Phase Changes

There are forces of attraction and repulsion that exist between molecules of all substances. These intermolecular forces allow molecules to pack together in the solid and liquid states.

When a pot of water is placed on a burner, it will soon boil. A phase change is occuring; the liquid water is changing to gaseous water, or steam. On a molecular level, the intermolecular forces between the water molecules are decreasing. The heat is providing enough energy for the water molecules to overcome these attractive forces.

All phase changes involve either an increase or decrease of intermolecular forces. For each of the phase changes below, choose whether intermolecular forces increase or decrease.

In this phase change, does the distance between molecules increase or decrease? When considering phase changes, the closer molecules are to one another, the stronger the intermolecular forces.

Good! For any given substance, intermolecular forces will be greatest in the solid state and weakest in the gas state.

The boiling points of ethyl ether and ethanol are 34.6ºC and 78.5ºC respectively. Which substance has stronger intermolecular forces?

As a liquid boils, it is undergoing the liquid to gas phase change. In order to do this, the intermolecular forces present in the liquid state must be overcome. Stronger intermolecular forces will require more energy to be overcome.

Good! A higher boiling point means more energy is required to overcome the intermolecular forces present in the liquid state.

The melting points of chlorine and iodine are -107ºC and 114ºC respectively. Which substance has stronger intermolecular forces?

As a substance melts, some of the intermolecular forces present in the solid state are overcome. More energy is required to overcome stronger intermolecular forces.

Good! A higher melting point means more energy is required to overcome some of intermolecular forces present in the solid state.

Intermolecular Forces

Phase Changes