Biology 107Biology 107

WaterWater

September 2, 2002September 2, 2002

WaterWaterStudent Objectives:Student Objectives: As a result of this lecture and the assigned As a result of this lecture and the assigned

reading, you should understand the following:reading, you should understand the following:

1.1. For all living organisms, life depends upon water, and living things are For all living organisms, life depends upon water, and living things are made up mostly of water molecules (Hmade up mostly of water molecules (H22O)O)

2.2. The dipoles produced by the polar covalent chemical bonds between the The dipoles produced by the polar covalent chemical bonds between the hydrogens and oxygen of the water molecule allow for hydrogen bonding hydrogens and oxygen of the water molecule allow for hydrogen bonding between water molecules, and the properties of water are related to these between water molecules, and the properties of water are related to these intermolecular bonds.intermolecular bonds.

Water molecules "stick" to each other,Water molecules "stick" to each other, cohesion cohesion, and to , and to charged charged substances, substances, adhesionadhesion. The cohesive and adhesive . The cohesive and adhesive properties of properties of water allow it to rise against the force of gravity water allow it to rise against the force of gravity through capillarity.through capillarity.

WaterWater

Related to cohesion is the property of surface tension - a Related to cohesion is the property of surface tension - a measure of how difficult it is to stretch or break the surface measure of how difficult it is to stretch or break the surface

of a of a liquid.liquid.

Water’s hydrogen bonds moderate temperature and Water’s hydrogen bonds moderate temperature and decrease decrease water evaporation and vaporization.water evaporation and vaporization.

Ice is less dense than liquid water.Ice is less dense than liquid water.

Water's versatility as a solvent results from the polarity of its Water's versatility as a solvent results from the polarity of its molecules; substances that dissolve in water are called molecules; substances that dissolve in water are called

hydrophilichydrophilic (i.e., water-liking), while those that do not dissolve (i.e., water-liking), while those that do not dissolve in in water are water are hydrophobichydrophobic (i.e., water-hating). (i.e., water-hating).

Water3.3. A certain proportion of the water molecules dissociate into ions A certain proportion of the water molecules dissociate into ions

because of their polarity. The two ions formed in the chemical because of their polarity. The two ions formed in the chemical reaction that dissociates water are the hydrogen cation (Hreaction that dissociates water are the hydrogen cation (H++), which ), which is simply a proton, and the hydroxide anion (OHis simply a proton, and the hydroxide anion (OH--))

4.4. Some substances when dissolved in water dissociate and release Some substances when dissolved in water dissociate and release HH++ ions. By definition, these substances that release H ions. By definition, these substances that release H++ ions when ions when dissolved in water are called dissolved in water are called acidsacids. Acids increase the H. Acids increase the H++ concentration of the solution. That is, concentration of the solution. That is, the more acidic a solution, the the more acidic a solution, the higher its concentration of Hhigher its concentration of H+ + ions, and the lower its ions, and the lower its pHpH. .

5.5. A A basebase is any substance that accepts H is any substance that accepts H++ ions when dissolved in ions when dissolved in water.water.

Water Molecule DipolesWater Molecule Dipoles

Hydrogen Bonding of Water MoleculesHydrogen Bonding of Water Molecules

Hydrogen Bonding of Water Molecules to Other Hydrogen Bonding of Water Molecules to Other MoleculesMolecules

Water Adhesion and Cohesion

Water as a Polar SolventWater as a Polar Solvent

Water as a Polar Solvent

Water as a Polar SolventWater as a Polar Solvent

Water as a Polar SolventWater as a Polar Solvent

Why Ice FloatsWhy Ice Floats

pH ScalepH Scale

pH measures the hydrogen pH measures the hydrogen ion concentration of a solutionion concentration of a solution

The higher the HThe higher the H+ +

concentration the lower the pH concentration the lower the pH numbernumber

AcidsAcids donate H donate H+ + , while , while basesbases accept Haccept H+ +