Heating Curves & Phase-Change Energy — Practice · Grade 11 Science

Chapter: States of Matter & Gases

What this practice covers

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

8 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: latent heat, plateau.

Problem 1

Melt 5 g ice at 0°C using Lf=334 J/g.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

1670 J

mL=5×334.

Problem 2

Why is q=mcΔT insufficient across a melting plateau?
Answer and explanation

It includes sensible heating but omits latent heat needed to change phase.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 3

Freeze 3 g water at its freezing point using Lf=334 J/g. Find heat released as a positive magnitude.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

1002 J

3×334=1002; sample heat would be negative.

Problem 4

Does boiling break water into H and O atoms?
Answer and explanation

No. Ordinary boiling primarily overcomes intermolecular attractions; the molecules remain H₂O.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 5

Find heat for 20 g material, c=2 J/(g·°C), warmed by 15°C. Show the reasoning, not just the conclusion.
Hints

Do not apply mcΔT alone to a phase-change plateau.

Answer and explanation

Q=20×2×15=600 J.

Problem 6

A sample melts while heating continues. What must a full energy calculation include?
Hints

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

Answer and explanation

Phase-change energy mL plus any separate warming before or after, with each segment counted once.

Problem 7

Latent energy changes phase fraction at the stated transitionLatent energy changes phase fraction at the stated transitionqphase = mL10.0 g ice × 334 J/g = 3340 JTemperature can remain near 0°C while melting.L depends on the substance, transition and pressure.
Original rounded water fusion example; it assumes the starting ice is already at the transition temperature and does not include container or environmental losses.
Interpret the supplied visual. Which sign describes heat for the sample during freezing if melting the same amount requires +3340 J?
Answer and explanation

Approximately −3340 J for the reverse transition under the corresponding conditions.

A specific latent energy L gives energy per mass for a particular phase change under specified conditions. Near ordinary-pressure melting, water’s fusion value is about 334 J/g. Melting ten grams of ice at its transition therefore needs about 3,340 J even while temperature is nearly constant. Vaporization has a different value, and reverse transitions have the opposite system heat sign. The energy concerns the full phase arrangement, not ordinary cleavage of every O–H covalent bond in the molecules.

Approximately −3340 J for the reverse transition under the corresponding conditions.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

For 20.0 g ice already at 0°C, calculate energy to melt it and warm the liquid to 10.0°C. Use Lf=334 J/g and c=4.18 J/(g·°C), neglecting apparatus/losses.
Answer and explanation

Melting: 20.0×334=6680 J. Warming: 20.0×4.18×10.0=836 J. Total=7516 J≈7.52 kJ. The sample receives positive heat under the stated boundary.

Melting: 20.0×334=6680 J. Warming: 20.0×4.18×10.0=836 J. Total=7516 J≈7.52 kJ. The sample receives positive heat under the stated boundary.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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