Which intermolecular forces are present in pure water? Which is the strongest?

Question

Lab Results:

Surface Tension

Needle floats on distilled water
Needle does not float on acetone
Needle does not float on vegetable oil

Viscosity

Time for water to drain from bottle: 28 sec, 27 sec, 26 sec
Time for veg oil to drain from bottle: 1 m 15sec, 1m 16sec, 1m 15sec

Time for 5w-30 Motor oil to drain: 11m 55sec, 11m 54sec, 11m 53sec
Time for 20W-50 motor oil to drain: 35 m, 34 m 98sec, 34m 99 sec

Answer 1

Hydrogen bonding forces (H-bonding forces):
Only in molecules with an O- H bond, an N- H bond, or an F- H bond.
Strongest of the intermolecular forces between neutral molecules.

Approximately 10% as strong as an actual covalent single bond.

Dipole-dipole forces:
Only in polar molecules
Depends on the size of the molecular dipole moment.

Approximately 5% as strong as H-bonding forces.

Dispersion forces:
Present in all molecules.
Roughly proportional to the square of the "volume" of the molecule.

Weakest of the intermolecular forces.
Approximately 1% as strong as dipole-dipole forces (except for large molecules where the forces can be comparable).

Examples:
H2, HCl, Cl2, H2S, C6H6, CH3OH, CO2, HCN, glycerine

Vapor Pressure and Boiling Points

We can use our knowledge of intermolecular forces to compare vapor pressures and boiling points of liquids.

Vapor Pressure

Vapor pressure is a measure of the ability of molecules to escape from the surface of a liquid (or a solid, for that matter). Vapor pressure is sometimes referred to as "escaping tendency."
We know that there is a competition between intermolecular forces (attracting molecules to each other) and the kinetic energy of the molecules (which is trying to separate them).

We also know that the average kinetic energy of the molecules is proportional to the Kelvin temperature, T. However, not all molecules have the average kinetic energy. There is a distribution of kinetic energies with some molecules having a kinetic energy higher than the average and some having an energy lower than the average.

In the liquid phase molecules at the high end of the kinetic energy distribution can still have enough energy to escape from the surface of the liquid. The pressure generated by these escaping molecules is called the vapor pressure of the liquid.
When the temperature is increased the fraction of molecules with sufficient kinetic energy to overcome the intermolecular forces increases so that the vapor pressure increases.
If the temperature is decreased the vapor pressure decreases because fewer molecules have enough kinetic energy to escape the intermolecular forces.

When the vapor pressure reaches one atmosphere the liquid boils and the temperature at which this happens is called the boiling point.
We can compare vapor pressures of two different substances at the same temperature.

At a given temperature, the substance with the stronger intermolecular forces will have the lower vapor pressure.
Examples
CH4, NH3, H2O
H2O, H2S, H2Se

Boiling points
A liquid boils when its vapor pressure reaches one atmosphere.

(Actually, the liquid boils when its vapor pressure reaches the pressure of the ambient atmosphere. In Tucson, at about 2500 ft in altitude, the pressure is less than one atmosphere - about 0.92 atm. On Mt. Lemmon the pressure is even lower - about 0.73 atm. Liquids boil at a lower temperature in less than one atmosphere pressure. It takes a long time to hard-boil an egg at the top of Mt. Lemmon.)
The boiling point of a liquid is defined to be its boiling point at one atmosphere.
We can use our knowledge of intermolecular forces to predict the relative boiling points of compounds.

Since compounds with large intermolecular forces have lower vapor pressures, we predict that one has to go to higher temperature to make them boil.

So we can use the following principles to predict relative boiling points:

Stronger intermolecular forces mean higher boiling points.
Other things being equal:

Polar molecules boil higher than nonpolar molecules.
Hydrogen-bonded molecules boil higher than nonhydrogen-bonded molecules.

Large molecules boil higher than small molecules.

"Spread out" molecules boil higher than "compact" molecules.
Examples: TBP
HCl - 82oC
Ar - 186oC

CH4 - 162oC
NH3 - 34oC
H2O 100oC

He 4.22 K
Ar 87.5 K

CH3(CH2)3CH3 36oC
C(CH3)4 9.5oC

Intermolecular Forces in Mixtures

In mixtures we get a variety of new combinations of molecular interactions. For example in water solutions of ionic compounds there are interactions of the dipole moment of the water molecules with the charge on the ions to produce "solvated ions."

Sometimes the solvation energy is strong enough to carry water molecules into the crystals when the ionic compounds are crystallized out. This is called "water of crystallization," and gives rise to formulas such as BaCl2×2H2O.

In nonionic solutions and in gas phase mixtures there are other combinations of intermolecular forces. For example, the interaction between a polar molecule and a nonpolar molecule is stronger than dispersion forces and weaker than dipole-dipole forces.

Intermolecular forces give us an indication of the solubility of a compound in another compound. The principle is that "like dissolves like." That is, nonpolar solvents tend to dissolve other nonpolar compounds but not, for example, hydrogen-bonded compounds. Polar solvents tend to dissolve other polar compounds, but not nonpolar compounds or hydrogen bonded compounds.


Superheated and Supercooled Liquids

Sometimes, if the sample and container are very clean and there are no dust particles in the liquid, you can heat a liquid above it's boiling point without it boiling. This is called a superheated liquid. A superheated liquid is not at equilibrium - the liquid is unstable. If you stir the liquid or drop in a grain of salt or something similar the system will release all of its excess energy all at once in a flash of vapor. (It might look like a small explosion.)

In the laboratory, when boiling liquids, say in distillation, we add "boiling stones" to the liquid. The boiling stones provide sites for the nucleation of bubbles and keep the liquid from overheating.

By the same token, liquids can be cooled below their freezing point without freezing - if it is carefully done and the system is very pure. This results in what is called a supercooled liquid. A supercooled liquid is not at equilibrium and will freeze immediately if the system is stirred or a seed crystal of the solid material is added to the system.

Answer 2

Hi. The surface tension is strong enough to hold a glob of water together in micro-gravity. You may have seen astronauts playing with liquid water in orbit. Put a drop of water on wax paper and it will try to pull itself into a sphere, again due to surface tension. Any time that water (or any liquid) is more attracted to itself than another substance this will happen. Since you asked about intermolecular forces we'll leave out forces like the strong and weak nuclear forces.

You seem to have an answer or two from pretty knowledgeable people and I'm sure their answers are much more in line with what you needed.

Answer 3

the strongest is the hydrogen bonding (between hydrogen and oxygen from different water molecules)

There are also dipole and van der vaals forces but they aren't as strong.