LaMotte ACID RAIN STUDY OUTFIT ARO - Manual (Page 4)

RAIN
Normal rain may have a pH as low as 5.6 due to the absorption of small amounts of
carbon dioxide (CO
2
) in the atmosphere. The carbon dioxide reacts with rainwater
to form carbonic acid (H
2
CO
3
). The pH of the rain is lowered because a small
portion (about 10% or less) of the carbonic acid dissociates into bicarbonate
(HCO
3
- ) and hydrogen ions (H + ):
H
2
O + CO
2
<=> H
2
CO
3
H
2
CO
3
<=> HCO
3
- + H +
Carbonic acid is a weak acid which can be easily neutralized by the buffering
substances found in streams, lakes, and soil.
ACID RAIN
Precipitation having a pH less than 5.6 is considered to be abnormally acidic. A pH
of less than 5 indicates the presence of strong acids in addition to carbonic acid.
CAUSES OF ACID RAIN
Acid rain is formed from air pollutants, sulfur dioxide and nitrogen oxides, which are
released as gases into the atmosphere during the refining of metal ores and the
burning of fossil fuels such as coal, heating oil and gasoline. Power plants, smelters,
automobiles, and even volcanic activity can contribute to this problem. These gases
may travel miles from their sources. Eventually, they combine with moisture in the
atmosphere to form sulfuric acid and nitric acid solutions which fall to earth in rain,
snow, dew, fog, frost, and mist.
SUL FUR DI OX IDE (SO
2
)
When fuel or coal that contains sulfur is burned, the sulfur compounds react with
oxygen during combustion to form sulfur dioxide. Sulfur dioxide is also produced
when sulfite ores such as sphalerite (ZnS), pyrite (FeS
2
), and chalcocite (Cu
2
S) are
roasted or heated in air during the production of the metals zinc, iron, and copper.
Sulfur dioxide also occurs in volcanic gases. The “rotten egg” odor of burning sulfur
is due to sulfur dioxide.
4
GLOS SARY OF TERMS
Absorption - The addition of one substance through the surface of another.
Acid - A compound that donates a hydrogen ion (H + ).
Acid Deposition - The depositing of acidic material from the atmosphere as gases,
particles, rain, snow, or fog.
Acidic - The degree or level of the acid content of a substance. An acidic substance
is below 7.0 on the pH scale.
Algae - Simple rootless plants that grow in sunlit waters in proportion to the
amounts of nutrients available. Algae have chlorophyll which is used to convert
solar energy (sunlight) to chemical energy. They are food for fish and small aquatic
animals.
Akalinity - A measure of the capacity of water to neutralize acids.
Aquatic - Living or growing in a water environment.
Bacteria - Microscopic living organisms which help break down complex substances,
such as dead animal and plant matter, by decay, and convert these substances to
simpler forms.
Base - A compound that accepts a hydrogen ion (H + ).
Basic - A basic substance is above 7.0 on the pH scale.
Buffer - A substance that resists pH changes when small amounts of acid or base are
added.
Carbon Dioxide (CO
2
) - A colorless, odorless, non-poisonous gas, which is a normal
part of the ambient air.
Corrosion - The deterioration of metal parts slowly eaten away by acid solutions or
acid water supplies.
Detritus - Dead plant and animal material.
Fungus - A primitive groups of organisms that lack chlorophyll; includes the
mushrooms, molds, mildews, yeasts, rusts and smuts which live primarily on dead or
living organic matter.
Groundwater
- Water that has soaked into the ground; often used to supply wells.
Hydroxyl Ion (OH - ) - A negatively charged particle containing an oxygen atom
and a hydrogen atom. See pH.
Hydrogen Ion (H + ) - A positively charged particle containing only a hydrogen
atom. See pH.
Indicator Solution - A liquid containing a chemical compound added to a test
sample to bring about a color reaction to show the presence or absence of a
particular substance.
Leaching - The removal of dissolved chemical compounds by the passage of water
through soil.
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