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Difficulty: MediumIdentifying Points of Disagreement

### The Mpemba Effect

The Mpemba effect is the observation that, under certain conditions, initially warm water freezes faster than initially cold water. Two students propose different explanations for this phenomenon.

Student 1

The Mpemba effect is caused by evaporation. When warm water is placed in a freezer, it evaporates rapidly. This evaporation reduces the total mass of the water sample. Because there is less mass of water remaining to be frozen, the remaining water freezes in less time than the cold water sample, which experiences negligible evaporation. The concentration of dissolved gases in the water has no influence on the freezing rate.

Student 2

The Mpemba effect is caused by dissolved gases. As water temperature increases, the solubility of gases decreases; thus, warm water contains a much lower concentration of dissolved gases than cold water. Dissolved gases lower the freezing point of water and inhibit convection currents that facilitate cooling. Therefore, the warm water freezes faster because it has fewer dissolved gases. The mass lost due to evaporation is too small to have any measurable effect on the freezing rate.

According to the passage, Student 1 and Student 2 disagree about the effect of which of the following factors on the freezing rate of the water?

  1. A
    The chemical composition of the water molecules
  2. The reduction of water sample mass due to evaporationAnswer
  3. C
    The relationship between water temperature and gas solubility
  4. D
    The initial temperature difference between the two water samples

Answer

The reduction of water sample mass due to evaporation
Student 1 states that rapid evaporation reduces the mass of the warm water sample, causing it to freeze in less time. In contrast, Student 2 states that the mass lost to evaporation is too small to have any measurable effect on the freezing rate. Therefore, the students disagree on whether the reduction of water sample mass due to evaporation affects how quickly the water freezes.

Step-by-Step Solution

1
Analyze Student 1's position on evaporation and mass loss.
Student 1 asserts that evaporation reduces the total mass of the warm water sample, which directly causes it to freeze in less time.
To identify points of disagreement, the specific mechanism and claims of each student must be determined.
2
Analyze Student 2's position on evaporation and mass loss.
Student 2 asserts that the mass lost due to evaporation is too small to have any measurable effect on the freezing rate.
Comparing Student 2's claim to Student 1's claim reveals whether they hold opposing views on this factor.
3
Evaluate the options to find the factor where the two students express opposing claims.
The two students directly disagree on whether the mass lost via evaporation affects the freezing rate of the water samples.
A point of disagreement requires one student to support a claim and the other student to contradict or deny that same claim.

Key Concept

Identifying points of disagreement between conflicting scientific viewpoints by comparing the mechanisms or variables each viewpoint proposes.
Estimated Time:1m 30s
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Topics

Identifying Points of Disagreement

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