Interactions between Molecules

The three states of matter, solid, liquid, and gas


What is the volume of 1.00 g water when it is completely changed into gaseous form (not the visible "moisture" or "steam")?
(Here is the information you need: water density, 1.00 g/cm3; gaseous water occupies 22.4 L at STP; molar mass; and compare a fixed amount of water and its gaseous form)

1.00 g water is 1.00 g ÷ 18.0 g/mol = 0.0556 mol
0.0556 mol water is changed into 0.0556 mol of gaseous water, which is
0.055g mol × 22.4 L/mol = 1.25 L

Remember that 1.00 g water is 1.00 mL since the density of water (at 4 deg-C) is 1.00 g/mL.

Attractive forces between molecules: secondary forces
van der Waals forces: the weakest attractive force
Dipole-Dipole interactions: between molecules with permanent dipole moments, like H2O, CO, and so on.

Bond polarity is caused by uneven distribution of electrons due to different electronegativities of different atoms.

London force: resulting from temporary dipole

    Non-polar molecules can condense to liquid due to this force (but at much lower temperature),

    e.g., CH4 condenses at –164 ºC, but H2CO at –21 ºC.

Hydrogen Bond: the kind of dipole interaction when H is involved, occurred when H is bounded to a very electronegative atom, like F, N, and O.

e.g., in proteins shown below

in DNA                                                         and in water and ice


Surface tension: caused by net attraction of molecules inward toward the interior of the liquid, that resists the expansion of the liquid surface and prevent a paper clip from sinking, but can be greatly decreased by detergents.


Chemistry within us, 6.1 and 6.2
(6.1) The bile salt, which acts like "detergent", emulsifies lipid in the intestine for the purpose of lipid digestion.
(6.2) A "surface-active agent" (surfactant) is synthesized in alveolar cells which decreases water surface tension of better expansion of the alveoli.

Formation of solution: "Like dissolves like."
e.g., salt in water, Na+ and Cl ions are surrounded by water molecules via dipole-dipole interaction

Vaporization: the "escape" of a molecule form a liquid

(Sublimation: escape from a solid, e.g., dry ice.)

A molecule must have enough kinetic energy in order to escape from a liquid. The higher the temperature, the easier a molecule can escape, thus the higher the vapor pressure of the liquid. Therefore,
            Liquid + heat ® vaporization into vapor
And,     Vapor ® liquid + heat

Influence of secondary forces on vapor pressure: The greater the attractive forces between molecules, the smaller the vapor pressure.
A liquid boils when its vapor pressure reaches atmospheric pressure. Thus, the higher the atmospheric pressure, the higher the boiling temperature is like in the case of a pressured cooker. The opposite is like in the case of cooking on the top of a high mountain.

Would be a joke if you say: "The boiling temperature of a liquid is the temperature when the liquid boils." Duh!! It does not define anything! Just like it is a joke by saying: "Liquid can vaporize because the temperature is high!" "One can make ice cube in a freezer because it is cold in the freezer."

Normal boiling point: the temperature at which boiling occurs under (an external pressure of) 1 atm.


Vapor pressure is influenced by H-bond (above right), size of the molecule (propane and higher b.p. than methane).

Relative humidity of water = (partial pressure/equilibrium vapor pressure) ´ 100%

The relative humidity at 25 ºC is 95%, what is the partial pressure of water under that condition? (The equilibrium vapor pressure of water at 25 ºC is 23.8 torr.)
        95% (= 0.95) = Vp/23.8
        Vp = ____ torr

Effect of H-bonds on water

           e.g., surfactant and beads of water on greasy surface             cf. Methane has a heat of vaporization of 122 cal/g.             H-bond strength: H2O > NH3 and H2S
            b.p.: H2O (100 °C) > NH3 (–33.3 °C) and H2S (–60.7 °C)

Read Section 6.9 about "attractive forces and the structure of solids": examples on amorphous; covalent crystals, etc.