Melting Point Sample Preparation

The melting point (mp) is a physical property of a substance. Melting points are often given in lab handbooks as single temperatures. Such a value may be assumed to be the temperature at which pure solid and liquid phases are in equilibrium. This temperature would be the same as the freezing point. To determine a freezing point, a liquid is slowly cooled with stirring until crystals begin to form. The temperature of the resulting solid-liquid mixture is the freezing point. It remains constant as long as pure solid and pure liquid are in equilibrium. Continued cooling (heat removal) results in complete solidification or freezing. Further cooling after solidification then lowers the temperature.

For practical reasons, the melting points of organic compounds are usually determined differently in the lab. The melting point is found by slowly raising the temperature of a small sample of solid. The observed temperature continues to rise during the transition from solid to liquid. The value recorded is a range of temperatures, or melting point range. Even though we record a melting point range, we still refer to it as the melting point.

A sufficiently accurate melting point determination can usually be made within about 15 minutes. Practice is needed in order to develop the necessary skill. Each mp determination involves parameter adjustments while the procedure is being executed. A set of instructions performed by rote is not adequate. Success requires careful observation, analysis of the situation, and timely adjustments. As such, taking a melting point is a good model for other organic laboratory procedures.

MELTING POINT MAINPAGE *The purpose of these pages is to help beginners obtain melting points accurately and efficiently*.
The melting point (mp) is a physical property of a substance. Melting points are often given in lab handbooks as single temperatures. Such a value may be assumed to be the temperature at which pure solid and liquid phases are in equilibrium. This temperature would be the same as the freezing point. To determine a freezing point, a liquid is slowly cooled with stirring until crystals begin to form. The temperature of the resulting solid-liquid mixture is the freezing point. It remains constant as long as pure solid and pure liquid are in equilibrium. Continued cooling (heat removal) results in complete solidification or freezing. Further cooling after solidification then lowers the temperature. For practical reasons, the melting points of organic compounds are usually determined differently in the lab. The melting point is found by slowly raising the temperature of a small sample of solid. The observed temperature continues to rise during the transition from solid to liquid. The value recorded is a range of temperatures, or melting point range. Even though we record a melting point range, we still refer to it as the melting point. A sufficiently accurate melting point determination can usually be made within about 15 minutes. Practice is needed in order to develop the necessary skill. Each mp determination involves parameter adjustments while the procedure is being executed. A set of instructions performed by rote is not adequate. Success requires careful observation, analysis of the situation, and timely adjustments. As such, taking a melting point is a good model for other organic laboratory procedures.
This is what you will see when you take a melting point. To the right is a time-compressed “movie”of lauric acid melting inside a melting point apparatus. The actual time involved was about 2 minutes, during which the temperature rose from 42 to 44 degrees.
Melting Point Subpages Melting Point Tips and Guidelines Melting Point Sample Preparation Mel Temp II Heating Curves and Instructions

MELTING POINT MAINPAGE

The purpose of these pages is to help beginners obtain melting points accurately and efficiently.