Question

Indicate whether each of the following statements is TRUE or FALSE 1. A sharp melting point for a crystalline organic material always indicates a pure single compound 2. A molecular size impurity always lowers the melting point of an organic compound 3. If the addition of a sample of compound A to compound B does not change the melting point of B, A and B must be identical 4. An eutectic mixture has a sharp melting point Answer 4 Melting and freezing points of a pure compound are identical

Answer 1

Answer:1. False, 2. False, 3. True, 4. True

Explanation:

1. FALSE. Pure crystalline solids have sharp melting points, while the present of impurity lower the melting point. This is True, but the exception is a eutectic mixture. A properly mixed eutectic mixture have a sharp melting point also.

2. FALSE, although the present of impurity lower the melting point of a organic compound, but if a molecular size impurity with a significantly higher melting point and is present in large quantities the melting point of the organic compound will not be lowered.

3. TRUE, two compounds with same melting point, when mixed together will not change the melting point of either compounds. But if A has higher MP than B and the addition of A to B creates mixture with same melting point as pure A then the statement will be false.

4. TRUE, a properly mixed eutectic mixture will have a sharp melting point.

Melting point and freezing point of a pure compound will be identical because the liquid form of a pure substance when cooled it forms a stable arrangement that supercooling is prevented, freezing occur at the same temperature at which the pure solid phase melts