text: Do you know about intermolecular forces? \nThere are four types of intermolecular forces (how attached one molecule is to another molecule) that you have to worry about: \n1. London dispersion\n2. Dipole-dipole\n3. Hydrogen bonding\n4. Ion-Dipole\nThese four are listed in order of increasing strength.  \n\nNow, when intermolecular forces are high, the molecules are really stuck to each other.  Such is the case in water molecules, H2O. This molecule experiences hydrogen bonding, which is a strong intermolecular force.  That's why water exists as liquid under normal conditions.  \n\nHowever, CO2 molecules (carbon dioxide) only experience London dispersion, the weakest form of intermolecular forces.  This is why carbon dioxide is a gas.  \n\nNow, we can make liquid molecules like water break away from each other if we supply the molecules with enough energy to do so.  We can do this by heating water until it starts boiling, which is, by definition, when water becomes gas.