Question

You are attempting a recrystallization of the world’s only sample of charliebionic acid with a small amount of an unknown impurity.   The acid is not soluble at all in water. The acid is very soluble in hot ethanol but only slightly soluble at room temperature. But as the solution cooled to room temperature, no precipitate formed. However, when the solution was cooled in an ice bath solid immediately formed. This solid was dried and the melting point measured. The melting point showed that the impurities remained.

What is the most likely reason that no crystals formed at room temperature?

Why is the charliebionic acid still impure after recrystallization?

Propose a last minute addition to this procedure that might correct this situation and form pure crystals at room temperature from this sample.

Answer 1

What is the most likely reason that no crystals formed at room temperature?

(1). We have to take in account of the colligative property that is the depression of freezing point.

When a non-volatile substance is added to a pure molecule, it decreases the freezing point of the molecule.

(2). Let us suppose the compound has hexagonal geometry and impurity has triangle geometry. If the solution is allowed to cool slowly, the triangle shaped molecule (impurities) may sit down briefly in the growing crystal lattice. But soon the triangle geometry (impurities) will leave, when the compound with a more suitable (hexagonal) geometry comes in, to take their place. Suitable geometry molecule (hexagons) stay more readily in the growing lattice, and eventually pure crystals of hexagons are formed.

On the other hand, if we cool the solution too quickly. The triangle (impurities) are trapped inside the crystals being formed by the hexagons, thus, the crystals isolated are impure.

(3) For recystalization slow cooling is required. Fast cooling will land up you impure crystal.

Since, the slow crystallization gives larger crystals than that of fast crystallization. Small crystals have a large surface area to volume ratio and impurities are located on the surface of the crystals as well as trapped inside the matrix.