Acids are an important group of chemicals.
All acids contain chemicals that are combined with hydrogen
in a specific way. Acids are present in aspirin, carbonated
drinks, and your stomach.
Another group of chemicals is called
bases. They contain oxygen and hydrogen that are
combined with other chemicals. Chemicals containing bases
are called alkaline. Examples of alkaline substances
are deodorants, ammonia cleaners, and the mortar around
Liquids are sometimes measured in terms
of acidity or alkalinity. A scale called the
pH scale is used for this measurement. The scale
ranges from zero to 14. Anything with a pH below 7 is considered
acidic. The lower the pH, the more acidic it is. Substances
above 7 are alkaline. The higher the pH, the more alkaline
it is. Liquids that are very acidic or alkaline are dangerous
to handle. A substance that is neither acidic or alkaline
would have a pH of 7. Absolutely pure water would have a
pH of 7.
Normal rainwater has a pH of around
5.6. This is because some carbon dioxide in the atmosphere
combines with atmospheric water to form a weak acid. The
term acid rain is used when rainwater becomes more
Acid precipitation occurs when water
(rain, snow, sleet and fog) combines with pollutants containing
sulfur or nitrogen. This may bring the pH down to between
4 and 5.
Many laws and regulations have been
created in United States and other countries to cut down
the amount of sulfur and nitrogen that is released into
the air by human activity. This seems to already be helping
to raise the pH of precipitation back to normal. An Illinois
State Water Survey study has shown that the amount of sulfur
in precipitation has decreased the last few years over Illinois
and much of the Northeastern United States.