Is it advisable to use the temperature of solidification as the melting point of a solid? Why?
Answer:
Yes, and here is the reason, when you are heating up your sample in the melting point apparatus the rate that the sample is heating up along with the temperature increase is hard to control precisely. If you go too slowly, you have to wait a long time, and if you go too rapidly you will shoot past your melting point, and get a blur of a melting point reading.
Now, if the material is already in a liquid state and you can switch the apparatus off and let it cool slowly, and catch the precise moment when the liquid solidifies, and this is identical with the temperature at which the solid melts. The melting point range will be much sharper if you do it in this manner.
The melting point* of a crystalline solid is the temperature range at which it changes state from solid to liquid*. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds actually melt over a range of a few degrees or less. At the melting point the solid and liquid phase exist in equilibrium. When considered as the temperature of the reverse change from liquid to solid*, it is referred to as the freezing point*.
The melting point and freezing point are usually the same temperature.
NO! In the absence of nucleating centers liquids can supercool to a metastable state - filtered water in a PET bottle in your freezer. Melting does not superheat except under extraodinarily rigged conditions and even then only for small particles in free space.
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