer)
  3. Sugar: separate molecules; Metal: separate molecules.
  4. Sugar: extended structure; Metal: extended structure, but both have countable molecules.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like C₁₂H₂₂O₁₁ for sugar), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula for metals often just lists the element since it's a network of identical atoms. Choice B is correct because it accurately identifies that molecules are discrete countable units while extended structures are continuous networks, correctly categorizing sugar as separate molecules and metal as an extended lattice. Choice A is incorrect because it reverses the structure types, calling the molecular substance (sugar) an extended lattice and the extended structure (metal) separate molecules. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 5

A model of solid table salt shows Na+^+ and Cl−^- ions arranged in a repeating pattern. There are no separate "NaCl molecules" drawn—just the pattern continuing.

Why can you not count "one NaCl molecule" inside the salt crystal the way you can count one H2_2O molecule in water vapor?

  1. Because the salt crystal is a continuous repeating network of ions, not separate NaCl molecules. (correct answer)
  2. Because water vapor is a crystal lattice, but salt is made of separate molecules.
  3. Because NaCl contains no atoms, only energy.
  4. Because H2_2O has no fixed formula, but NaCl does.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Choice A is correct because it properly explains the key difference: you cannot count individual NaCl molecules in salt because it is a continuous network, unlike discrete H₂O molecules in vapor. Choice B is incorrect because it reverses the structure types, calling water vapor a crystal lattice and salt separate molecules. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 6

Two students describe a substance:

Student 1: "It is made of identical groups of atoms, and each group stays together as a unit. There is space between the groups." Student 2: "It is made of atoms/ions bonded in a repeating pattern that continues throughout the solid, so there are no separate units."

Which student is describing a molecular substance, and which is describing an extended crystal structure?

  1. Student 1: extended crystal; Student 2: molecular substance.
  2. Student 1: molecular substance; Student 2: extended crystal structure. (correct answer)
  3. Both students are describing molecular substances.
  4. Both students are describing extended crystal structures.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Choice B is correct because it accurately identifies that molecules are discrete countable units while extended structures are continuous networks, correctly categorizing Student 1 as describing molecular (separate groups with space) and Student 2 as extended (continuous pattern with no separate units). Choice D is incorrect because it claims both students are describing extended crystal structures, when Student 1 clearly describes separate units typical of molecular substances. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 7

A student says: "NaCl is one molecule made of 1 sodium atom and 1 chlorine atom, just like H2_2O is one molecule."

Which statement best corrects the student using the idea of molecules vs. extended crystal structures?

  1. NaCl usually forms an extended crystal lattice of Na+^+ and Cl−^- ions; NaCl shows a ratio, not a single molecule like H2_2O. (correct answer)
  2. NaCl is made of separate NaCl molecules floating around with space between them, like water vapor.
  3. H2_2O is an extended lattice, but NaCl is made of small molecules.
  4. Both NaCl and H2_2O are always extended networks, so neither has individual molecules.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Choice A is correct because it properly explains the key difference: separated molecules vs bonded-throughout crystal, correctly categorizing NaCl as an extended ionic lattice with a ratio and H₂O as discrete molecules. Choice B is incorrect because it claims you can count individual molecules in salt when actually salt is a continuous network with no discrete molecules. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 8

Two particle models are shown.

Model 1: Several separate groups shaped like a "V", each group has 3 atoms labeled H–O–H, and the groups are not connected to each other. Model 2: A repeating checkerboard pattern of circles labeled Na+^+ and Cl−^- that continues in all directions, with each Na+^+ touching several Cl−^-.

Which statement correctly identifies the structure types shown in Model 1 and Model 2?

  1. Model 1 is an extended crystal network; Model 2 is made of separate molecules.
  2. Model 1 is made of separate molecules; Model 2 is an extended crystal structure (lattice). (correct answer)
  3. Both models show separate molecules that can be counted.
  4. Both models show continuous networks with no discrete units.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. For water vs salt: Water exists as individual H₂O molecules—you can see separate molecules in the liquid or gas, and each molecule has exactly 2 H atoms bonded to 1 O atom, forming discrete units that can move independently, as shown in Model 1 with separate H–O–H groups; Salt (NaCl), however, is an extended structure where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a continuous 3D pattern—there's no such thing as 'one NaCl molecule' because every sodium is surrounded by chlorides and every chloride by sodiums in a network extending throughout the crystal, so the formula NaCl just tells us the 1:1 ratio, as in Model 2 with the repeating checkerboard. Choice B is correct because it accurately identifies that Model 1 (water) is made of separate molecules while Model 2 (salt) is an extended crystal structure (lattice). Choice A incorrectly reverses the structure types, calling the molecular substance (Model 1) an extended crystal network and the extended structure (Model 2) made of separate molecules. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 9

Two models are shown.

Model A: Several separate ring-shaped groups labeled C, H, and O (each group represents one sugar molecule), and the groups do not connect to each other. Model B: A solid made of identical circles packed in a repeating pattern, each circle is labeled Cu, and the pattern continues across the whole piece.

Which statement correctly matches each model to "molecular substance" or "extended structure"?

  1. Model A is an extended structure; Model B is a molecular substance.
  2. Model A and Model B are both molecular substances because they contain atoms.
  3. Model A is a molecular substance (separate molecules); Model B is an extended structure (metal lattice). (correct answer)
  4. Model A is an ionic lattice; Model B is separate Cu molecules.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. For water vs salt: Water exists as individual H₂O molecules—you can see separate molecules in the liquid or gas, and each molecule has exactly 2 H atoms bonded to 1 O atom, forming discrete units that can move independently, similar to Model A with separate sugar molecules; Salt (NaCl), however, is an extended structure where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a continuous 3D pattern—there's no such thing as 'one NaCl molecule' because every sodium is surrounded by chlorides and every chloride by sodiums in a network extending throughout the crystal, so the formula NaCl just tells us the 1:1 ratio, while for metals like Cu in Model B, it's a lattice of copper atoms in a repeating pattern. Choice C is correct because it correctly categorizes Model A (sugar) as a molecular substance with separate molecules and Model B (Cu metal) as an extended structure (metal lattice). Choice D incorrectly describes Model A as an ionic lattice and Model B as separate Cu molecules, when Cu is a continuous metallic network and sugar is molecular. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 10

A sample of oxygen gas is made of O2_2 units moving around with space between them. A sample of diamond is made of carbon atoms bonded in a repeating network that extends throughout the solid.

What is the main structural difference between oxygen gas and diamond?

  1. Oxygen gas is a continuous network, but diamond is made of separate molecules.
  2. Oxygen gas is made of discrete molecules (O2_2); diamond is a continuous network with no individual molecules to count. (correct answer)
  3. Both oxygen gas and diamond are made of separate molecules that can be counted easily.
  4. Diamond is made of O2_2 molecules packed tightly, while oxygen gas is made of single carbon atoms.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like O₂ with 2 oxygen atoms), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like C for diamond) gives the ratio of atoms, not the composition of one separate molecule. Choice B is correct because it accurately identifies that molecules are discrete countable units while extended structures are continuous networks, correctly categorizing oxygen gas as discrete O₂ molecules and diamond as a continuous carbon network. Choice A is incorrect because it reverses the structure types, calling the molecular substance (oxygen gas) a continuous network and the extended structure (diamond) separate molecules. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 11

A model shows carbon dioxide as separate O=C=O units with space between them. Another model shows quartz (SiO2_2) as a connected network where each atom is bonded to several neighbors in a repeating pattern.

Which statement is correct?

  1. CO2_2 is an extended structure, while SiO2_2 (quartz) is made of separate molecules.
  2. Both CO2_2 and SiO2_2 are made of separate molecules that can be counted.
  3. CO2_2 is made of discrete molecules; SiO2_2 (quartz) is an extended network with no individual molecules. (correct answer)
  4. Neither CO2_2 nor SiO2_2 has atoms bonded together.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like CO₂ with 1 carbon and 2 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like SiO₂ for quartz) gives the ratio of atoms, not the composition of one separate molecule. Choice C is correct because it accurately identifies that molecules are discrete countable units while extended structures are continuous networks, correctly categorizing CO₂ as discrete molecules and SiO₂ as an extended network. Choice A is incorrect because it reverses the structure types, calling the extended structure (SiO₂) separate molecules and the molecular substance (CO₂) extended. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 12

A student breaks a crystal of table salt into two smaller pieces.

Which statement best describes what happens at the particle level, based on the idea of extended crystal structures vs. discrete molecules?

  1. Breaking salt separates pre-existing NaCl molecules from each other without breaking any bonds.
  2. Breaking salt cuts through a continuous ionic lattice, creating new surfaces; it does not separate pre-made NaCl molecules. (correct answer)
  3. Breaking salt turns the ions into separate H2_2O molecules.
  4. Breaking salt removes the empty space between NaCl molecules inside the crystal.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Choice B is correct because it properly explains the key difference: breaking salt cuts through a continuous ionic lattice since there are no pre-existing separate NaCl molecules. Choice A is incorrect because it describes the crystal as having separated molecules, when crystals are continuous with each atom bonded to neighbors throughout. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 13

A class compares two formulas:

  • H2_2O: the subscripts show the number of atoms in one molecule.
  • NaCl: the formula shows a 1:1 ratio of ions repeated throughout a crystal.

Which choice correctly uses these formulas to identify the structure types?

  1. H2_2O is an extended crystal structure, and NaCl is made of discrete molecules.
  2. Both H2_2O and NaCl formulas describe one molecule each.
  3. H2_2O is molecular (discrete molecules); NaCl is an extended crystal structure (ratio in a lattice). (correct answer)
  4. NaCl is molecular because it has two letters, while H2_2O is an extended structure because it has a subscript.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Choice C is correct because it accurately identifies that molecules are discrete countable units while extended structures are continuous networks, correctly categorizing H₂O as molecular (discrete with composition) and NaCl as extended (ratio in lattice). Choice B is incorrect because it treats the formula NaCl as if it represents one molecule, when it actually represents the ratio of ions in the extended crystal structure. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 14

Two models are shown:

Model 1: H–O–H H–O–H H–O–H (three separate water molecules with space between them)

Model 2: Na⁺ Cl⁻ Na⁺ Cl⁻ Cl⁻ Na⁺ Cl⁻ Na⁺ Na⁺ Cl⁻ Na⁺ Cl⁻ (a repeating pattern that continues)

Which statement correctly identifies the structure types in Model 1 and Model 2?

  1. Model 1 is an extended crystal structure; Model 2 is made of separate molecules.
  2. Both Model 1 and Model 2 are made of separate, countable molecules.
  3. Model 1 is made of separate H2_2O molecules; Model 2 is an extended crystal lattice of ions. (correct answer)
  4. Both Model 1 and Model 2 are extended structures with no separate units.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Choice C is correct because it accurately identifies that molecules are discrete countable units while extended structures are continuous networks, correctly categorizing Model 1 (water) as separate H₂O molecules and Model 2 (salt) as an extended ionic lattice. Choice A is incorrect because it reverses the structure types, calling the molecular substance (Model 1) an extended structure and the extended structure (Model 2) molecular. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 15

A sample of oxygen gas is made of many separate O2\mathrm{O_2} units moving around with space between them. A sample of table salt is a solid where Na+\mathrm{Na^+} and Cl−\mathrm{Cl^-} repeat in a pattern throughout the whole crystal. Which statement correctly compares their structure types?

  1. Oxygen gas is an extended network, and salt is made of separate molecules.
  2. Both oxygen gas and salt are made of separate, countable molecules.
  3. Oxygen gas is made of discrete molecules, while salt is an extended crystal structure with a repeating pattern. (correct answer)
  4. Oxygen gas cannot be counted as molecules, but salt can be counted as individual molecules.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like O₂ with 2 oxygen atoms), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. Oxygen gas consists of individual O₂ molecules—each molecule has exactly 2 oxygen atoms bonded together, and these molecules move around freely with space between them, which is why oxygen is a gas at room temperature. Table salt (NaCl), however, is an extended structure where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a continuous 3D pattern—there's no such thing as "one NaCl molecule" because every sodium is surrounded by chlorides and every chloride by sodiums in a network extending throughout the crystal. Choice C is correct because it accurately identifies that oxygen gas is made of discrete molecules while salt is an extended crystal structure with a repeating pattern. Choice A incorrectly reverses the structure types, calling oxygen gas an extended network when it's actually made of separate O₂ molecules, and claiming salt is made of separate molecules when it's actually a continuous ionic lattice.

Question 16

A substance has the formula SiO2\mathrm{SiO_2} and forms quartz (a common mineral in sand). In quartz, silicon and oxygen atoms are bonded in a continuous pattern throughout the solid (not as separate SiO2\mathrm{SiO_2} molecules). Which classification is correct?

  1. Molecular substance: separate SiO2\mathrm{SiO_2} molecules with space between them
  2. Extended crystal/network: atoms bonded in a repeating structure throughout the solid (correct answer)
  3. Gas made of SiO2\mathrm{SiO_2} molecules moving freely
  4. Mixture: separate silicon molecules and oxygen molecules not bonded to each other
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together, and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula gives the ratio of atoms, not the composition of one separate molecule. Quartz (SiO₂) is an extended structure where each silicon atom bonds to 4 oxygen atoms and each oxygen bonds to 2 silicon atoms in a continuous 3D network—the question explicitly states that silicon and oxygen atoms are "bonded in a continuous pattern throughout the solid (not as separate SiO₂ molecules)." This continuous bonding pattern with no discrete molecular units is the defining characteristic of an extended crystal/network structure, similar to diamond (carbon network) or salt (ionic lattice). Choice B is correct because it accurately identifies quartz as an extended crystal/network with atoms bonded in a repeating structure throughout the solid. Choice A incorrectly identifies quartz as a molecular substance with separate SiO₂ molecules with space between them, when the question explicitly states it's NOT made of separate SiO₂ molecules but rather a continuous bonding pattern.

Question 17

A solid sample has a pattern that repeats throughout the material, and each atom is bonded to several neighbors so there are no separate groups you can point to as "one molecule." Which type of structure best matches this description?

  1. Molecular substance with separate, countable molecules
  2. Mixture of different molecules with large spaces between them
  3. Extended crystal/network structure (correct answer)
  4. Single giant molecule only found in gases
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together, and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the key characteristic described in the question is that "each atom is bonded to several neighbors" and there are "no separate groups you can point to as one molecule." The description clearly indicates an extended crystal/network structure: the repeating pattern throughout the material and the fact that atoms are bonded to several neighbors with no discrete molecular units are defining features of extended structures like diamond (carbon network), salt (ionic lattice), or metals (metallic bonding network). Choice C is correct because it accurately identifies this as an extended crystal/network structure based on the continuous bonding pattern with no separate molecules. Choice A incorrectly identifies this as a molecular substance with separate, countable molecules, when the description explicitly states there are no separate groups you can point to as "one molecule"—this is the opposite of what defines a molecular substance.

Question 18

A student breaks a crystal of table salt into smaller pieces. Which statement best describes what happens at the particle level?

  1. The salt breaks between pre-existing NaCl\mathrm{NaCl} molecules, leaving each molecule unchanged.
  2. The salt breaks through a continuous ion lattice, creating new surfaces; it is not separating individual molecules. (correct answer)
  3. The salt turns into separate Na\mathrm{Na} and Cl\mathrm{Cl} atoms that are no longer charged.
  4. The salt becomes a gas of NaCl\mathrm{NaCl} molecules because crystals cannot be broken.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together, and these molecules are separated from each other by space—you can identify and count individual molecules because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count. Table salt (NaCl) is an extended ionic structure where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a continuous 3D lattice—when you break a salt crystal, you're breaking through this continuous network of ionic bonds, not separating pre-existing molecules. The breaking creates new surfaces where the ionic lattice has been cleaved, exposing ions that were previously in the interior of the crystal—you're not separating "NaCl molecules" because such discrete molecules don't exist in salt. Choice B is correct because it accurately describes that salt breaks through a continuous ion lattice, creating new surfaces, and explicitly states it is not separating individual molecules (which don't exist in salt). Choice A incorrectly claims that salt breaks between pre-existing NaCl molecules, when actually salt has no discrete molecules—it's a continuous ionic network, so breaking creates new surfaces through the lattice rather than separating molecules.

Question 19

Which substance is best described as an extended (crystal) structure rather than a set of separate molecules?​

  1. Oxygen gas (O2_2)
  2. Water vapor (H2_2O)
  3. Table salt (NaCl) (correct answer)
  4. Carbon dioxide gas (CO2_2)
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. For the listed substances: Gases like O₂, H₂O vapor, and CO₂ exist as individual molecules (O₂ with 2 O atoms, H₂O with 2 H and 1 O, CO₂ with 1 C and 2 O), separate from each other, allowing them to move freely; table salt (NaCl), however, is an extended structure with Na⁺ and Cl⁻ in a continuous network, no discrete molecules, which is why it's a solid crystal at room temperature. Choice C is correct because it accurately identifies that molecules are discrete countable units while extended structures are continuous networks. Choice A incorrectly claims you can count individual molecules in salt when actually salt is a continuous network with no discrete molecules. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.

Question 20

A sample of oxygen gas is made of many O2_2 units. Each unit has exactly 2 oxygen atoms bonded together, and the units are not bonded to each other. Is oxygen gas (O2_2) a molecular substance or an extended (crystal) structure?​

  1. Extended (crystal) structure, because it contains oxygen atoms.
  2. Extended (crystal) structure, because O2_2 repeats throughout the sample as one connected network.
  3. Molecular substance, because it is made of separate, countable O2_2 molecules. (correct answer)
  4. Neither, because gases cannot be made of molecules.
Explanation: This question tests understanding of the important distinction between molecular substances (made of discrete individual molecules) and extended structures like crystals (continuous networks of atoms or ions). Molecular substances are composed of separate individual molecules, where each molecule is a specific group of atoms bonded together (like H₂O with 2 hydrogen and 1 oxygen), and these molecules are separated from each other by space—you can identify and count individual molecules (one molecule, two molecules, three molecules) because they exist as discrete units. Extended structures like crystals, on the other hand, are continuous networks where atoms or ions are bonded to their neighbors in a repeating pattern that extends throughout the entire substance with no discrete molecules to count—the formula (like NaCl for table salt) gives the ratio of atoms or ions, not the composition of one separate molecule. For oxygen gas: Oxygen gas exists as individual O₂ molecules—you can identify separate molecules in the gas, and each molecule has exactly 2 O atoms bonded together, forming discrete units that can move independently; unlike extended structures like salt, there's no continuous network connecting all O₂ units, which is why oxygen is a gas at room temperature with molecules free to move. Choice C is correct because it properly explains the key difference: separated molecules vs bonded-throughout crystal. Choice B incorrectly reverses the structure types, calling the molecular substance an extended structure. To distinguish molecular from extended structures, ask these questions: (1) Can you identify and count individual molecules? (yes → molecular, no → extended), (2) Are there distinct separate groups of bonded atoms? (yes → molecular, no → extended), (3) Does the formula represent one molecule's composition or just a ratio? (composition → molecular, ratio → extended). Examples to remember: molecular substances include most gases (O₂, CO₂, N₂), many liquids (water H₂O, gasoline), and some solids (sugar, ice - though ice is crystalline arrangement of molecules); extended structures include table salt (NaCl network), diamond and graphite (carbon networks), most metals (metal atom networks), and sand/quartz (SiO₂ network)—the structure type affects properties like melting point (crystals often higher due to continuous bonding), state at room temperature, and how the substance dissolves or breaks apart.