| What is the Difference Between Melting Point and Freezing Point? |
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Most of us are familiar with the terms solid and liquid. We not only recognize these words but have little trouble classifying most materials we encounter as one of these two states of matter or as a gas. We are also rather comfortable with the terms melting and freezing. We recognize when something melts and when it freezes. The scientific view of these two processes, however, is a little more complicated than one might at first imagine. |
| To a chemist, the melting point of a solid is the temperature at which it changes from a solid to a liquid. The transition between the solid and the liquid is so sharp for small samples of a pure substance that melting points can be measured to 0.1oC. The melting point of solid oxygen, for example, is -218.4oC. The temperature at which the reverse happens, a liquid changes to solid, is referred to as the freezing point. |  |
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For most substances, melting point and freezing point refer to the same temperature. For example, the melting point and freezing point of mercury is -38.83 °C or -37.89 °F. However, for certain substances the transition temperatures are different. For example, agar melts at 85 °C (185 °F) and changes from a liquid to a solid from 32 °C to 40 °C (89.6 °F to 104 °F); this process is known as hysteresis. Certain materials, such as glass, may harden without crystallizing; these are called amorphous solids. You may have heard that glass is a liquid because it continues to flow but, in point of fact, it is an amorphous solid. |
| We all know that the melting point of water is 0 °C (32 °F). The freezing point of water is the same as the melting point as ling as the water contains what is called a nucleating substance. , but in the absence of nucleators water can supercool to -42 °C (-43.6 °F) before freezing. It is difficult, if not impossible, to heat a solid above its melting point because the heat that enters the solid at its melting point is used to convert the solid into a liquid. It is possible, however, to cool some liquids to temperatures below their freezing points without forming a solid. When this is done, the liquid is said to be supercooled. |  |
| Unlike boiling point, melting point does not fluctuate in response to changes in pressure. |
| The melting point of a pure substance is always higher than the melting point of that substance when a small amount of an impurity is present. This is why salt is added to icy roads. The salt is, in this case, the impurity that lowers the freezing point of the water. The more impurity is present, the lower the melting point. |  |
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Because it is difficult to heat solids to temperatures above their melting points, and because pure solids tend to melt over a very small temperature range, melting points are often used to help identify compounds. We can distinguish between the three sugars known as glucose (MP = 150oC), fructose (MP = 103-105oC), and sucrose (MP = 185-186oC), for example, by determining the melting point of a small sample. |
Measurements of the melting point of a solid can also provide information about the purity of the substance. Pure, crystalline solids melt over a very narrow range of temperatures, whereas mixtures melt over a broad temperature range. Mixtures also tend to melt at temperatures below the melting points of the pure solids. |
It is, no doubt, because of this greater reliability of melting point as an identifying property, that TEA included it, and not freezing point, in the TEKS associated with this concept. (5.7D observe and measure characteristic properties of substances that remain constant such as boiling points and melting points.) |
Adapted from materials found at the following links.
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