Structural Biochemistry/Melting Point

Melting Point

Melting point is defined as the temperature at which the bond within a solid collapse and the solid is converted into a liquid. The stronger the force of attraction between molecules or atoms, the higher the melting point will be, because more energy is required to break these bonds and change the state of the molecule.

Since melting points are varied across the periodic table, there is not a distinguishable trend.

1. As the atomic number of elements increases, the melting point mostly increases because there are more electrons around the nucleus, which creates stronger intermolecular forces. With stronger forces, the melting point rises, as it takes more energy to break these forces. But the melting point also varies according to the type of structure, with giant covalent or metallic structures having a higher melting point than simple molecular structures.

2. Metals usually have high melting point due to the metallic bonding. This is an electrostatic attraction between positive ions and delocalised electrons (strong bonds).

3. Non-metals usually have low melting points, as they normally have simple molecular structures.

Chart of Melting Points of Elements

The melting point of a molecule or compound is a very important characteristic. Many times we can determine that identity of a compound or molecule using just this characteristics. This would be called melting point determination where a compound is slowly heated to find the temperature at which it melts. The unknown compound's melting temperature is compared to other known melting points, where it can be either extrapolated that it is the correct compound or a different one. In order to further investigate, a mixed melting point determination is done. This is done by mixing two compounds and testing the resulting melting point. If the compounds are different then the melting point will always lower. If the two compounds are identical the melting point stays the same. The use of a melting point apparatus and care is needed for this. This experiment shows the importance of melting point characteristic in chemistry. Not only as a characteristic, but also as a means to identifying unknown compounds.

Examples

Melting points and boiling points of the first eight carboxylic acids (°C) For most substances, melting and freezing points are approximately equal. For example, the melting point and freezing point of the element mercury is 234.32 kelvin (−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 31 °C to 40 °C (89.6 °F to 104 °F); such direction dependence is known as hysteresis. The melting point of ice at 1 atmosphere of pressure is very close to 0 °C (32 °F, 273.15 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 °C (−43.6 °F, 231 K) before freezing. The chemical element with the highest melting point is tungsten, at 3683 K (3410 °C, 6170 °F) making it excellent for use as filaments in light bulbs. The often-cited carbon does not melt at ambient pressure but sublimes at about 4000 K; a liquid phase only exists above pressures of 10 MPa and estimated 4300–4700 K. Tantalum hafnium carbide (Ta4HfC5) is a refractory compound with a very high melting point of 4488 K (4215 °C, 7619 °F). At the other end of the scale, helium does not freeze at all at normal pressure, even at temperatures very close to absolute zero; pressures over 20 times normal atmospheric pressure are necessary.