Fractional Crystallization

Fractional crystallization is a method of refining substances based on differences in solubility

If a mixture of two or more substances in solution is allowed to crystallize, for example by allowing the temperature of the solution to decrease, the precipitate will contain more of the least soluble substance.

 

Is it possible to separate mixtures of different ionic compounds by dissolving them in water and adjusting the temperature of the solution so that one compound crystallizes out and the other does not?

YES!! You are given a mixture containing 80 g of K2SO4(s) and 80 g of KNO3(s) and are asked to produce as much pure KNO3 as possible. How do you do it?

Step 1: Dissolve salts in 100 g distilled water at 80°C.

By looking at the solubility curves, you will see that 100 g of KNO3 and 170 g of K2SO4 will dissolve at this temperature.

Thus, everything will dissolve.

Step 2: Cool the solution to 0°C.

K2SO4

At 0°C about 150 g of K2SO4 is soluble in 100 g of water.

We only dissolved 80 g of K2SO4 so NONE of it has precipitated out.

KNO3

At 0°C only 12 g of KNO3 can remain in 100 g of water.

We dissolved 80 g of KNO3 so (80g-12g=68g) of pure KNO3 precipitated from the solution.

A steeper solubility curve indicates that a greater mass of precipitate will form during a reduction of temperature.

Conversely, a steeper solubility curve indicates that a greater mass of solid will dissolved during an increase of temperature.

You are given a dry mixture containing 60 g of KNO3(s) and 60
g of KClO3(s) and are asked to produce as much pure KClO3(s)
as possible. How?

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