A mixture contains solid naphthalene (melting point 80°C) and solid sodium chloride. Which method is most appropriate to separate and purify the naphthalene?
Show the answer and explanation
Correct answer
C. Heat the mixture gently; collect and cool the sublimate.
Principle or equation
Naphthalene sublimes (turns from solid to vapor) when heated; sodium chloride does not. Sublimation separates a volatile solid from a non-volatile solid.
Why this answer is correct
Heating the mixture causes naphthalene to vaporize; the vapor can be collected and cooled to re-form pure solid naphthalene. Sodium chloride remains behind. Water would dissolve NaCl but not naphthalene, but naphthalene is insoluble and would remain as a solid; filtration would separate it, but not purify from other insoluble impurities. Sublimation is more direct for purification.
Example
Place mixture in a beaker, cover with a cold watch glass, heat gently. Naphthalene vapor condenses on the watch glass as pure solid.
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