Final answer:
In melting point analysis, the correct range to record is from the initial melting to the disappearance of the solid (option b). Melting point ranges provide information on a substance's purity and are influenced by its crystalline or amorphous structure. A melting temperature midpoint is often used for substances like DNA.
Step-by-step explanation:
In a melting point analysis, the range that should be recorded is the range from the initial melting to the disappearance of the solid phase, which is option (b). The melting point is not a single temperature but a range over which a substance transitions from a solid to a liquid. This range is indicative of the purity of a substance; a pure substance typically has a narrow melting point range, while an impure substance has a broader range due to the presence of impurities that disrupt the orderly structure and lower the melting point.
As an example, ice has a melting temperature of 0 °C, but an amorphous solid like butter softens over a range of temperatures due to its less ordered structure. The difference between crystalline and amorphous solids illustrates why recording the melting range is important in analyzing the substance's properties. A consistent mid-point temperature, or 'melting temperature,' can also be used as a reference point for substances such as DNA, where the melting temperature reflects the stability of the double-stranded structure.