soils 3a

the water molecule

  • oxygen wants to gain 2 electrons

  • two hydrogen atoms want to lose one electron each

  • h2o

    • two H share electrons w O

    • everyone is happy

    • charge is balanced

  • covalent bond

  • oxygen holds the electrons closer to its nucleus than the H

  • causes the bend in the molecule

  • results in “dipole moment”

    • zones within the molecule that are more negatively charged or more positively charged than the rest of the molecule

water molecules act like magnets

  • opposite charges attract

    • + attracts -

    • - attracts +

  • results in polarity

importance of polarity

  • explains why water molecules are attracted to charged ions and to colloidal surfaces

  • positively charged ions are attracted to oxygen side of water (neg charged side)

  • negatively charged ions are attracted to hydrogen side of water (pos charged side)

    • ionic compounds have greater attraction for water molecules than each other

    • makes water the “universal solvent”

nonpolar compounds

  • have an equal charge balance throughout the compound

    • no dipole moments

  • nonpolar substances can only be dissolved in nonpolar solvents

  • that is why grease (organic compound) does not dissolve in water without soap

hydrogen bonds

  • hydrogen atom from one water molecule is attracted to the oxygen atom of another

  • forms a low energy bond between the two molecules

    • hydrogen bond

  • h attracted to other o, vice versa

cohesion and adhesion

  • cohesion

    • attraction of water molecules for each other

    • “cohorts”

  • adhesions

    • attraction of water molecules to solid surfaces

    • “adhere”

surface tension

  • liquid air interface surface tension results from the greater attraction of water molecules to each other than for the air

  • results in an inward force at the surface

    • water molecules act as if there was a stretched elastic membrane covering the surface

capillary action

  • due to two forces

    • attraction of water for the solid (adhesion)

    • surface tension of the water (due strongly to cohesion)

  • in glass tubes

    • water molecules are attracted to the glass (adhesion)

    • simultaneously cohesive forces hold water molecules together and create surface tension

      • causes a curved surface

    • lower pressure below meniscus

    • allows higher pressure in free tube to push water up

  • height of rise in capillary tube is inversely proportional to tube inside radius, r

  • with water at 20 degrees C, height, h can be found with:

capillary action in soils

  • capillary forces are at work in most soils

  • rate of movement and rise in height are less than you’d expect based on pore size alone

    • pores aren’t straight smooth tubes like glass

    • some pores are filled with air

      • could be entrapped air

      • slows down movement of water and decreases rise in height.

  • the pore size distribution that controls capillary rise

    • sands have abundance of medium to large sized pores

      • permits rapid capillary rise

      • limits ultimate height of rise

    • clays have abundance of small sized pores

      • takes a long time for water to rise up the small pores

      • ultimately rises to a height greater than in sands