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a student performs two melting-point determinations on a crystalline product. in one...

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thewakulam | eNotes Newbie

Posted September 1, 2013 at 1:10 PM via web

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a student performs two melting-point determinations on a crystalline product. in one determination, the capillary tube contains a sample about 1-2 mm in height and the melting range is found to be 141 degree - 142 degree celsius. in the other determination, the sample height is 4-5 mm and the melting range is found to be 141 degree -145 degree. explain the broader melting-point range observed for the second sample. the reported melting point for the compound is 143 degree celsius.

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llltkl | College Teacher | Valedictorian

Posted September 1, 2013 at 5:42 PM (Answer #1)

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During the process of melting, the crystalline sample collects latent heat of fusion from the atmosphere it is kept in. When a little sample is taken, the required amount of latent heat is quite small, and is  swiftly acquired by the crystals by conduction from the heat source through the surface of the glass capillary. As soon as the melting point is reached, the sample exhibits actual melting and the corresponding temperature, or a range close to it is recorded.

On the contrary, when a large amount of sample is taken, the required amount of latent heat is appreciable, and it takes quite some time for the requisite amount of latent heat to be transferred to the crystals. Result is that, after the melting point is reached, part of the sample starts melting and the melting process takes some time for completion for obvious reasons. By this time, the temperature rises and hence the recorded melting temperature range widens.

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