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The melting point of a 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. Because of the ability of some substances to supercool, the freezing point is not considered to be a characteristic property of a substance. For most substances, melting and freezing points are essentially equal. For example, the melting point and freezing point of the element mercury is 234.32 kelvin (-38.83&_160;°C or -37.89&_160;°F). However, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85&_160;°C (185&_160;°F) and solidifies from 31&_160;°C to 40&_160;°C (89.6&_160;°F to 104&_160;°F); this process is known as hysteresis. Certain materials, such as glass, may harden without crystallizing; these are called amorphous solids. Amorphous materials as well as some polymers do not have a true melting point as there is no abrupt phase change at any specific temperature. Instead, there is a gradual change in their viscoelastic properties over a range of temperatures. Such materials are characterized by a glass transition temperature which may be roughly defined as the "knee" point of the material's density vs. temperature graph. The melting point of water at 1 atmosphere of pressure is very close [1] to 0&_160;°C (32&_160;°F, 273.15&_160;K), this is also known as the ice point. In the presence of nucleating substances the freezing point of water is the same as the melting point, but in the absence of nucleators water can supercool to -42&_160;°C (-43.6&_160;°F, 231&_160;K) before freezing.
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