Why is the melting point of a compound reported as a range of temperatures?
To understand why melting points are given as a range of temperatures, it is important to first understand what the melting point of a compound is. The melting point of a solid, as defined by the University of Wisconsin-Madison Department of Chemistry, is the "temperature at which the compound's solid and liquid state are in equilibrium" with one another at "1 atmosphere" of "total pressure."
Essentially, this means the compound's solid structure is broken down, and the compound is in equilibrium between the liquid and solid state.
Melting points can be determined via specific heating of the compound using a number of methods. Different melting point determination techniques require different amounts of compound to be run effectively.
The problem with determining the exact melting point of a compound, however, is two-fold, hence the reason melting points are typically given in a melting range instead of at a distinct point.
One problem is that the actual apparatus used to heat the compound may lead to slight variations in measurement of the temperature when compared to the actual temperature of the compound. This means slight inaccuracies in measurement will occur.
The second problem has to due with purity of the compound. It is very difficult to isolate most compounds into a completely pure form. These small impurities will lead to slight, but possibly measurable, changes in the melting point of the compound.
It is for these two reasons that, in most settings, such as a college or high school chemistry lab, melting points will be reported as a range rather than a specific temperature. Hope this helps!