Passage III

In a chemistry class, the teacher placed a beaker containing a clear green liquid on a heat source. Over the next 12 min, the volume of the liquid steadily decreased and the liquid darkened in color. At 12 min, a white solid appeared that floated on top of the remaining liquid. At 16 min, all of the liquid was gone and only the white solid remained.

The teacher asked 4 students to provide explanations for what occurred over the 16 min.

Student 1

The beaker initially contained a solution made by dissolving a white solid in a pure, green solvent. Over the first 12 min, the solvent evaporated, causing the concentration of the dissolved solid to increase. At 12 min, the volume of solvent had decreased to a point at which the solid was no longer soluble, so it started to exit the solution. At 16 min, all of the solvent had evaporated.

Student 2

The beaker initially contained a solution made by dissolving a white solid in a pure, colorless solvent. Each molecule of the dissolved solid paired with (but did not react with) a solvent molecule to form a green-colored solute-solvent complex (SSC). Over the first 12 min, the solvent evaporated, causing the concentration of the SSC to increase. At 12 min, the volume of solvent had decreased to a point at which the solid was no longer soluble, so it started to exit the solution. At 16 min, all of the solvent had evaporated.

Student 3

The beaker initially contained a solution made by dissolving a green solid in a pure, colorless solvent. Over the first 12 min, the solvent evaporated, causing the concentration of the dissolved solid to increase. At 12 min, the high concentration and heat caused all of the dissolved solid to instantly react with some of the solvent to form an insoluble white solid. At 16 min, all of the remaining solvent had evaporated.

Student 4

The beaker initially contained a pure, green liquid. Over the first 12 min, the liquid slowly decomposed, but did not evaporate. At 12 min, the volume of liquid had decreased to a point at which the product of the decomposition was no longer soluble, so it started to exit the solution. At 16 min, all of the green liquid had decomposed.

17. Suppose that after the demonstration, a colorless liquid had been mixed with the solid remaining in the beaker and the liquid then turned green. This observation would have been most consistent with the explanation given by which student?

Answer and Explanation

Your Answer is

Correct Answer is B

Explanation

Precipitated solid + colorless liquid = green solution, which is in line with Student 2's statement (because Student 2 said that no real chemical changes occurred during the whole process, and the precipitated solid is the original solute)