Question

1) What is the effect of an insoluble impurity, such asa sodium sulfate, on the observed melting point of a compound?
2)Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. How could you prove that the test tubes contain three different chemical compounds?

Answer 1

1)

Soluble impurities (that is, impurities that are included within the crystal matrix) tend to lower the observed melting point and broaden the melting point range. Insoluble impurities have no effect on the melting point.

There is no effect on the melting point of this particular compound, Because it has to be soluble in order to depress the melting point. If it is insoluble it is just like having 100 % of compound X and 0 % of compound Y, assuming there is only two compounds in this impure compound. Thus the vapor pressure is not effected and melting point will not change.

2)

It may not be possible to find the difference by simply looking at the tubes. However there are several chemical or physical tests may be able to prove if these are same or different.

1. Solubility in solvents like water, dilute HCl, dilute NaOH, Ethyl alcohol, etc.

2. Smell.

3. Mixed melting point. Make mixtrure of A+B, A+C,B+C an find melting point of the mixtures. If mixture has same mp then the compounds are same otherwise they are different.