Video: Identifying the Set of Equipment Appropriate for the Recrystallization of a Compound with Benzene in a List of Equipment

A student used benzene to recrystallize a compound. As the hot solution cooled to room temperature, very few crystals appeared. Which pieces of laboratory equipment would be appropriate for this recrystallization? I. Stirring rod II. Filter funnel III. Conical flask [A] II only [B] III only [C] I and III only [D] II and III only [E] I, II, and III

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Video Transcript

A student used benzene to recrystallize a compound. As the hot solution cooled to room temperature, very few crystals appeared. Which pieces of laboratory equipment would be appropriate for this recrystallization? (I) Stirring rod, (II) filter funnel, (III) conical flask. And we are given answer options (A) (II) only, (B) (III) only, (C) (I) and (III) only, (D) (II) and (III) only, or (E) (I), (II), and (III).

Recrystallization is a technique used to purify a substance. The recrystallization process that the student would’ve used is as follows. The compound, which was impure and needed to be recrystallized and purified, is drawn in orange with little pink dots to represent impurities. The impure compound is dissolved in a minimum amount of hot solvent.

The question told us that the solvent used by the student is benzene. Benzene, C6H6, is a nonpolar, colorless liquid. And because it is used as the solvent here, we know that the impure substance must also be nonpolar. The impure compound would be stirred and dissolved using a glass rod to form a saturated solution of the desired compound and with some impurity. So, a glass rod used to stir is necessary to form the initial saturated solution.

The student then would have cooled the solution, in this case, to room temperature as shown in the question. And as the temperature of the solution decreased, so the solubility of the desired compound would also decrease. We know this because, in general, for a solid substance dissolving in a solvent, temperature is proportional to solubility. In other words, the higher the temperature of the solvent, the more solid substance dissolves. And the lower the temperature of the solvent, the less solid substance is able to dissolve.

And since we started with a saturated solution, as soon as that solution starts to cool, it will not be able to hold the same amount of dissolved solid solute but less. So, some of the solute will precipitate out and no longer be dissolved in solution. So, cooling will induce some precipitation, otherwise known as crystallization, the formation of tiny solid crystals.

Now, the question tells us that very few crystals appeared. This makes sense because the solution was only cooled to room temperature. If the student had instead used an ice bath to cool the solution to a very, very cold temperature, this would’ve caused the solubility of the compound to decrease rapidly, forming many more crystals and, thus, a higher yield of desired pure product.

But, in this case, cooling just to room temperature, the student would’ve ensured the formation of some pure crystals. But some of the desired compound would still remain dissolved in solution with the impurity. The last step would have been to filter the pure crystals from the solution. Let’s clear some space to have a look at that in more depth.

Filtration removes solid particles from a solution. The contents of the beaker are poured through a filter funnel with filter paper leaving the purified crystals on the filter paper, while the rest of the solution filters through and is called a filtrate. The filtrate contains the dissolved impurity and, in this case, some dissolved desired compound which did not crystallize out of solution.

From the diagram, we can see that a filter funnel and a conical flask are necessary for the filtration step. The question asked, which pieces of laboratory equipment would be appropriate for this recrystallization? And we have seen that the stirring rod, filter funnel, and conical flask are used in this process. So, the correct answer is (I), (II), and (III), answer option (E).

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