Water & pH A few more details…

Rain Water Normal rain water is not pH 7 as you might expect…

Rain Water …Instead, it is slightly acidic, usually around pH 5.6. Water vapor tends to combine with CO 2 in the atmosphere, forming carbonic acid: CO 2 + H 2 O  H 2 CO 3 But, carbonic acid is a weak acid, and partially dissociates: CO 2 + H 2 O  H + + HCO 3 -

Acid Rain

Acid rain refers to rain water that has a pH below 5.6! Water vapor in the atmosphere combines with pollutants like sulfur dioxide (SO 2 ) to form Sulfurous Acid: SO 2 + H 2 O  H 2 SO 3 …Which can also dissociate: H 2 SO 3  H + + HSO 3 -

Acid Rain Also, sulfur dioxide in pollution can be oxidized into sulfur trioxide: 2SO 2 + O 2  2SO 3 Sulfur trioxide then reacts with water to form sulfuric acid: SO 3 + H 2 O  H 2 SO 4 …which can donate lots of H + to rain water!

Acid Rain The same sorts of reactions can occur with other pollutants, like nitrogen oxide (NO 2 ): 2NO 2 + H 2 O  HNO 2 + HNO 3 …forming nitrous acid and nitric acid, both of which can further dissociate = more H +

Acid Rain The resulting low pH for rain water has many effects: –Increased leaching of important nutrients OUT of the soil. –Increased solubility of harmful metals (like aluminum) in soils. –Disrupted photosynthesis due to poor production of chlorophyll. –Killing of plants unable to secure nutrients. –Pollution of groundwater.

Isn’t there only 1 kind of water?

Tap Water “Government-treated water”. Contains many minerals, like Calcium & Magnesium. Contains certain amounts of Chloride or Fluoride (for “sanitation”). Often contains dissolved organic compounds. Can contain metals.

Spring Water From a “Spring” – a point where groundwater meets the surface. Minerals become dissolved in the water as it moves through underground rock. Generally have high amounts of dissolved oxygen. Have impurities.

Distilled Water Distilled water has gone through the distillation process in a still: –Water is boiled, and the re-condensed steam is collected as a liquid into a clean container Most contaminants are left behind.

Deionized Water Water is purified by going through an ion exchange process, where the ionic impurities are removed. –Non-ionic impurities (like organic compounds) remain. –Certain ions remain (OH - and H 3 O + ). What is this?

pH H2OH2O H+H+ OH - What we pictured happening:

pH What actually happens: These Hydronium ions are VERY reactive, and THEY are technically what is measured by pH. H 2 O +H 2 O  H 3 O + + OH - Hydronium Ion

Water