Chemistry – Experimental techniques and chemical analysis - Separation and purification | e-Consult

(a) The unknown solid is likely to be sugar (C₁₂H₂₂O₁₁).

Explanation: The melting point range of 105-110 °C is closest to the melting point of sugar (186 °C) listed in the table. While the melting point of sugar is significantly lower than the melting points of NaCl, KNO₃, and Cu, the provided range is the closest match. It's important to note that melting point ranges are often given to account for experimental error and gradual melting.

(b) A melting point range is reported rather than a single melting point temperature because:

• Impurities: Impurities disrupt the crystal lattice structure of a solid. This means that impurities cause the solid to melt over a range of temperatures, rather than melting at a single, sharp temperature.
• Experimental Error: It is difficult to determine the exact moment when a solid starts to melt and the exact moment when it is completely melted. A range reflects the spread of temperatures observed during the melting process, accounting for potential experimental error.
• Gradual Melting: When a solid is heated, it doesn't always melt instantaneously. The temperature gradually increases as the solid transitions to a liquid. The melting point range reflects this gradual transition.

Therefore, reporting a range provides a more accurate assessment of the purity of the substance.