Final answer:
Insoluble barium sulfate is used in upper gastrointestinal x-ray studies because it does not dissolve in the stomach or intestinal fluids, allowing for a clear outline of the digestive organs. Barium carbonate is not used because it is soluble in plain water and would dissolve in the stomach's acidic environment.
Step-by-step explanation:
Insoluble barium sulfate is routinely used in upper gastrointestinal x-ray studies because it is so insoluble that it does not dissolve in the stomach or intestinal fluids and therefore does not get absorbed into the tissues. This allows for a clear outline of the digestive organs to appear on x-rays. Barium carbonate is not used because it is soluble in plain water and would dissolve in the stomach's acidic environment, making it ineffective for imaging purposes.