A mixture contains solid iodine and solid potassium iodide. Iodine sublimes readily when heated gently, while potassium iodide does not. Which method is most appropriate to obtain pure iodine from the mixture?
Show the answer and explanation
Correct answer
B. Heat the mixture gently and collect the sublimate on a cold surface.
Principle or equation
Sublimation separates a volatile solid from a non-volatile solid. Iodine sublimes on heating, leaving potassium iodide behind. The iodine vapor condenses back to solid on a cold surface, yielding pure iodine.
Why this answer is correct
Iodine has a high vapor pressure and sublimes at moderate heating. Potassium iodide is ionic and has a high melting point. Sublimation exploits this difference. Water would dissolve KI and some iodine (slightly soluble) but not separate them effectively. Ethanol dissolves both. Nitric acid would react with KI.
Example
A mixture of iodine and KI is heated in a beaker covered with a cold watch glass. Purple iodine vapor deposits as shiny crystals on the glass, while KI remains in the beaker.
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