What does the melting and boiling point depend on?

Answer 1

In general, the melting and boiling points of substances depends on the strength of the forces holding the molecules or ions together, the intermolecular bonds.

For example, sodium chloride is a giant ionic crystal lattice consisting of Na+ and Cl- ions. This structure is very strong due to the attractive forces between the ions, so a lot of energy is required to break the ionic bonds and melt NaCl, hence the melting/boiling point being very high.

Conversely, a molecule like methane (CH4) has very little localised charge, it is not polar, and it is a small molecule so the attractive forces between molecules are weak and methane has a low melting and boiling point.

Answer 2

MP's & BP's depend on temperature and pressure.

Taking water as an example

water's MP is 0 oc and its BP is 100 oC. This is at atmospheric pressure at sea- level ( 760 mmHg). #

However, if you go to the top of Mount Everest ( 29,000 ft above sea-level), then water boils at a much lower temperature, because the atmospheric pressure is much lower.

Another example is Carbon Dioxide gas. If it is cooled a sea-level atmospheric pressure it sublimes, turns directly to a solid ( dry-ice). However, if the pressure and temperature are altered in such a way, then carbon dioxide will form a liquid.

Answer 3

The melting and boiling point of a substance is largely determine by the intermolecular forces which hold its molecules together. These may included Van der Waals forces (dispersion forces), dipole-dipole interactions, as well as hydrogen bonding. The stronger these forces act between molecules, the high the melting point and boiling point of the substance.

Answer 4

The melting and boiling points are independent on the size.

Answer 5

The boiling point depends on:

- pressure

- purity of the substance