Phases of Matter — Chapter Practice, Grade 8

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

Problem 1

In which state do particles vibrate about relatively fixed positions in the introductory particle model?
  1. Gas
  2. Liquid
  3. Solid
  4. Plasma
Answer and explanation

Solid

Solids have tightly packed particles that vibrate in place rather than moving around freely.

Problem 2

While a pure liquid boils at fixed pressure, what happens to its temperature during the phase change?
  1. It stays the same until all the liquid has boiled away
  2. It rises steadily
  3. It drops suddenly
  4. It becomes negative
Answer and explanation

It stays the same until all the liquid has boiled away

At fixed pressure, added energy changes particle interactions during boiling rather than raising temperature while liquid and vapor coexist.

Problem 3

Which phase change describes water vapor turning into liquid water on a cold glass?
  1. Melting
  2. Evaporation
  3. Condensation
  4. Sublimation
Answer and explanation

Condensation

Gas turning into liquid is condensation.

Problem 4

Explain the difference between evaporation and boiling.
Answer and explanation

Evaporation happens slowly at the surface of a liquid at almost any temperature, while boiling happens throughout the entire liquid at one specific temperature, the boiling point.

Problem 5

A heating curve shows a solid's temperature rising, then flattening out at 0°C while it melts, then rising again as a liquid. What temperature does that flat section occur at?
Answer and explanation

0 °C

The flat section on a heating curve occurs at the melting point, given here as 0°C.

Problem 6

Dry ice (solid carbon dioxide) turning directly into a gas without becoming a liquid is an example of…
  1. melting
  2. sublimation
  3. condensation
  4. freezing
Answer and explanation

sublimation

Sublimation is the direct change from solid to gas.

Problem 7

Ice and liquid water coexist at about 0°C at normal atmospheric pressure while heat is added. Why does the temperature stay nearly constant? Explain the reasoning in your own words.
Answer and explanation

Energy is used in melting while both phases remain; temperature rises after the ice has melted if heating continues.

Identify melting: the substance is changing from solid to liquid.

Added energy changes particle interactions instead of mainly increasing average kinetic energy.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

A puddle disappears at 18°C. Explain why this does not require boiling. Show the reasoning, not just the conclusion.
Hints

The particles remain the same chemical substance through the change.

Answer and explanation

Evaporation occurs at the surface below boiling temperature as some particles escape. The liquid need not reach its boiling point.

Problem 9

Ice melts in a sealed container. Must its molecules split into hydrogen and oxygen?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Melting changes arrangement and interactions; water molecules retain their chemical identity.

Problem 10

Evaporation and boiling can both make water vaporEvaporation and boiling can both make water vaporEvaporation: surfaceBoiling: vapor bubbles in bulkBoth retain water identity. Visible fog above a cup is mainly liquid droplets.Schematic selected molecules, not literal bubble size or an experiment to perform.
Evaporation selects surface particles escaping from liquid and can occur below boiling. In the specified boiling-water model, vapor bubbles form in the liquid’s bulk. Both are phase changes retaining water identity. Bubble size and selected particles are schematic, and neither drawing says visible white fog is invisible gas molecules or that all bubbling processes are boiling.
For this unit’s illustrated case, which feature distinguishes these two modeled phase changes, and why would a bubble from an unknown mixture need other evidence?
Answer and explanation

Surface-only escape versus vapor-bubble formation within the liquid distinguishes these cases. Unknown bubbles could arise from other processes, so gas identity and conditions must be established.

Bulk means the material away from only its outer surface. Water vapor is gaseous water and is invisible; visible mist consists mainly of small liquid droplets. The model separates these ideas without asking the learner to boil water.

Surface-only escape versus vapor-bubble formation within the liquid distinguishes these cases. Unknown bubbles could arise from other processes, so gas identity and conditions must be established.

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

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 12

Heat transfers overall from…
  1. colder to warmer
  2. warmer to colder
  3. smaller to larger, always
  4. lighter to heavier, always
Answer and explanation

warmer to colder

Temperature difference sets the net direction.

Problem 13

Why does temperature alone not tell the total thermal energy of a sample?
Answer and explanation

The amount and type of matter also matter.

Problem 14

Thermal equilibrium means interacting objects have the same…
  1. mass
  2. volume
  3. temperature
  4. number of particles
Answer and explanation

temperature

At equal temperature there is no net heat transfer between them.

Problem 15

Two equal-mass water samples at 20°C and 60°C mix in an insulated container. Estimate the final temperature. Explain the reasoning in your own words.
Answer and explanation

About 40°C, assuming no heat loss, evaporation or container energy change.

Equal masses of the same material exchange equal amounts of energy in this ideal model.

The final temperature lies halfway between: (20+60)/2=40°C.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 16

Equal masses of water at 10°C and 50°C mix in an ideal insulated container. Estimate the final temperature and name an assumption. Show the reasoning, not just the conclusion.
Hints

Keep temperature, material and temperature rise distinct from mass.

Answer and explanation

30°C, the midpoint for equal masses of the same material, assuming no energy lost or absorbed by the container and no phase change.

Problem 17

A metal spoon and room air begin at different temperatures. What determines the direction of heat transfer?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Energy transfers from warmer to cooler regions because of their temperature difference, until equilibrium in an isolated idealization.

Problem 18

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 19

Two supplied water samples at 25 °C have masses 100 g and 400 g. A second pair has equal mass but temperatures 25 and 40 °C. Explain what temperature compares and why equal temperature alone does not imply equal internal energy. Predict the direction of net thermal transfer for the second pair in contact.
Answer and explanation

For the same substance, temperature is related to average particle motion, while internal energy also depends on amount and interactions. The larger 25 °C sample can have greater internal energy despite equal temperature. Net thermal transfer is from the 40 °C sample toward the 25 °C sample under contact conditions, not a flow of a material called cold.

For the same substance, temperature is related to average particle motion, while internal energy also depends on amount and interactions. The larger 25 °C sample can have greater internal energy despite equal temperature. Net thermal transfer is from the 40 °C sample toward the 25 °C sample under contact conditions, not a flow of a material called cold.

Response criteria

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

Problem 20

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 21

Net heat transfer follows a temperature differenceNet heat transfer follows a temperature differenceWarmer object80 °CCooler object20 °CNet energy transferFor equal masses of the same substance in an ideal isolated comparison:equilibrium is 50 °C, not 100 °C; energy transfers are not temperature addition.
The model compares equal masses of the same substance initially at 80 and 20 °C, with no phase changes or net exchange with other objects. Net energy transfer is from warmer to cooler, reaching 50 °C under the stated ideal assumptions. The red arrow shows energy transfer, not flow of a substance called heat or cold. Different masses or substances would require a different final-temperature account.
For this unit’s illustrated case, would simply adding the initial temperatures give the equilibrium temperature? Explain what is held comparable here.
Answer and explanation

No. For this ideal equal-mass, same-substance case the result is the midpoint 50 °C, not 100 °C. Amount, substance, phase and outside transfers affect the prediction.

Heat names energy transferred because of a temperature difference. Thermal energy names an energy contribution within an account. When temperatures match, the simple model has no net heat transfer between these two objects, though microscopic exchanges can still occur.

No. For this ideal equal-mass, same-substance case the result is the midpoint 50 °C, not 100 °C. Amount, substance, phase and outside transfers affect the prediction.

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 22

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 23

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 24

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 25

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 26

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 27

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 28

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 29

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 30

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 31

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 32

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 33

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 34

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 35

Why compare with a room-temperature cup?
Answer and explanation

It helps test the role of cooling while keeping the cup construction similar. Clear outside water on the cold cup supports an air-water source rather than a colored leak.

It helps test the role of cooling while keeping the cup construction similar. Clear outside water on the cold cup supports an air-water source rather than a colored leak.

Response criteria

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

Problem 36

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 37

Which supplied cup is more likely to condense water?
Answer and explanation

The 8 °C surface is below the supplied 12 °C dew point; the 18 °C surface is above it. The prediction depends on those air conditions.

The 8 °C surface is below the supplied 12 °C dew point; the 18 °C surface is above it. The prediction depends on those air 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 38

Why is greater loss from a sunny wide tray not proof of an area effect alone?
Answer and explanation

Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Response criteria

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

Problem 39

Illustrative records show a cold intact cup with clear outside droplets. An open tray loses 6 g while a covered matched tray loses 1 g in an hour. Explain both observations and identify a limit of the mass comparison.
Answer and explanation

The droplets can be condensed water from surrounding air; the tray loss can be water evaporating into that air. The open tray loses 5 g more under the stated comparison. Covering changes exchange and can also change temperature or airflow, so the result does not isolate every mechanism. Neither observation requires water creation or destruction.

The droplets can be condensed water from surrounding air; the tray loss can be water evaporating into that air. The open tray loses 5 g more under the stated comparison. Covering changes exchange and can also change temperature or airflow, so the result does not isolate every mechanism. Neither observation requires water creation or destruction.

Response criteria

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

Problem 40

A white cloud above warm water is called visible water vapor. Correct the statement.
Answer and explanation

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

Response criteria

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

Problem 41

Evaluate and correct: Cold cups create new water on their outside.
Answer and explanation

Existing water vapor from the air can condense on the cold surface; identify its source and energy transfer.

Existing water vapor from the air can condense on the cold surface; identify its source and energy transfer.

Response criteria

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

Problem 42

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

Evaporation can cool the remaining liquid because escaping particles remove energy. Water and energy accounts therefore need separate arrows: a water arrow indicates where matter moves; an energy arrow shows a transfer, not a stream of a material called heat. Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Evaporation can cool the remaining liquid because escaping particles remove energy. Water and energy accounts therefore need separate arrows: a water arrow indicates where matter moves; an energy arrow shows a transfer, not a stream of a material called heat. Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Response criteria

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

Problem 43

Air dew point is supplied as 15 °C. An intact cup surface is 18 °C and another is 9 °C. Predict the condensation comparison and account for water origin.
Hints

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

Answer and explanation

The 9 °C surface is below the dew point and more likely to collect droplets; the 18 °C surface is above it. Existing water vapor from air can supply the condensate. The prediction uses the stated air conditions.

The 9 °C surface is below the dew point and more likely to collect droplets; the 18 °C surface is above it. Existing water vapor from air can supply the condensate. The prediction uses the stated air conditions.

Problem 44

A white cloud above warm water is called visible water vapor. Correct the statement. Explain a condition that would change this conclusion.
Hints

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

Answer and explanation

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

The gas water vapor itself is normally invisible. The visible white cloud mainly consists of small condensed liquid droplets, and its appearance alone does not identify every particle present.

Problem 45

Evaporation occurs at the liquid surfaceEvaporation occurs at the liquid surfaceLiquid: many interacting particlesSelected surface particles escapeBelow boiling is possibleRemaining average motion may fallParticle motion varies; the drawing does not give every particle one speed.
Selected particles leave the surface while the remaining liquid still contains interacting particles. Evaporation can occur below the boiling temperature and often removes particles with above-average kinetic energy, lowering the remaining average. This is a schematic selected-particle view, not a universal rule that every particle moves upward or that molecules break into new atoms.
For this unit’s illustrated case, why can a puddle dry at room temperature without the whole puddle boiling?
Answer and explanation

Some surface particles can escape into the gas phase under those conditions. Boiling is a different bulk process involving vapor bubbles throughout the liquid.

A surface is the boundary between liquid and its surroundings. Molecules in a liquid have a range of motions; an escaping molecule retains its chemical identity. Matter leaving also carries energy, linking evaporation to cooling.

Some surface particles can escape into the gas phase under those conditions. Boiling is a different bulk process involving vapor bubbles throughout the liquid.

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 46

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 47

What does the graph directly record?
Answer and explanation

Temperature at recorded times. A constant-rate assumption connects time with energy input; the graph does not directly measure every energy pathway.

Temperature at recorded times. A constant-rate assumption connects time with energy input; the graph does not directly measure every energy pathway.

Response criteria

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

Problem 48

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 49

What evidence would show change despite constant temperature?
Answer and explanation

A decreasing solid fraction and increasing liquid fraction while energy is supplied. Temperature alone is insufficient to describe the complete state.

A decreasing solid fraction and increasing liquid fraction while energy is supplied. Temperature alone is insufficient to describe the complete state.

Response criteria

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

Problem 50

Can a temperature of 0 °C alone tell us the fraction of liquid water?
Answer and explanation

No. Pure ice, a solid–liquid mixture or liquid at that temperature may be possible under the stated conditions; additional state evidence is needed.

No. Pure ice, a solid–liquid mixture or liquid at that temperature may be possible under the stated conditions; additional state evidence is needed.

Response criteria

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

Problem 51

Illustrative ice–water data remain near 0 °C for four minutes while a heater supplies 100 J each minute. Account for the 400 J input during the plateau and state an assumption.
Answer and explanation

400 J enters during the four-minute interval. In the simplified model it supports melting and changed particle interactions while the temperature stays approximately constant. It is not missing energy. Some real input can also reach the container or surroundings, so attributing all 400 J to the sample requires an idealized boundary and negligible losses.

400 J enters during the four-minute interval. In the simplified model it supports melting and changed particle interactions while the temperature stays approximately constant. It is not missing energy. Some real input can also reach the container or surroundings, so attributing all 400 J to the sample requires an idealized boundary and negligible losses.

Response criteria

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

Problem 52

Two equal masses reach the same temperature, one partly melted and one fully liquid. Must their energy histories be identical?
Answer and explanation

No. Temperature is not a complete measure of internal energy or phase fraction. Different arrangements and energy transfers can accompany the same temperature.

No. Temperature is not a complete measure of internal energy or phase fraction. Different arrangements and energy transfers can accompany the same 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 53

Evaluate and correct: A flat heating curve means the heater adds no energy.
Answer and explanation

Continued input can change phase fraction and particle interactions while temperature remains nearly steady.

Continued input can change phase fraction and particle interactions while temperature remains nearly steady.

Response criteria

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

Problem 54

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

Use 0 °C for pure water melting in this simplified ordinary-pressure model, not every substance. A plateau at another temperature could belong to another material. No visible plateau in a brief household observation does not by itself invalidate the particle model. No. Pure ice, a solid–liquid mixture or liquid at that temperature may be possible under the stated conditions; additional state evidence is needed.

Use 0 °C for pure water melting in this simplified ordinary-pressure model, not every substance. A plateau at another temperature could belong to another material. No visible plateau in a brief household observation does not by itself invalidate the particle model. No. Pure ice, a solid–liquid mixture or liquid at that temperature may be possible under the stated conditions; additional state evidence is needed.

Response criteria

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

Problem 55

A model heater adds 160 J/min for 3 min during a melting plateau. Find input and explain why no temperature rise does not mean no energy transfer.
Hints

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

Answer and explanation

Input is 480 J. Energy can change phase fraction and particle interactions during the plateau. In a real apparatus some energy can warm the container or leave to surroundings, so an ideal sample-only account needs stated assumptions.

Input is 480 J. Energy can change phase fraction and particle interactions during the plateau. In a real apparatus some energy can warm the container or leave to surroundings, so an ideal sample-only account needs stated assumptions.

Problem 56

Two equal masses reach the same temperature, one partly melted and one fully liquid. Must their energy histories be identical? Explain a condition that would change this conclusion.
Hints

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

Answer and explanation

No. Temperature is not a complete measure of internal energy or phase fraction. Different arrangements and energy transfers can accompany the same temperature.

No. Temperature is not a complete measure of internal energy or phase fraction. Different arrangements and energy transfers can accompany the same temperature.

Problem 57

Within one phase, energy input can increase average motionWithin one phase, energy input can increase average motionCooler sampleWarmer same-phase sampleSame substance and particle count; longer arrows show faster average motion.Particles do not become larger when this sample warms.
Both panels represent the same number and kind of particles in one phase. The warmer panel uses longer motion arrows to show a higher average kinetic energy in this introductory model. The arrows are not forces and the circles do not expand. Actual particle motions vary; equal arrow styles are a drawing simplification rather than a claim that all particles share one speed.
For this unit’s illustrated case, what should change and what should stay the same in a particle drawing of this same-phase warming?
Answer and explanation

Motion patterns/average kinetic energy can change, while particle identity, count and size stay the same. This comparison is not a phase-change plateau.

Within a phase, a rising temperature can track increasing average particle kinetic energy. During a phase change, energy can change the arrangement instead; that requires a different model and a different reading of the curve.

Motion patterns/average kinetic energy can change, while particle identity, count and size stay the same. This comparison is not a phase-change plateau.

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 58

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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