Question

1. Explain why impurities broaden the melting point of a compound.

2. In the recrystallization process, when removing insoluble impurities why is it important to keep the filtration apparatus on the hot plate?

3. In the recrystallization process, why is it important to cool the solution slowly after heating?

Answer 1

(1)

Solid compund or elements we come across are crystalline solids in nature i.e. they have no. of molecules or ions arranged in regular and repeating fashion thus forming a crystal lattice.

These crytal lattices are held together by various inter molecular forces. When a solid is heated these intermolecular forces are disrupted by absorbing the required amount of heat energy. This amount of heat energy absorbed is measured in terms of temperature and gives us the Melting Point of the solid compound (stronger inermolecular forces means higher melting point).

But when a soluble impurity (insoluble impurity doesn't effect meting point) is added to the solid this definite structure of crytal lattice is disturbed and thus weakening the intermolecular forces. Therefore, as the forces weaken less amount of heat energy is required to disrupt the crytal lattice. This results in lowering of Melting Point of the sloid compound.

Note: More the impuity broader the range of Melting Point.

(2).

In recrytallization process hot filtration process is used to seperate impurities or insoluble matter from the compound. For this the compound is dissolved in minimum amount of hot solvent and then filterted to remove the insoluble matter/ impurities from the compound.

But while removing these impurities it is necessary to keep the filtration apparatus hot in order to prevent the dissolved compound to crytallize from the solution resulting in the formation of crytals on the funnel or filter paper and thus disrupts the filtration process.

(3)

The key step in the procees of recrystallization process is cooling the hot solution with solute (containing impurities) dissolved in it.

When the hot solution is allowed to cool down, with the decreasing temperature the solubility of the impurities and the compound decreases. Thus, the compound crytallizes before the impurities which gives us the pure compound.

The slow cooling is preferred so that impurities are not trapped in the crytals of pure compound thus formed. Also,slower cooling rate results in formation of large crytals which are easier to be filtered.