Answer to Question #288838 in Organic Chemistry for thomas

Question

A pure white crystalline compound was found to melt at 131.5-132.0oC when taken on a

melting point apparatus, and on further heating, the liquid was found to start bubbling at ca.

155-160

oC. The capillary containing the melted compound was set aside to cool, and

resolidify. When the sample was retried using that same capillary, the sample melted at

120.6-154.5oC, along with some bubbling. What is the cause of the bubbles? Why is the

melting point so wide the second time? Explain by stating what you have learned in this

laboratory about melting points

For the same solid compound as the question above, if the 1 mm sample was heated at 10o

per minute on a melting point apparatus, what would you expect for an approximate

melting point? (note that at this heating rate, the sample will melt rather quickly, but again

give an actual numerical approximation)

Answer 1

Atmospheric gases such as nitrogen and oxygen can dissolve in water. The amount of gas dissolved depends on the temperature of the water and the atmospheric pressure at the air/water interface. Hence bubbles along the insides of your water glass.

Impurities, even when present in small amounts, usually lower the melting point and broaden the melting point range. A wide melting point range (more than 5°C) usually indicates that the substance is impure; a narrow melting point range (O. 5-2°C) usually indicates that thesubstance is fairly pure.