Biology 107 Water

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Biology 107Water

• September 2, 2002

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Water

• Student Objectives As a result of this lecture
  and the assigned reading, you should understand
  the following
• 1. For all living organisms, life depends upon
  water, and living things are made up mostly of
  water molecules (H2O)
• 2. The dipoles produced by the polar covalent
  chemical bonds between the hydrogens and oxygen
  of the water molecule allow for hydrogen bonding
  between water molecules, and the properties of
  water are related to these intermolecular bonds.
• Water molecules "stick" to each other,
  cohesion, and to charged substances, adhesion.
  The cohesive and adhesive properties of water
  allow it to rise against the force of gravity
  through capillarity.

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Water

• Related to cohesion is the property of surface
  tension - a measure of how difficult it is to
  stretch or break the surface of a liquid.
• Waters hydrogen bonds moderate temperature and
  decrease water evaporation and vaporization.
• Ice is less dense than liquid water.
• Water's versatility as a solvent results from
  the polarity of its molecules substances that
  dissolve in water are called hydrophilic (i.e.,
  water-liking), while those that do not dissolve
  in water are hydrophobic (i.e., water-hating).

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Water

• 3. A certain proportion of the water molecules
  dissociate into ions because of their polarity.
  The two ions formed in the chemical reaction that
  dissociates water are the hydrogen cation (H),
  which is simply a proton, and the hydroxide anion
  (OH-)
• Some substances when dissolved in water
  dissociate and release H ions. By definition,
  these substances that release H ions when
  dissolved in water are called acids. Acids
  increase the H concentration of the solution.
  That is, the more acidic a solution, the higher
  its concentration of H ions, and the lower its
  pH.
• A base is any substance that accepts H ions when
  dissolved in water.

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Water Molecule Dipoles
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Hydrogen Bonding of Water Molecules
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Hydrogen Bonding of Water Molecules to Other
Molecules
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Water Adhesion and Cohesion
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Water as a Polar Solvent
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Water as a Polar Solvent
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Water as a Polar Solvent
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Water as a Polar Solvent
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Why Ice Floats
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pH Scale
pH measures the hydrogen ion concentration of a
solution The higher the H concentration the
lower the pH number Acids donate H , while
bases accept H