The Miracle of Water

 

 

Structure & Related Properties of Water

Water influences so many soil processes because of its structure

This structure is also responsible for the fact that water is present primarily as a liquid at earth surface temperatures

Water is, with the exception of mercury, the only inorganic (not carbon-based) liquid found on earth

 

Polarity:

  • instead of lining up symmetrically on either side of the oxygen atom ( H - O - H ), the hydrogen atoms are attached to the oxygen in a V-shaped arrangement at an angle of 105o.
  • water is therefore an asymmetrical molecule with the shared electrons bonded covalently* and attracted nearer the oxygen atom, a polar covalent bond.
  • consequently, water molecules exhibit polarity, that is the charges are not evenly distributed
  • the hydrogen side tends to be positive and the oxygen side negative

 

*Covalent Bond: a chemical bond where electrons are shared by atoms

 

 

     Water molecule showing a large oxygen
     atom and much smaller hydrogen atoms

 

 

Hydrogen Bonding

Because of the polarity of water molecules, a hydrogen atom of one water molecule is attracted to an oxygen molecule of a neighboring water molecule

This forms a low energy bond between the two molecules

This hydrogen bond accounts for water's relatively high:

  • boiling point
  • specific heat
  • viscosity

compared to the same properties of other hydrogen-containing compounds

 

Cohesion vs. Adhesion

Hydrogen bonding accounts for two basic forces responsible for water retention and movement in soils:

  • cohesion - the attraction of water molecules to each other
  • adhesion (adsorption): the attraction of water molecules for solid surfaces

 

By adhesion, some water molecules are held rigidly at the surfaces of soil solids

In turn, these tightly bound water molecules hold, by cohesion, other water molecules further away from the solid surface

Together, cohesion and adhesion, make it possible for soil solids to retain and control the movement of water

Adhesion and cohesion also make possible the property of plasticity in clays

 

 

 

 

 

 

 

 

Surface Tension

Water molecules have a greater attraction (cohesion) for each other than for the air.

The net effect is an inward force at the surface that causes water to behave as if its surface were covered with a stretched elastic membrane

Because of the relatively high attraction of water molecules for each other, water has a high surface tension compared to other liquids

 

 

 

 

 

Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces opposite the force of gravity

Adhesion (absorption) and surface tension cause capillary action

Capillary action can be demonstrated by placing a small diameter glass tube in water

The smaller the tube diameter, the higher the water rises

Water molecules are attracted (adhesion) to the sides of the tube and start to spread out

At the same time, cohesive forces hold the water molecules together and create surface tension, causing a curved surface (called a meniscus) to form at the interface of air and water

Lower pressure at P2 allows higher pressure at P1 to  push water up the tube.

The height of the rise is inversely proportional to the tube diameter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hydrophilic: water loving

Hydrophobic: water hating