CDS Chemistry · Chemistry in Everyday Life
Antiseptics and the Barium Meal
What tincture of iodine is made of, and why insoluble barium sulphate is safe to swallow for an X-ray while soluble barium salts are poisons.
Why this matters
Three CDS questions, two of them on tincture of iodine (2017 and 2022) and one on the barium meal (2022).
Concept 1 of 2: Tincture of iodine and other antiseptics
Definition
The facts:
- Tincture of iodine = a dilute (2–3%) solution of iodine with potassium iodide, in an alcohol–water mixture. It contains no acetone.
- It is an antiseptic for fresh wounds.
- Dettol (chloroxylenol and terpineol) and boric acid are other common antiseptics.
- Antacids (milk of magnesia, sodium hydrogen carbonate) neutralise excess stomach acid.
| Component of tincture of iodine | Present? |
|---|---|
| Iodine (dilute) | Yes |
| Potassium iodide | Yes — keeps iodine dissolved |
| Alcohol (ethanol) | Yes |
| Water | Yes |
| Acetone | NoQ |
Practice this concept3 quick reps
The same idea in a real exam question:
Example 1 · Chemistry in Everyday Life · Medicines and Health Chemistry
No acetone in tincture of iodine
The iodine is dilute
An antacid is a base
Antiseptic, not antibiotic
Concept 2 of 2: The barium meal
Definition
The facts:
- Barium sulphate, BaSO₄, is given as a meal before an X-ray of the alimentary canal.
- It is opaque to X-rays and practically insoluble, so no toxic barium ions are absorbed.
- Soluble barium salts such as barium chloride are poisonous.
| Salt | Soluble? | Safe as an X-ray meal? |
|---|---|---|
| Barium sulphate | No | YesQ |
| Barium chloride | Yes | No — poisonous |
Practice this concept2 quick reps
The same idea in a real exam question:
Example 2 · Chemistry in Everyday Life · Medicines and Health Chemistry
Barium sulphate, not barium chloride
It works because barium blocks X-rays
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Tincture of iodine and other antiseptics5 rows
| Component of tincture of iodine | Present? |
|---|---|
| Iodine (dilute) | Yes |
| Potassium iodide | Yes — keeps iodine dissolved |
| Alcohol (ethanol) | Yes |
| Water | Yes |
| Acetone | NoQ |
The barium meal2 rows
| Salt | Soluble? | Safe as an X-ray meal? |
|---|---|---|
| Barium sulphate | No | YesQ |
| Barium chloride | Yes | No — poisonous |
Watch out for (6)
- No acetone in tincture of iodine→ Tincture of iodine and other antiseptics
- The iodine is dilute→ Tincture of iodine and other antiseptics
- An antacid is a base→ Tincture of iodine and other antiseptics
- Antiseptic, not antibiotic→ Tincture of iodine and other antiseptics
- Barium sulphate, not barium chloride→ The barium meal
- It works because barium blocks X-rays→ The barium meal
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