Melting point
The
melting point of a
crystalline
solid is the
temperature at which it changes
state from solid to
liquid. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the
freezing point.
For most substances,
melting and
freezing points are equal. For example, the melting point
and freezing point of the
element mercury is 234.32
kelvins (−38.83
°C or −37.89
°F). However, certain substances possess differing solid-liquid transition temperatures. For example,
agar melts at 85 °C (185 °F) and solidifies 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.
The melting point of
water is 0 °C (32 °F, 273 K). 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 °C (−43.6 °F, 231 K) before freezing.
Unlike the
boiling point, the melting point is relatively insensitive to
pressure.
Melting points are often used to characterise organic compounds and to ascertain the
purity. 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. The more impurity is present, the lower the melting point. Eventually, a minimum melting point will be reached. The mixing ratio that results in the lowest possible melting point is known as the
eutectic point.
The
chemical element with the highest melting point is
tungsten, at 3695 K (3422 °C, 6191 °F). The often-cited
carbon does not melt at ambient pressure but
sublimates at about 4000 K; a liquid phase only exists above pressures of 10
MPa and estimated 4300–4700 K. Tantalum hafnium carbide (Ta
4HfC
5) is a
refractory compound with a very high melting point of 4488 K (4215 °C, 7619 °F).
[hafnium entry at Britannica.com] At the other end of the scale,
helium does not freeze at all at normal pressure, even at
absolute zero.
*
Phases of matter*
Triple point*
Freezing-point depression*
Boiling point}}}
