CDS (II) 2022 General Knowledge · Question 15 of 120

Correct — C, 3. All three statements are true of tincture of iodine. A tincture is by definition a solution of something in alcohol, and NCERT introduces this very substance as an example when teaching solutions: 'A solution of iodine in alcohol known as tincture of iodine, has iodine (solid) as the solute and alcohol (liquid) as the solvent.' The pharmacopoeial preparation is iodine together with an iodide salt dissolved in aqueous alcohol, which is why the second statement's phrasing 'alcohol-water mixture' is right — the water is there to dissolve the iodide that in turn keeps the iodine in solution. The concentration is genuinely low; the mild strength sold over the counter is about 2 per cent iodine, and it is that dilute solution which is painted on a cut. And iodine kills bacteria on contact, so the preparation is an antiseptic — a substance applied to living tissue, as distinct from a disinfectant used on surfaces.

• (a)1 — Accepts only the antiseptic claim. The solvent and the concentration statements are just as sound, and the first of them is the textbook's own worked example of a solid dissolved in a liquid.
• (b)2 — The common near miss. Candidates usually keep the first two and reject the third, on a vague feeling that a medicine that stings must be strong. Strength of sensation is not concentration; the solution is about 2 per cent.
• (d)None — Rejects the whole set, including a definition — a tincture is a solution in alcohol, so statement 2 cannot be false.

A solution is a homogeneous mixture of a solute in a solvent, and the two need not be liquid and liquid. Tincture of iodine is the standard textbook case of a solid solute in a liquid solvent, and the word tincture itself is the pharmacist's term for an alcoholic extract or solution. Iodine is only slightly soluble in water but dissolves readily in alcohol and in a solution of potassium or sodium iodide, which is why the preparation combines both.

Three statements, none of them numerical in a checkable way, and the pressure on the candidate is to find something wrong. Take them one at a time and there is nothing. The one that causes hesitation is the third, because 'very low' sounds like an overstatement — but a 2 per cent solution is very low by any ordinary reading, and it has to be, since iodine at higher strength irritates and stains the skin badly. A related point worth carrying is the difference between an antiseptic and a disinfectant: both kill microbes, but an antiseptic is safe on living tissue while a disinfectant is for floors and instruments. Iodine preparations, hydrogen peroxide and dilute alcohol are antiseptics; phenol at strength is a disinfectant.

• NCERT gives tincture of iodine as its example of a solution with a solid solute, iodine, and a liquid solvent, alcohol.
• The pharmacopoeial preparation dissolves iodine with sodium or potassium iodide in aqueous alcohol.
• The mild preparation sold for household use is about 2 per cent iodine.
• An antiseptic is applied to living tissue; a disinfectant is used on non-living surfaces.
• Iodine is only slightly soluble in water but dissolves readily in alcohol and in iodide solutions.

• Rejecting the low-concentration statement because the sting feels strong.
• Calling tincture of iodine a suspension; it is a true solution, unlike milk of magnesia.
• Using antiseptic and disinfectant as if they meant the same thing.

As a count-the-statements item like this one, as a solute-and-solvent identification question, or as a match of substance to medical use.

• practice — not a real PYQ

  In tincture of iodine, the solute and the solvent are respectively

  • (a)alcohol and iodine
  • (b)iodine and alcohol
  • (c)iodine and water
  • (d)water and alcohol

  Answer(b) iodine and alcohol — a solid solute dissolved in a liquid solvent, which is the textbook example of that combination.

• practice — not a real PYQ

  A chemical substance applied to living tissue to prevent infection is called

  • (a)a disinfectant
  • (b)an antiseptic
  • (c)an antibiotic
  • (d)an analgesic

  Answer(b) an antiseptic — a disinfectant is for non-living surfaces, an antibiotic acts inside the body against bacteria, and an analgesic relieves pain.