Temperature & Thermal Energy — Practice · Grade 8 Science

Chapter: Phases of Matter

What this practice covers

By the end of Temperature & Thermal Energy you should be able to:

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

Vocabulary used: temperature, heat, thermal equilibrium, model, evidence, system boundary, assumption.

Problem 1

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 2

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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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 11

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.

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