Melting point
(Redirected from Freezing point)
Categories: Chemical properties
The melting point of a 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 and solidifies from 32 to 40 °C; this phenomenon is known as hysteresis.
Certain materials, such as glass, may harden without crystalizing; this is called an amorphous solid.
Unlike the boiling point, the melting point is relatively insensitive to pressure.
The material with the highest known melting point at atmospheric pressure is graphite, with a melting point of 3948 kelvins.
Melting point is often used to ascertain purity of and characterise organic compounds. The melting point of a pure substance is always higher than the melting point of an impure sample of that particular substance. When two chemical substances are mixed, the melting point of the resultant mixture will be lower than the melting point of either constituent. The mixing ratio that results in the lowest possible melting point is known as the eutectic point.
DNA
When referring to oligonucleotides or DNA, the melting temperature Tm refers to the temperature where 50% of the DNA is in a duplex form. In other words, 50% of the DNA has been denatured into single strands.
See also
ar:نقطة إنصهار bg:Температура на топене ca:Punt de fusió cs:Teplota tání da:Smeltepunkt de:Schmelzpunkt et:Sulamistemperatuur es:Punto de fusión eo:Frostopunkto fr:Température de fusion ko:녹는점 hr:Talište is:Bræðslumark it:Punto di fusione he:נקודת התכה lv:Kušanas temperatūra lt:Lydymosi temperatūra hu:Olvadáspont nl:Smeltpunt ja:融点 no:Smeltepunkt nn:Smeltepunkt pl:Temperatura topnienia pt:Ponto de fusão ro:Punct de topire ru:Температура плавления sl:Tališče sr:Температура топљења fi:Sulamispiste sv:Smältpunkt th:จุดหลอมเหลว uk:Температура плавлення zh:熔点