WATER MOLECULE

A Whirlwind Tour of the Amazing Water Molecule
and Some Remarkable Effects of Its Properties

by Steve Horstmeyer, Meteorologist

GREATLY ENLARGED WATER MOLECULE

Looking very much like the shape of Mickey Mouse

Water Molecule

Because a water molecule has a slight positive charge on one end and a slight
negative charge on the other...the attraction of the opposite charges, (electro-static charges)
creates what is called surface tension, the weak attraction is called a hydrogen bond.

WATER MOLECULES AT THE SURFACE IN A GLASS OF WATER

Water molecules at the surface

Please note that the illustration above is a gross simplification, the molecules are in constant
motion and it would be rare for this very simple situation to occur exactly as above.
The effect over a sufficiently long period of time is for the attraction of positive and negative
charges to create a surface harder to penetrate than if surface tension did not exist.

This is the reason, if your take care, that you can float a needle or razor blade on
the surface of the water.

CLOUD DROPS WITHIN A CLOUD

Water molecules at the surface

Again the illustration is a simplification. A water molecule inside the drop is attracted equally in all
directions by neighboring molecules. A molecule at the surface is pulled inward, but not outward because there are no
water molecules to pull the surface molecule outward. So....a molecule from the interior of the drop must do work
against the "cohesive" forces (forces of attraction) between water molecules.

To minimize the amount of energy expended in creating a surface a mass of water will assume the shape that gives
minimum surface area. That shape is a sphere. We say it has minimum surface area to volume ratio. For a small
amount of water, that is a very small drop, the electro-static forces (hydrogen bonds)
are stronger than external forces, like air turbulence and gravity. As the amount of water increases and the drop grows it
will be deformed by the external forces which eventually will overwhelm the electrostatic forces.

Water - The Universal Solvent???

Water has been called the “universal solvent”. It is not. Many substances will not dissolve in water, many oils, for example. However water is very effective at dissolving molecules with “exposed” electrical charges, like NaCl.

 

Water seems to act mysteriously in dissolving materials. Look at the following graphs:

 

 

 

The solubility of sucrose (a sugar) and NaCl both increase with increasing temperatures.

The increase of solubility of sucrose is great, for NaCl the increase is slight.

Hypothesis: The solubility of solids increases with increasing temperature, but all solids are not the same. Sucrose which does NOT dissolve into ions (charged atoms) shows a great increase, NaCl which does dissolve into charged ions (Na+, Cl-).

 

The solubility of both CO2 and O2  decrease with increasing temperature.

Hypothesis: The solubility of gasses decreases with increasing temperature.

More Data:

 

SUBSTANCE

0oc

20oc

40oc

60oc

80oc

100oC

CHANGE

0o to 100o

Solids – concentration in grams / 100ml

NaCl

35.7

36.0

36.6

37.5

38.5

39.6

+10.6%

Sucrose

179.2

203.9

238.1

294

378

498

+178%

Liquids – concentration in mg / L (ppm)

NH3 (ammonia)

75

55

37

22

12

6

-92.0%

O2

14.6

9.65

6.11

4.67

3.23

2.44

-83.3%

CO2

1.34

0.690

0.421

0.294

0.226

0.184

-86.3%

 

 

 

 

From you lecture notes on kinetic theory, be able to explain these observations by answering these questions:

 

1.    Do all solids get more soluable as temperature increases?

2.    Do all gases get less soluable as temperature increases?

3.    Why does sucrose get so much more soluable with increasing temperature than table salt?

 

 

 

 


 

 

 


© 2004 Steven L. Horstmeyer, all rights reserved