水污染 英文版.ppt

It covers 71% of the earth's surface and makes up 65 % of our bodies.
Without the seemingly invaluable compound comprised of hydrogen and oxygen, life on Earth would be non-existent
Water is Essential for Life
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Three fourths of the surface of the Earth is covered with water. Nearly 98% of the water on Earth, though, is seawater‑unfit for drinking and not even suitable for most industrial purposes. Something less than 2% of the water is frozen in the polar ice caps, leaving less than 1% available as fresh water.
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Water: Unusual Properties
Many chemical substances are soluble in water. Thus, they are easily dispersed and eventually are scattered to nearly infinite dilution in the ocean. Those properties of water that make it uniquely suited for the support of life also make it easy to pollute.
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Natural Water Isn’t All H2O
Rainwater carries dust particles from the atmosphere to the ground. Rainwater also dissolves a little oxygen, nitrogen, and carbon dioxide as it falls through the atmosphere. During electrical storms, lightning causes nitrogen, oxygen, and water vapor to combine to form nitric acid.
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As water moves along or beneath the surface of the Earth, it dissolves minerals and matter from decaying plants and animals.
The principal positive ions (cations) in natural water are sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), and sometimes iron (Fe2+ or Fe3+).
The negative ions (anions) are usually sulfate (SO42-), bicarbonate (HCO3-), and chloride (Cl-).
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hard water
Water containing these calcium, magnesium, and iron ions is called hard water.
The positive ions react with the negative ions in soap to form a scum that clings to clothes and leaves them dingy looking.
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Water pollution
Water pollution is any chemical, physical or biological change in the quality of water that has a harmful effect on any living thing
Water pollution caused by human activities is a