Properties of Matter — Problem set, Grade 8

Use particle models, measurements, and material properties to explain and compare the matter around us.

Review the key ideas

Atomic Structure

Protons and neutrons occupy the nucleus; electrons are described by an electron cloud around it. Proton count identifies the element.

Physical vs. Chemical Properties

If a new substance with new properties forms, it's a chemical change; if the same substance just looks or feels different, it's a physical change.

Density & Floating

Density is mass per volume. Floating depends on the object’s average density compared with the fluid.

The Periodic Table

The periodic table organizes elements by atomic number; elements in the same group share valence electrons and similar properties.

Materials: Properties, Uses & Impacts

Choose materials using properties and evidence, and consider benefits and environmental trade-offs.

Problem 1

A particle has eight protons and ten electrons. Is it oxygen, and is it neutral?
Answer and explanation

It is oxygen because it has eight protons. It has two excess negative charges, so it is an ion with charge −2, not a neutral atom.

It is oxygen because it has eight protons. It has two excess negative charges, so it is an ion with charge −2, not a neutral atom.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 2

If only electrons rise to 10, what entries change?
Answer and explanation

The charge becomes −2 while atomic number 8, mass number 16 and oxygen identity remain unchanged.

The charge becomes −2 while atomic number 8, mass number 16 and oxygen identity remain unchanged.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 3

Draw two water molecules and a larger repeating salt patch. Compare total atom counts with a formula ratio, and explain why a two-ball salt molecule is an incomplete crystal model.
Hints

Use the illustrated study titled Molecules and extended structures. Keep the observations, model and conditions separate.

Answer and explanation

Two water molecules contain four H and two O atoms. A salt patch represents part of an extended arrangement; its bulk Na:Cl ratio is one-to-one rather than a claim that the crystal is a collection of separate NaCl molecules. Retain element/ion labels and note that colors, bonding lines and distances are conventions.

Two water molecules contain four H and two O atoms. A salt patch represents part of an extended arrangement; its bulk Na:Cl ratio is one-to-one rather than a claim that the crystal is a collection of separate NaCl molecules. Retain element/ion labels and note that colors, bonding lines and distances are conventions.

Response criteria

  • Distinguish molecule from extended arrangement.
  • Count water atoms correctly.
  • Explain formula ratio and a model limit.

Problem 4

Visual-model chapter review: An atom has 12 protons, 13 neutrons and 10 electrons. State atomic number, mass number and net charge, and explain whether losing electrons changed its element.
Answer and explanation

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Read proton count for atomic number.

Add 12+13 for mass number; compare 12 positive with 10 negative charges.

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 5

Why is reversibility alone an unreliable rule?
Answer and explanation

Some physical changes are hard to undo, such as breaking glass, and some chemical reactions can be reversed under appropriate conditions. Chemical identity is the deciding idea.

Some physical changes are hard to undo, such as breaking glass, and some chemical reactions can be reversed under appropriate conditions. Chemical identity is the deciding idea.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 6

Which case changes substance identity?
Answer and explanation

Rusting forms different substances; melting retains water identity. Appearance alone cannot establish the classification.

Rusting forms different substances; melting retains water identity. Appearance alone cannot establish the classification.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 7

Plan and, if safe materials are available, conduct the sand/salt comparison. Use the supplied recovery record to argue whether it is consistent with a physical mixture, and explain why a mass shortfall is not automatically a reaction.
Hints

Use the illustrated study titled Mixtures and physical separation. Keep the observations, model and conditions separate.

Answer and explanation

The separate-component tests and recovery of sand and salt are consistent with a physical combination. The illustrative recovered masses are 4.8+2.9=7.7 g versus 8.0 g originally added, leaving 0.3 g unaccounted for in that collection. Transfer residues, remaining dissolved salt or reading uncertainty can explain a shortfall. Better rinsing, complete collection and repeated matched tests can improve the account; the data do not establish a new compound.

The separate-component tests and recovery of sand and salt are consistent with a physical combination. The illustrative recovered masses are 4.8+2.9=7.7 g versus 8.0 g originally added, leaving 0.3 g unaccounted for in that collection. Transfer residues, remaining dissolved salt or reading uncertainty can explain a shortfall. Better rinsing, complete collection and repeated matched tests can improve the account; the data do not establish a new compound.

Response criteria

  • Specify components and a property-based method.
  • Use recovered identity and mass evidence.
  • Explain a limitation or controlled improvement.

Problem 8

Visual-model chapter review: Two colorless samples have the same density but melt at different temperatures under comparable conditions. Does matching density establish identical substance identity? Explain the role of a second property.
Answer and explanation

No. Their distinct melting behavior provides another characteristic-property comparison. One matching density is not unique identification.

Compare multiple characteristic properties under stated conditions.

Separate an observed property from an inference of identity.

No. Their distinct melting behavior provides another characteristic-property comparison. One matching density is not unique identification.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 9

What error comes from using 42 mL as the object volume?
Answer and explanation

It includes the original water volume. The object displaced only 12 mL; using 42 mL would underestimate its density.

It includes the original water volume. The object displaced only 12 mL; using 42 mL would underestimate its density.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 10

Why does B not have twice A’s density?
Answer and explanation

Its mass and volume both double, so 48/16 equals 24/8, or 3 g/cm³.

Its mass and volume both double, so 48/16 equals 24/8, or 3 g/cm³.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 11

An illustrative unknown has mass 54 g and displacement 20 cm³. References A/C are both 2.7 g/cm³ and B is 7.9. Argue which candidates remain and plan one additional property comparison.
Hints

Use the illustrated study titled Density as identification evidence. Keep the observations, model and conditions separate.

Answer and explanation

Density is 54/20=2.7 g/cm³. A and C remain consistent while B is inconsistent under matched conditions and adequate measurement precision. Density alone cannot distinguish A from C; choose a safe characteristic-property comparison with supplied reference data, such as solubility for approved samples. Name uncertainty and whether the object is homogeneous.

Density is 54/20=2.7 g/cm³. A and C remain consistent while B is inconsistent under matched conditions and adequate measurement precision. Density alone cannot distinguish A from C; choose a safe characteristic-property comparison with supplied reference data, such as solubility for approved samples. Name uncertainty and whether the object is homogeneous.

Response criteria

  • Calculate with correct units.
  • Distinguish support from unique identification.
  • Choose relevant further evidence and controls.

Problem 12

Visual-model chapter review: A completely submerged object raises a cylinder reading from 35 to 47 mL and has mass 30 g. Calculate density, state the volume assumptions and distinguish this from claiming an exact substance identity.
Answer and explanation

Volume 12 cm³; density 30/12 = 2.5 g/cm³. Assume no trapped bubbles, spill or incomplete submersion. Density alone does not uniquely identify the substance.

Use the change 47−35, not the final reading.

Divide the independently measured mass by that volume.

Volume 12 cm³; density 30/12 = 2.5 g/cm³. Assume no trapped bubbles, spill or incomplete submersion. Density alone does not uniquely identify the substance.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 13

Does being in the same group make two elements identical?
Answer and explanation

No. It suggests related chemical behavior for many main-group elements, but the elements retain different proton counts and can differ strongly in other properties.

No. It suggests related chemical behavior for many main-group elements, but the elements retain different proton counts and can differ strongly in other properties.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 14

What fixes the order shown?
Answer and explanation

Atomic number, the proton count, orders Na 11, Mg 12 and Al 13. The display omits most elements and does not claim identical reactivity.

Atomic number, the proton count, orders Na 11, Mg 12 and Al 13. The display omits most elements and does not claim identical reactivity.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 15

Visual-model chapter review: A learner reads atomic number 17 and average atomic mass about 35.45 from a chlorine card and reports 18.45 neutrons in one atom. Correct the interpretation for a specified chlorine-37 atom.
Answer and explanation

Chlorine-37 has 17 protons and 37−17 = 20 neutrons. The decimal 35.45 is an isotope-average mass, not one atom’s particle count.

Use the specified isotope mass number, not the decimal average.

Subtract atomic number from mass number.

Chlorine-37 has 17 protons and 37−17 = 20 neutrons. The decimal 35.45 is an isotope-average mass, not one atom’s particle count.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 16

Why is natural versus synthetic insufficient for choosing a container?
Answer and explanation

Those labels do not supply evidence of leakage, durability, safety in intended use, or disposal impacts. The actual material properties and use conditions matter.

Those labels do not supply evidence of leakage, durability, safety in intended use, or disposal impacts. The actual material properties and use conditions matter.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 17

Which stage is measured by a one-minute leak test?
Answer and explanation

One use-performance feature. It does not measure extraction, manufacturing energy, lifetime reuse or disposal impacts.

One use-performance feature. It does not measure extraction, manufacturing energy, lifetime reuse or disposal impacts.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 18

Use the supplied two source records to explain one benefit, one concern and a needed follow-up for a film/container choice. Do not treat a resource label as a full safety or impact rating.
Hints

Use the illustrated study titled Natural resources and different plastics. Keep the observations, model and conditions separate.

Answer and explanation

The reuse record supports longer tested service for the rigid container, while the disposal record warns that acceptance depends on product and local processing. Resource origin alone does not establish total impacts. Compare the same task, include cleaning and replacement, and obtain specific production/disposal information. Describe chains and processing as a plausible property explanation rather than proof from the label plastic.

The reuse record supports longer tested service for the rigid container, while the disposal record warns that acceptance depends on product and local processing. Resource origin alone does not establish total impacts. Compare the same task, include cleaning and replacement, and obtain specific production/disposal information. Describe chains and processing as a plausible property explanation rather than proof from the label plastic.

Response criteria

  • Connect a synthetic material to a resource and processing.
  • Use two different source records.
  • Assess benefits, concerns and missing evidence fairly.

Problem 19

Visual-model chapter review: Cup A can be used 20 times and needs 8 material-units per cup; B is single-use and needs 2 units each time. Compare raw material for 20 uses, then name two omitted impacts before declaring an environmental winner.
Answer and explanation

A uses 8 initial material-units; B uses 40 for 20 cups. Cleaning energy/water, production, transport, disposal and actual lifetime are omitted, so initial material alone is not a complete impact ranking.

Compare the same amount of service.

Keep the measured raw-material comparison separate from the full life cycle.

A uses 8 initial material-units; B uses 40 for 20 cups. Cleaning energy/water, production, transport, disposal and actual lifetime are omitted, so initial material alone is not a complete impact ranking.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.
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