Melting point of the naphthalene [something went wrong]

  • Thread starter germangb
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hello,

Last day in school we were heating solid naphthalene until it melted, but the temperature didn't stop at the melting point.

if anybody has any idea of what could happen, please tell me, maybe we didn't do it correctly, or the test tube was dirty or whatever...

Thanks
 
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  • #2
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Occam razor... It can be the temperature readout, the heating device or the compound being tested.

Devices to measure MP generally consist of a temperature measuring device (simple liquid filled thermometer, digital temperature readout, thermocouple, or capillary tubes containing known MP substances or a mixture of the unknown and known MP substance being checked) and the associated heating device ( a metal block with electric heater, an oil bath, or a constant boiling liquid of known boiling temperature). Heating too fast will allow all but the last type of heater to shoot past the temperature, before everything can equilibrate in temperature- the last device is a single point temperature bath. Overshooting the temperature will cause the MP to appear too low for an unknown, but if known MP tubes are side by side with the unknown (whatever they may be) the temperatures will be experimentally the same. This can allow for determining the identity of an unknown if known pure compounds are available, even with an irregular temperature readout, or if an uncalibrated thermometer is available This covers the heater.

With glass thermometers, in a heated metal block, the thermometer can be at fault if it has a break in the liquid column (reads too high or erratically), or it has overtemped and burst the bulb (may not move at all). Thermometers should be calibrated against ice + water and/or in the vapor of steam above boiling water to double check that they are working- thermocouples can exhibit a break or corrosion that causes the digital readout to behave erratically or not move. These should also be calibrated to verify that they work. This covers the temperature device.

A known pure compound has the highest, shortest range MP, and any organic impurities cause a lower MP and broadening of the range. This is why a tube of the unknown mixed with the known MP can be expected to melt before the tube of authentic MP known material (if it represents an impurity when mixed); if you have the correct guess and have mixed an unknown of the same product with the known compound, the melt remains the shortest range and highest MP, consistent with the known. In this case organic impurities or wrong guesses only provide lower and broadened MP.

Caveat, if you have isolated a inorganic salt of high MP by filtering a precipitate from a crystallization or other reaction,then the MP will never be reached on the typical MP apparatus. It will not mix with authentic naphthalene, and will remain as an intractable non melting portion in the tube. This happens more often than you might imagine, so you back track on the source of the isolated impurity. It may have been in the crystallizing solvent, or added as an impurity. Simple tests for inorganic salts (solubility in water, tests for common anions such as sulfate, halides, carbonates) may help narrow down what went wrong. If a filtrand was tested, an aliquot of the filtrate can be boiled down in a watch glass to solid, and it tested by MP and mixed MP. It can also be tested for inorganic anions like above.
 
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  • #3
Borek
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Last day in school we were heating solid naphthalene until it melted, but the temperature didn't stop at the melting point.
Why do you think it should stop?
 

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