1. Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other. Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other. –The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H + ). –The water molecule that lost a proton is now a hydroxide ion (OH - ). Dissociation of Water Molecules

2. A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion: A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion: –H 2 O H + + OH - This reaction is reversible. This reaction is reversible. At equilibrium the concentration of water molecules greatly exceeds that of H + and OH -. At equilibrium the concentration of water molecules greatly exceeds that of H + and OH -. In pure water only one water molecule in every 554 million is dissociated. In pure water only one water molecule in every 554 million is dissociated.

3. Acids and Bases An acid is a substance that increases the hydrogen ion (H+) concentration in a solution. An acid is a substance that increases the hydrogen ion (H+) concentration in a solution. Any substance that reduces the hydrogen ion concentration in a solution is a base. Any substance that reduces the hydrogen ion concentration in a solution is a base. –Some bases reduce H + directly by accepting hydrogen ions. Strong acids and bases completely dissociate in water. Strong acids and bases completely dissociate in water. Weak acids and bases dissociate only partially and reversibly. Weak acids and bases dissociate only partially and reversibly.

4. pH Scale Measures the degree of acidity (0 – 14) Measures the degree of acidity (0 – 14) Most biologic fluids are in the pH range from 6 – 8 Most biologic fluids are in the pH range from 6 – 8 Each pH unit represents a tenfold difference (scale is logarithmic) Each pH unit represents a tenfold difference (scale is logarithmic) –A small change in pH actually indicates a substantial change in H + and OH - concentrations. The pH scale in any aqueous solution : –[ H + ] [OH - ] = 10 -14

5. Neutralizing an Acid or Base A solution with a pH of 7 is considered neutral A solution with a pH of 7 is considered neutral If a solution is considered acidic it can be neutralized by an equally basic solution If a solution is considered acidic it can be neutralized by an equally basic solution –ie solution with pH of 4 can be neutralized by a solution with a pH of 10 Acid/Base neutralization results in the formation of a salt Acid/Base neutralization results in the formation of a salt

6. Acid Precipitation Rain, snow or fog with more strongly acidic than pH of 5.6 Rain, snow or fog with more strongly acidic than pH of 5.6 West Virginia has recorded 1.5 West Virginia has recorded 1.5 East Tennessee reported 4.2 in 2000 East Tennessee reported 4.2 in 2000 Occurs when sulfur oxides and nitrogen oxides react with water in the atmosphere Occurs when sulfur oxides and nitrogen oxides react with water in the atmosphere –Lowers pH of soil which affects mineral solubility – decline of forests –Lower pH of lakes and ponds – In the Western Adirondack Mountains, there are lakes with a pH <5 that have no fish.