MELTING POINT
The melting point of a solid is the temperature at which the substance begins to change from a solid to a liquid. Pure organic compounds have very defined melting points. Through the use of these melting points, the purity of the compound can be determined. Contaminants normally lower the melting point and broaden the melting point range (the temperature at which melting starts and the temperature at which the substance is melted). Narrow range melting points are normally indicative of the purity of an organic compound. Extremely pure compounds have ranges of 0.1 to 0.3 C. Commercially available compounds (technical grade) have ranges of 2-3 C. The normal laboratory grade organic (ACS) compounds have a 1 C range. The larger the melting point range, the less pure the compound.
The melting point range is the temperature range between where a compound starts to melt and completely melts (becomes liquid). It is highly recommended that during a melting point determination, the temperature be slowly increased. This enables one to observe the change and temperature range.
Sometimes, mixtures of two organic compounds with the same individual melting point have a much lower melting point. This temperature depression is useful for determining unknowns. When an unknown is identified as a suspected compound, this compound can be mixed with a known compound of identical melting point. The melting point depression can confirm whether the suspected organic compound is indeed the correct assumption.
Not all organic compounds melt. Some substance will decompose, discolor, soften and/or shrink as they are heated. If possible, a reference compound should be compared. If a compound decomposes, this temperature is normally a reliable indicator. The temperature is followed by the letter "d" to indicate decomposition (198 d).