Question

LAB Discussion: Why is it important to add a solevent to a solid during recrystalization and not the other way around? Why is the melting point of a mixture of two different compounds lower and broader than the melting point of a pure compound? Why would a percent recovery be lower when measuring the weight of the recrystalized over the impure ratio? I got 73 perecent recovery

Answer 1

1). The basic principle of crystallization is to dissolve the compound in the minimum amount of solvent. If you take your compound in a flask and add solvent slowly then we can stop when the compound is completely dissolved in the solvent, hence using the least amount of solvent. The other way If you take a particular amount of solvent in flask and then add certain amount of compound in it, then the solvent may be excess than the required to dissolve it completely. So this way we may or may not get crystals, hence this process can't be used.

2). It is also known as depression in melting point. Since we are allowing two compounds to interact with each other by mixing them together, which will stabilize or destabilize the molecules (by non covalent interactions) and can change their physical properties. So melting point may vary accordingly.

3). before we crystallize the compound it had contained some impurities already (lets say 10%) which is later removed in the process of rerystallization. Percentage recovery depends on many facotrs and not just on the contained impurity.