The Modern Chemistry Lab

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What affects melting and boiling points of chemical compounds?

Intermolecular (between molecules) forces govern the melting point of a chemical substance. These include Van der Waal’s interactions, dipole-dipole interactions, hydrogen bonding, and, in the case of ionic compounds or ionic solutions, ionic bonds or Coulombic interactions. The stronger the intermolecular forces between the molecules in a solid, the harder they will be to melt, so stronger intermolecular forces lead to higher melting points. The same is true with boiling points: stronger intermolecular forces make the molecules harder to separate, leading to higher boiling points.

For solids, the shape of a molecule can also affect its ability to pack into an ordered lattice. Having a shape that allows a well-ordered lattice to form will tend to stabilize the solid phase of a compound, leading to a higher melting point. The shape of a molecule can also affect the boiling point of a compound. In liquids that are able to form hydrogen bonds, the location of the hydrogen bond donor or acceptor can affect its spatial availability to serve as a donor or acceptor. In organic liquids, where Van der Waal’s interactions are important, molecules with larger surface areas will have stronger Van der Waal’s interactions, leading to a higher boiling point.


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