Final answer:
The student mistakenly identified the solid as barium carbonate due to the formation of a pale yellow precipitate, but the behavior of the precipitate in hydrochloric acid suggested it was actually calcium carbonate.
Step-by-step explanation:
The student believed the solid to be barium carbonate (BaCO3) because a pale yellow precipitate formed upon adding potassium chromate, which could indicate the presence of barium ions since barium chromate is yellow. However, this assumption was incorrect.
The confusion may have arisen because both calcium and barium form insoluble carbonates and sulfates. When barium chloride is added to a solution, a white precipitate will form if a sulfate or carbonate is present. To accurately identify BaCO3, one would expect the precipitate to be insoluble in hydrochloric acid (HCl), matching the description of test tube Y.
However, if the precipitate dissolved in HCl and released a gas, as in tube X, this would match the expected behavior of calcium carbonate (CaCO3), which reacts with HCl to produce soluble calcium chloride, carbon dioxide gas, and water. Therefore, the teacher corrected the student, as the results suggest the presence of calcium rather than barium.