Green Chemistry and the Ten Commandments

Gas solubilities decrease with increasing temperature. You can see evidence of this
by heating water in a pan and observing the air bubbles that come out of the water while
it is warming, but still substantially below the boiling temperature.

7.7. (Weak) Acid from the Sky

Acids and bases and their reactions are of the utmost importance in water and
are involved with many aquatic chemical phenomena. An excess of either can be very
detrimental to water quality. An acid is a substance that contains or produces H+ ion in
water, whereas a base is a substance that accepts H

• ion in water or contains or produces
  hydroxide ion, OH

-

. Whether water is acidic or basic is expressed by a parameter called
pH. It is defined as

pH = -log [H+] (7.7.1)

where [H+] is the molar concentration of H+ in water, that is, the number of moles of
this ion per liter of water. Several pH values corresponding to various concentrations of
H+ are given below:

[H_________________________+], mol/L log[H+] pH

0.100 -1.00 1.00

1.00 × 10

- 3

-3.00 3.00

1.00 × 10

- 5

-5.00 5.00

1.00 × 10

- 9

_____-9.00 9.0^0

The value of [H+] in pure water at 25 ̊C is 1.00 × 10

- 7

mol/L and the pH is 7.00.
Such water is said to be neutral, neither acidic nor basic. Water with a pH less than 7.00
is acidic, whereas water with a pH greater than 7.00 is basic.
The average global concentration of CO 2 gas in air in the year 2005 is about 380
parts per million by volume, and going up by about 1 ppm per year. The concentration of
dissolved carbon dioxide, [CO 2 (aq)], in water in equilibrium with 380 ppm atmospheric
air at 25 ̊C is 1.24 × 10 -^5 mol/L. This low concentration is enough to make the water
slightly acidic because the dissolved carbon dioxide reacts with water as shown by the
following reaction,

CO 2 + H 2 O ←→ H

+

+ HCO 3

-

(7.7.2)

to a sufficient extent to make [H

• ] = 2.4 × 10 -^6 mol/L. This corrresponds to a pH of
  5.6. This pH is lower than the neutral pH of 7, and the water is slightly acidic. Since
  unpolluted rainfall formed in the atmosphere is in equilibrium with atmospheric CO 2 , it
  is slightly acidic.

Chap. 7. Water, The Ultimate Green Solvent 169