An unknown compound gave a melting point of 230 Cwhen the liquid solidifiedthe melting point was redetermined and found to be 131Cwhat might have caused this change?

Answer 1

Pressure can affect melting points.

However, assuming that your conditions are the same, you may have changed the crystal structure of your compound.

The crystal structure of a compound can affect its melting point. For example, diamond and graphite are both forms of elemental carbon. Diamond is a face-centered cubic structure. Graphite, however, is hexagonal. Graphite has a melting point of 3652°C, whereas diamond melts at 3550°C. When you liquified your compound, it because amorphous and lost its crystal structure. It may have recrystallized into a different structure.

Heating may also have effected a chemical change. If you saw bubbles or a color change, you likely made a new compound.

Answer 2

The initial high melting point of 230°C could have been due to impurities present in the unknown compound. Upon redetermination, these impurities might have been removed, leading to the lower, more accurate melting point of 131°C. Alternatively, the unknown compound could have undergone some form of chemical degradation or decomposition between the initial and subsequent melting point measurements, affecting its melting point.

Answer 3

Yes, generally the answer is correct; but for some substances occurs the strange phenomenon of hysteresis and the freezing point and the melting point has different values.