Melting and Boiling Point

The nature of the particles, and therefore the strength of forces between the particles, can be used to explain trends in melting and boiling points. Melting and boiling require the breaking of bonds. The stronger the bonds/forces between the particles, the higher the melting and boiling points will be.

The water example has an interesting outcome when it comes to boiling. While it lowers the melting point of water (ice), it increases the boiling point of water. This is because liquid water primarily contains only permanent dipole-permanent dipole forces (a type of intermolecular bonds). Salt water contains ions, dissolved in (surrounded by) water. These have a stronger ionic-permanent dipole forces between the particles. These take more energy to break than hydrogen bonds and a lot more energy to break than permanent dipole-permanent dipole forces.

This is why we often add salt to vegetables when we cook them. The water can get a lot hotter before boiling, so cooks the vegetables more quickly. It also has the added bonus of seasoning the food (adding flavour).