AQA A-Level Chemistry AS Paper 1, November 2021: Question 22

✅ Correct Answer: C

In medicine to produce an X-ray image

💡 Key Knowledge

• Barium meal: Barium sulfate ( BaSO₄ ) is administered as a "barium meal" to patients undergoing X-ray imaging of the gut.
• Opacity: Barium is a heavy element that absorbs X-rays strongly, making the digestive tract clearly visible on the resulting images.
• Toxicity safety: Although soluble barium compounds (like barium chloride) are toxic, barium sulfate is safely ingested because it is completely insoluble in water and bodily fluids, preventing absorption into the bloodstream.

🧠 Exam Technique

• Elimination: Cross out agricultural uses immediately—barium compounds are not used as fertilisers or soil neutralisers (farmers use calcium carbonate or calcium oxide for acidic soils).
• Medical Context: Distinguish between different Group 2 medicinal uses. Magnesium hydroxide ( Mg(OH)₂ ) and calcium carbonate are used as antacids, whereas barium sulfate is exclusively used for medical imaging.

❌ Common Errors

• Confusing the medical uses of Group 2 compounds (e.g., mixing up the antacid role of magnesium hydroxide with the imaging role of barium sulfate).
• Failing to recognise that insolubility is the exact reason why barium sulfate can be safely swallowed despite barium's inherent toxicity.