Question

A solid was dissolved in water and analyzed using thin layer chromatography. All the necessary precautions were taken to get a good result. The chromatogram had three spots with R_f values of 0.25, 0.38, 0.74 in the column for the solid. If a fresh sample of the solid were tested to measure the melting point, would the melting point have been sharp or have been a broad range? Explain your answer.

Answer 1

The melting point can be used to identify a substance or compound and get a note of its purity. The melting point of a solid is a temperature at which a solid exists in equilibrium state with its liquid. A transition from solid to liquid state under external pressure of 1 atm.

Sharp range and broad range melting point: A pure crystalline solid possesses sharp melting point and it melts completely over a temperature range of not more than 0.5-1.0 degree difference, provided the technique ia accurate. A broad range M.P shows that the product is impure(contaminated and not 100% pure).

Modern lab facilities use melting point apparatus instead of capillary tube method, where sample is loaded to sealed capillary and placed in socket where the sample is electrically heated and melting point is noted upon liquification.

From 3 Rf values , it is found 3 solids are present and it a mixture. Thin layer chromatography (TLC) is seperation technique to separate component mixtures and is used for identification , purity and impurity identification.