Question

17. A pure compound melts at 48-49 degrees Celsius. If a trace amount of an impure compound, whose melting point is 80-82 degrees Celsius, is mixed in with the first compound, what happens to the melting point of the pure compound?

A. The second compound would lower the melting point of the first one

B. It is impossible to determine without knowing the identity of the two compounds.

C. The second compound would raise the melting point of the first one.

D. The second compound would have no effect on the melting point of the first one.

Answer 1

In order to look at this question objectively, let's assume that the pure compound is a crystalline solid where the atoms or groups of atoms are arranged in a crystal such that they form a repeating pattern known as a crystal lattice. When the compound is pure, the crystal lattice is formed by all the same atoms or the same groups of atoms, this means that all the atoms or groups of atoms are held together by the same intermolecular force throughout the crystal lattice, or to put it in simple terms, all atoms or groups of atoms are bonded together by the bonds of similar strength.

But, when an impurity is added to the system, the atoms or groups of atoms of impurity get into the crystal lattice and weaken the strength of the bonds, thus changing the previously similar strength of the bond which ran through the crystal. Now, when the compound, where the impurity is mixed, is melted, the bonds would break at a lower temperature as compared to the pure compound.

This concept is often referred to as the melting point depression.

So, the correct choice in this question is option A. The second compound would lower the melting point of the first one.