Why do different substances have different melting and boiling points?

Different substances have different melting and boiling points due to the varying strengths of their intermolecular forces.

In more detail, the melting and boiling points of a substance are determined by the type and strength of the forces between its particles. These forces are known as intermolecular forces. There are three main types of intermolecular forces: dispersion forces, dipole-dipole forces, and hydrogen bonding.

Dispersion forces, also known as London forces, are the weakest type of intermolecular force. They occur between all molecules, whether they are polar or non-polar. The strength of dispersion forces increases with the size and shape of the molecule, so larger molecules generally have higher melting and boiling points than smaller ones.

Dipole-dipole forces occur between polar molecules, i.e., molecules with a positive end and a negative end. These forces are stronger than dispersion forces, so polar substances generally have higher melting and boiling points than non-polar substances of similar size.

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. Hydrogen bonds are much stronger than other types of intermolecular forces, so substances that can form hydrogen bonds usually have much higher melting and boiling points.

In summary, the melting and boiling points of a substance depend on the type and strength of the intermolecular forces between its particles. The stronger the intermolecular forces, the more energy is required to overcome them and change the substance from a solid to a liquid (melting) or from a liquid to a gas (boiling). Therefore, substances with stronger intermolecular forces have higher melting and boiling points.