Introduction

-Why do some solids dissolve in water but others do not?

-Why are some substances gases at room temperature, but others are liquid or solid?

-What gives metals the ability to conduct electricity, what makes non-metals brittle?

The answers to these questions all have to do with: 

INTERMOLECULAR FORCES

Intermolecular forces: is a general term describing the fact that all atoms, and molecules share a certain inherent attraction for each other

What are forces that exist between molecules of the same compound?

Intermolecular forces are the forces of attractions that exist between molecules in a compound. These cause the compound to exist in a certain state of matter: solid, liquid, or gas; and affect the melting and boiling points of compounds as well as the solubilities of one substance in another.

How do these forces affect the chemical and physical properties of molecular compounds?

The strength of intermolecular forces present in 
a substance is related to the boiling point and melting point of the substance.  Stronger 
intermolecular forces cause higher melting and boiling points since more kinetic energy is needed to break the bonds.   
EXAMPLES:   
CH4 - Methane:  has only very weak London dispersion forces (lowest b.p. & 
m.p.) 
CHCl3 - Chloroform:  has dipole-dipole interaction (moderate b.p. & m.p.)
NH3 - Ammonia:  has hydrogen bonding and dipole-dipole interaction (high b.p. 
& m.p.)