Heating Curves & Energy During Phase Change — Practice · Grade 8 Science

Chapter: Phases of Matter

What this practice covers

By the end of Heating Curves & Energy During Phase Change you should be able to:

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

Vocabulary used: heating curve, phase, plateau, latent heat, coexistence, model, evidence, system boundary, assumption.

Problem 1

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 2

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 3

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 4

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 5

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 6

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 7

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 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

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 9

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 10

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 11

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 12

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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