Gas Particles, Diffusion & Pressure — Practice · Grade 8 Science

Chapter: Phases of Matter

What this practice covers

By the end of Gas Particles, Diffusion & Pressure you should be able to:

12 questions, written answer, each with a worked explanation.

Vocabulary used: diffusion, concentration, gas pressure, compression, absolute pressure, model, evidence, system boundary, assumption.

Problem 1

Does diffusion require every particle to move in the same direction?
Answer and explanation

No. Individual particles move in varying directions. The overall distribution changes because more particles start in one region than another.

No. Individual particles move in varying directions. The overall distribution changes because more particles start in one region than another.

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

Why does trapped air resist being squeezed?
Answer and explanation

The particles have less space and collide with the walls more frequently under comparable temperature conditions, increasing the pressure.

The particles have less space and collide with the walls more frequently under comparable temperature conditions, increasing the pressure.

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

What stayed fixed in this volume comparison?
Answer and explanation

Particle number and temperature are assumed fixed. Volume changes, while particle size does not. A leaking syringe would not meet the fixed-particle assumption.

Particle number and temperature are assumed fixed. Volume changes, while particle size does not. A leaking syringe would not meet the fixed-particle assumption.

Response criteria

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

Problem 4

Why not compare a warm large vessel with a cool small vessel to isolate temperature?
Answer and explanation

Both temperature and volume change. Either can influence pressure, so the difference would not isolate temperature.

Both temperature and volume change. Either can influence pressure, so the difference would not isolate temperature.

Response criteria

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

Problem 5

Illustrative constant-temperature records give 80 kPa at 60 mL and 120 kPa at 40 mL for one fixed gas sample. Explain the trend and test whether pressure × volume stays constant.
Answer and explanation

80×60=4,800 and 120×40=4,800 kPa·mL. The smaller volume gives greater pressure while the model product stays constant. The gas particles do not shrink; more frequent wall impacts explain the increased pressure. This evidence is conditional on fixed temperature and particle number.

80×60=4,800 and 120×40=4,800 kPa·mL. The smaller volume gives greater pressure while the model product stays constant. The gas particles do not shrink; more frequent wall impacts explain the increased pressure. This evidence is conditional on fixed temperature and particle number.

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

A leaky bag becomes smaller with no pressure rise. Does it disprove the fixed-particle model?
Answer and explanation

No. Escaping gas changes particle number, so the fixed-amount assumption no longer applies. Account for matter crossing the boundary before testing that prediction.

No. Escaping gas changes particle number, so the fixed-amount assumption no longer applies. Account for matter crossing the boundary before testing that prediction.

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

Evaluate and correct: Squeezing air makes the individual molecules smaller.
Answer and explanation

The space between gas particles decreases; the individual molecules retain their sizes in this model.

The space between gas particles decreases; the individual molecules retain their sizes in this model.

Response criteria

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

Problem 8

Choose one prediction from this unit. Explain a comparison that could test it, specifying observations, changed and matched conditions, and a result that would challenge it.
Answer and explanation

Do not heat closed household containers. Instead compare supplied records of the same rigid vessel at two temperatures. Keep the vessel size and particle number explicit, and ask whether a different-size vessel would be a fair temperature test. Both temperature and volume change. Either can influence pressure, so the difference would not isolate temperature.

Do not heat closed household containers. Instead compare supplied records of the same rigid vessel at two temperatures. Keep the vessel size and particle number explicit, and ask whether a different-size vessel would be a fair temperature test. Both temperature and volume change. Either can influence pressure, so the difference would not isolate temperature.

Response criteria

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

Problem 9

A fixed gas at matched temperature is modeled at 80 kPa in 30 mL. Predict pressure at 15 mL using the constant-product model and name an assumption.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

The product is 2,400 kPa·mL, so the predicted pressure is 160 kPa at 15 mL. Amount and temperature must remain fixed; the particles do not shrink.

The product is 2,400 kPa·mL, so the predicted pressure is 160 kPa at 15 mL. Amount and temperature must remain fixed; the particles do not shrink.

Problem 10

A leaky bag becomes smaller with no pressure rise. Does it disprove the fixed-particle model? Explain a condition that would change this conclusion.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

No. Escaping gas changes particle number, so the fixed-amount assumption no longer applies. Account for matter crossing the boundary before testing that prediction.

No. Escaping gas changes particle number, so the fixed-amount assumption no longer applies. Account for matter crossing the boundary before testing that prediction.

Problem 11

Diffusion mixes particles without a one-way conveyorDiffusion mixes particles without a one-way conveyorInitial: marked gas near left sideLater: marked gas more spread outMarked-particle count stays four. Individual motion remains in many directions.Diffusion: spreading by particle motion; no chemical reaction is implied.
Both closed-container snapshots contain twelve schematic background particles and four marked particles. The marked gas initially occupies a smaller region and is later more spread out. Particle identities and numbers remain fixed; individual motion and collisions, not a conveyor aimed toward empty space, produce spreading. Counts represent selected particles rather than an actual gas sample size.
For this unit’s illustrated case, does more even spreading mean particles were created, or that every particle moved only to the right?
Answer and explanation

Neither. The four marked particles remain four. Individual motion can occur in many directions while the overall distribution becomes more even.

Diffusion is the spread of particles through their motion. The snapshots show distributions, not each particle’s exact path. Red is a tracking label for this model, not a literal color or proof of a chemical change.

Neither. The four marked particles remain four. Individual motion can occur in many directions while the overall distribution becomes more even.

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 12

At fixed temperature and gas amount, the supplied ideal model gives pressure×volume = 180. Predict pressure at volume 3 and at volume 6, then explain why particles do not need to shrink.
Answer and explanation

60 and 30 in the given pressure units. Particle size is unchanged; different spacing and wall-collision patterns explain the pressure change under the stated model.

Divide the supplied constant by each volume.

Hold temperature and amount comparable before attributing the result to compression.

60 and 30 in the given pressure units. Particle size is unchanged; different spacing and wall-collision patterns explain the pressure change under the stated model.

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.

Browse all courses