Biology 107 Water September 1, 2003

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 (H 2 O) 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.

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. Water’s 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).

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 - ) 4.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. 5.A base is any substance that accepts H + ions when dissolved in water.

Water Molecule Dipoles

Hydrogen Bonding of Water Molecules

Hydrogen Bonding of Water Molecules to Other Molecules

Water Adhesion and Cohesion

Water as a Polar Solvent

Why Ice Floats

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 +