Kinetic Molecular Theory & Phase Changes — Practice · Grade 11 Science

Chapter: States of Matter & Gases

What this practice covers

By the end of Kinetic Molecular Theory & Phase Changes you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: kinetic molecular theory (KMT), kinetic energy, phase change, heating curve, heat of fusion, heat of vaporization, phase diagram, sublimation.

Problem 1

According to the kinetic molecular theory, the average kinetic energy of gas particles is directly proportional to the…
  1. number of particles
  2. Kelvin temperature
  3. pressure of the gas
  4. volume of the container
Answer and explanation

Kelvin temperature

A core assumption of KMT is that average kinetic energy is directly proportional to the absolute (Kelvin) temperature.

Problem 2

In the ideal-gas particle model, which state has particles far apart with intermolecular forces treated as negligible?
  1. solid
  2. liquid
  3. gas
  4. they are identical in all states
Answer and explanation

gas

In a gas, kinetic energy dominates: particles spread far apart and move quickly with almost no attraction between them.

Problem 3

Convert 27°C to kelvin.
Answer and explanation

300 K

K = °C + 273 = 27 + 273 = 300 K.

Problem 4

On a heating curve, what is happening during a flat (horizontal) plateau?
  1. The substance is cooling down
  2. The particles have stopped moving completely
  3. An error was made recording the data
  4. A phase change is occurring; added energy breaks particle attractions instead of raising the temperature
Answer and explanation

A phase change is occurring; added energy breaks particle attractions instead of raising the temperature

During a phase change the temperature holds steady because the incoming energy goes into breaking attractions (potential energy), not speeding particles up (kinetic energy).

Problem 5

Explain why the temperature of a substance stays constant while it is melting, even though energy is still being added.
Answer and explanation

The added energy is used to overcome the attractive forces holding the particles in the solid arrangement — it raises their potential energy, not their kinetic energy. Since temperature measures average kinetic energy, it stays constant until all of the solid has melted.

Problem 6

Which group lists ONLY exothermic phase changes (ones that release energy)?
  1. freezing, condensation, and deposition
  2. melting, boiling, and sublimation
  3. melting, freezing, and boiling
  4. sublimation, deposition, and condensation
Answer and explanation

freezing, condensation, and deposition

Freezing, condensation, and deposition all move to a more ordered, tightly held state and release energy; melting, vaporization, and sublimation absorb it.

Problem 7

Why is Kelvin temperature used in ideal-gas proportionalities? Show the reasoning, not just the conclusion.
Hints

Specify fixed volume and amount before applying the relation.

Answer and explanation

It is an absolute scale compatible with the kinetic-energy relation, unlike Celsius's arbitrary zero.

Problem 8

During melting at fixed pressure, must a pure substance's temperature keep rising?
Hints

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

Answer and explanation

No. Energy can drive the phase change while temperature stays nearly constant.

Problem 9

Absolute temperature connects with mean energy, not identical particle speedsAbsolute temperature connects with mean energy, not identical particle speedsAbsolute temperature (K)Sample A300Sample B600Mean translational kinetic energy: B / A = 2For equal mass, characteristic speed ratio = √2, not 2.
Original ideal-gas temperature comparison. The bars show T, not a speed distribution or total energy for samples of unspecified amount.
Interpret the supplied visual. At 600 K versus 300 K, does a characteristic speed of the same gas double?
Answer and explanation

No. Mean translational kinetic energy doubles, while a characteristic speed increases by √2, approximately 1.41.

For an ideal gas, mean translational kinetic energy per particle is proportional to absolute temperature in kelvin: 3kBT/2. The corresponding molar quantity uses R rather than kB. At a given temperature particles have a distribution of speeds; none of these statements requires every molecule to move at one speed. Doubling T doubles the mean translational energy, while a characteristic speed for the same particle mass scales as the square root of T. Celsius ratios do not express this relation.

No. Mean translational kinetic energy doubles, while a characteristic speed increases by √2, approximately 1.41.

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 10

Two equal-mass ideal-gas particles are compared through samples at 250 K and 500 K. Give the ratio of mean translational energy and characteristic speed, and explain why Celsius values cannot replace kelvin in these ratios.
Answer and explanation

The energy ratio is 2 and the characteristic speed ratio √2≈1.41. Kelvin is an absolute scale proportional to this mean energy; Celsius has an offset and its numeric ratios do not express the same physical relation.

The energy ratio is 2 and the characteristic speed ratio √2≈1.41. Kelvin is an absolute scale proportional to this mean energy; Celsius has an offset and its numeric ratios do not express the same physical relation.

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.

Problem 11

OpenSciEd drawing of a glass of water with selected particles above the surface and arrows showing their outward motion during evaporation.
Actual OpenSciEd surface-evaporation drawing with selected outgoing molecules; it omits return flow and the full speed distribution. Particles leaving a liquid during evaporation · OpenSciEd · 6.2 Thermal Energy, Lesson 15, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
Use the selected evaporation source drawing to explain what its outward arrows show, what they do not show about chemical bonds or all molecule speeds, and what must be added for a closed-vessel equilibrium model.
Answer and explanation

The arrows show selected molecules leaving the surface. They are not broken O–H bonds, a literal complete inventory or a claim that all molecules share one speed. Add vapor and returning condensation with equal opposing rates at the stated closed-vessel equilibrium; a constant liquid amount need not mean motion has stopped.

Identify the source’s visible labels, scales and unfinished or omitted information.

Use the local complete explanations to connect the observation with an explicit mechanism.

State assumptions and a limit before extending the model.

Response criteria

  • Interpret the source symbols and boundary correctly.
  • Explain the relevant mechanism using quantities/species consistently.
  • State a meaningful omission, assumption or evidence limitation.

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