CDS (II) 2022 General Knowledge · Question 61 of 120

Correct — A, Barium sulphate. Two properties have to hold at once for a contrast agent that is swallowed, and barium sulphate is the salt that holds both. First, it must stop X-rays. Barium has atomic number 56, and heavier nuclei absorb X-rays far more strongly than the light elements that make up soft tissue, so a coating of barium sulphate throws a sharp white shadow that outlines the inside wall of the gut. Second, it must not be absorbed. Barium ions are toxic, but barium sulphate is practically insoluble in water — about 2·4 milligrams per litre at 20 degrees Celsius — so the suspension travels through the alimentary canal and leaves the body almost unchanged, releasing no meaningful quantity of barium into the blood. That combination of high atomic number and near-zero solubility is why the barium meal has been standard radiology for a century.

• (b)Barium chloride — Barium chloride would give the same X-ray contrast, since the barium is the same, but it is freely soluble in water. Soluble barium salts are poisonous — the barium ion interferes with potassium channels and can cause severe muscle weakness and cardiac effects — so a barium chloride meal would be a poisoning, not an investigation.
• (c)Strontium sulphate — Strontium sulphate is also very sparingly soluble, so it clears the safety test, but strontium's atomic number is 38 against barium's 56. It absorbs X-rays distinctly less well, gives a weaker outline, and is not an approved contrast medium.
• (d)Magnesium chloride — Magnesium chloride fails both tests. It is highly soluble, so it would be absorbed rather than coating the gut, and magnesium's atomic number of 12 is close to that of the tissues themselves, so it would cast no useful shadow.

An X-ray image is a map of how much radiation each path through the body absorbed. Bone shows up because calcium and phosphorus are heavier than the carbon, hydrogen, oxygen and nitrogen of soft tissue; the stomach and intestine do not show up at all, because their walls and contents absorb about as much as everything around them. A contrast medium fixes that by introducing an element heavy enough to stand out. Swallowed as a thick suspension, barium sulphate coats the lining and makes its folds, narrowings and ulcers visible.

The item is really a two-filter puzzle and rewards applying both filters instead of one. Filter on solubility alone and strontium sulphate survives alongside barium sulphate; filter on heaviness alone and barium chloride survives alongside barium sulphate. Only the compound that passes both is the answer, and only one option does. The safety point is worth remembering in its own right, because it is a favourite of chemistry papers: the toxicity of barium belongs to the barium ion in solution, so the same element is a poison in one salt and a routine hospital preparation in another.

• Barium has atomic number 56, high enough to absorb X-rays strongly and cast a clear shadow.
• Barium sulphate's solubility in water is about 2·448 milligrams per litre at 20 degrees Celsius — effectively insoluble.
• Soluble barium salts such as barium chloride are toxic; the insolubility of the sulphate is what makes it safe to swallow.
• Barium sulphate is given as a suspension of fine particles, orally as a barium meal or rectally as a barium enema.
• The white precipitate formed when barium chloride solution meets a sulphate solution is barium sulphate — the same insoluble compound.

• Choosing barium chloride because the barium is what matters; the anion decides whether the compound dissolves and therefore whether it poisons.
• Rejecting strontium sulphate for solubility reasons — it is insoluble too, and fails instead on being too light and not being an approved agent.
• Assuming a heavy metal compound must be dangerous; barium sulphate is safe precisely because it does not dissolve.

As a pick-the-contrast-agent item, as a why-is-it-safe reasoning item, or inside a solubility question where barium sulphate is the insoluble product.

• practice — not a real PYQ

  When a solution of barium chloride is added to a solution of sodium sulphate, the white precipitate formed is

  • (a)Barium hydroxide
  • (b)Barium sulphate
  • (c)Sodium chloride
  • (d)Barium oxide

  Answer(b) Barium sulphate — it is almost insoluble in water, which is why it drops out of solution as a white solid.

• practice — not a real PYQ

  Bone appears white on an ordinary X-ray photograph mainly because

  • (a)it is harder than soft tissue
  • (b)it contains elements of higher atomic number that absorb X-rays strongly
  • (c)it reflects X-rays back to the source
  • (d)it emits its own radiation

  Answer(b) it contains elements of higher atomic number that absorb X-rays strongly — chiefly calcium and phosphorus.