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

Intermolecular forces are weaker attractions that hold molecules or noble gas particles close together when they are in a liquid or solid form. Gas particles have broken away from the intermolecular forces that hold liquids and solids together.

In liquids, the intermolecular forces are strong enough to keep the particles tied upon to each other but not strong enough to keep them in fixed positions. In gases, the intermolecular forces are negligible (extremely weak), and the constituent particles are free to move.

Answer: The molecules of a gas move apart when they collide. The average kinetic energy of the particles in a liquid (or solid) is small enough that the forces of attraction between them is sufficient to hold the particles close together. The molecules in a liquid (or solid) do not move apart.

3) F2, Cl2, Br2 and I2 are non-polar molecules, therefore they have London dispersion forces between molecules. The molar mass increases from F2 to I2, therefore the srentgth of the Lodon dispersion forces also increases.

Hydrogen Bonding (H-Bonding) Hydrogen bonds are caused by highly electronegative atoms. They only occur between hydrogen and oxygen, fluorine or nitrogen, and are the strongest intermolecular force.

hydrogen bonding

dipole-dipole

London-dispersion force

to be weaker than dipole-dipole interactions, unless the dipoles are very small. H- bonding tends to be the strongest force, if present. Chlorine is more electronegative than bromine so the dipole moment in CH2Cl2 is larger than that in CH2Br2 leading to stronger dipole-dipole forces.

4. Iodine consists of I2 molecules, and the only attractions between the molecules are van der Waals dispersion forces. There are enough electrons in the I2 molecule to make the temporary dipoles creating the dispersion forces strong enough to hold the iodine together as a solid.