Phases of Matter — Problem set, Grade 8

Explain solids, liquids, gases, and changes of state using particle motion and energy transfer.

Review the key ideas

States of Matter & Phase Changes

Solid, liquid, and gas differ in particle spacing and motion; during a phase change, added or removed energy changes the state, not the temperature.

Temperature & Thermal Energy

Temperature describes an average; the total thermal energy also depends on how much material there is.

Gas Particles, Diffusion & Pressure

The space between gas particles decreases; the individual molecules retain their sizes in this model. 80×60=4,800 and 120×40=4,800 kPa·mL.

Evaporation, Condensation & a Wet Cold Cup

Existing water vapor from the air can condense on the cold surface; identify its source and energy transfer. The droplets can be condensed water from surrounding air; the tray loss can be water evaporating into that air.

Heating Curves & Energy During Phase Change

Continued input can change phase fraction and particle interactions while temperature remains nearly steady. 400 J enters during the four-minute interval.

Problem 1

How can water disappear from an open dish without reaching 100°C?
Answer and explanation

Surface molecules can escape by evaporation below boiling temperature. They enter the surrounding air as water vapor.

Surface molecules can escape by evaporation below boiling temperature. They enter the surrounding air as water vapor.

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

What changes without changing particle identity?
Answer and explanation

Spacing, arrangement and allowed motion can change. In the gas compression model, volume changes while particle number and temperature are held fixed.

Spacing, arrangement and allowed motion can change. In the gas compression model, volume changes while particle number and temperature are held fixed.

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

Visual-model chapter review: A labeled pure-water sample has a nearly constant temperature during melting while energy still enters. What changes at particle and whole-sample levels, and which identity stays the same?
Answer and explanation

More of the sample becomes liquid and particle arrangement/freedom of movement changes. Water molecules remain water; a plateau does not mean zero energy input or motionless particles.

Interpret the plateau with the stated phase-change conditions.

Track substance identity separately from arrangement and phase fraction.

More of the sample becomes liquid and particle arrangement/freedom of movement changes. Water molecules remain water; a plateau does not mean zero energy input or motionless particles.

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 4

What changes when a warm cup is wrapped in insulation?
Answer and explanation

Insulation reduces the rate of energy transfer to cooler surroundings. It does not necessarily change the starting temperature or create thermal energy.

Insulation reduces the rate of energy transfer to cooler surroundings. It does not necessarily change the starting temperature or create thermal energy.

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

Does four times the mass imply four times the temperature?
Answer and explanation

No. Temperature is not total energy or amount. Both are 25 °C under the supplied conditions.

No. Temperature is not total energy or amount. Both are 25 °C under the supplied conditions.

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

Design separate mass and material comparisons using the table. Explain the two trends and specify how a real investigation would verify matched energy input and limit environmental transfers.
Hints

Use the illustrated study titled Compare heating by mass and material. Keep the observations, model and conditions separate.

Answer and explanation

50 versus 100 g water isolates mass in the supplied model and gives rises 4 versus 2 °C. The 50 g water versus 50 g model solid comparison isolates material and gives rises 4 versus 8 °C. Equal input does not impose equal temperature change. Verify input method, starting state, container and surroundings, repeat readings, and label the supplied values rather than claim actual collection.

50 versus 100 g water isolates mass in the supplied model and gives rises 4 versus 2 °C. The 50 g water versus 50 g model solid comparison isolates material and gives rises 4 versus 8 °C. Equal input does not impose equal temperature change. Verify input method, starting state, container and surroundings, repeat readings, and label the supplied values rather than claim actual collection.

Response criteria

  • Choose separately controlled pairs.
  • Explain amount/material response without equating heat with temperature.
  • Specify input, tools, records and uncertainties.

Problem 7

Visual-model chapter review: Equal masses of the same substance start at 70 and 30 °C and reach equilibrium in the ideal isolated same-phase comparison. Predict the final temperature and explain why changing one mass would change the calculation.
Answer and explanation

50 °C in the stated equal-mass model. A different mass changes the energy amounts associated with each temperature change, so the simple midpoint would not generally apply.

Use equal amounts and the same substance/phase assumptions.

Do not add temperatures or apply the midpoint to unmatched masses.

50 °C in the stated equal-mass model. A different mass changes the energy amounts associated with each temperature change, so the simple midpoint would not generally apply.

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 8

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 9

What changes without changing particle identity?
Answer and explanation

Spacing, arrangement and allowed motion can change. In the gas compression model, volume changes while particle number and temperature are held fixed.

Spacing, arrangement and allowed motion can change. In the gas compression model, volume changes while particle number and temperature are held fixed.

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

Visual-model chapter review: 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.

Problem 11

Does a drying cloth need to reach 100 °C?
Answer and explanation

No. Surface evaporation occurs below boiling. Water transfers from the cloth into the air, rather than vanish.

No. Surface evaporation occurs below boiling. Water transfers from the cloth into the air, rather than vanish.

Response criteria

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

Problem 12

What energy pathway should a fuller model add?
Answer and explanation

Energy transfers between the warmer air/vapor and cool surface as conditions favor condensation. Water and energy require distinct accounts.

Energy transfers between the warmer air/vapor and cool surface as conditions favor condensation. Water and energy require distinct accounts.

Response criteria

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

Problem 13

Visual-model chapter review: An uncovered sample loses 2 g and a covered sample 0.3 g over the same interval. Explain the supported matter-loss comparison and why these balance readings alone cannot establish the whole energy account.
Answer and explanation

The uncovered sample loses 1.7 g more. Escaping matter can carry energy, but temperature, evaporation energy and other transfer paths need additional evidence for an energy balance.

Compare mass changes for the same boundary and interval.

Keep the matter measurement distinct from an energy measurement.

The uncovered sample loses 1.7 g more. Escaping matter can carry energy, but temperature, evaporation energy and other transfer paths need additional evidence for an energy balance.

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 14

Were ice particles motionless at −10 °C?
Answer and explanation

No. They vibrate in the solid. Warming changes their motion; melting later changes the larger arrangement and freedom to move.

No. They vibrate in the solid. Warming changes their motion; melting later changes the larger arrangement and freedom to move.

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

What changes during the level interval?
Answer and explanation

Liquid fraction increases while energy continues to enter; average temperature remains nearly steady under the model assumptions.

Liquid fraction increases while energy continues to enter; average temperature remains nearly steady under the model assumptions.

Response criteria

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

Problem 16

Visual-model chapter review: A supplied heating curve is level near 12 °C while the sample’s liquid fraction increases. Can you call the material pure water at ordinary pressure solely from the plateau? Explain what the record supports.
Answer and explanation

No. A phase-change interpretation is supported by the growing liquid fraction, but 12 °C is not ordinary-pressure pure water’s melting point. Composition, pressure and other properties need to be checked.

Use both temperature and phase evidence.

Do not treat every plateau as water melting at a universal temperature.

No. A phase-change interpretation is supported by the growing liquid fraction, but 12 °C is not ordinary-pressure pure water’s melting point. Composition, pressure and other properties need to be checked.

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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