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Cation Formation

Loss of electrons results in an ion

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Drinkable Water

Drinkable Water

Credit: Jeff Hilton, US Navy
Source: http://commons.wikimedia.org/wiki/File:Balzam-class_general_intelligence_collector_ship_-_Ocean_Sarafi_85_-_DN-ST-86-02554.JPEG
License: CC BY-NC 3.0

Water, water, everywhere,

And all the boards did shrink;

Water, water, everywhere,

Nor any drop to drink.

These lines from the poem Rime of the Ancient Mariner by Coleridge describe the fate of a ship after one of the sailors has killed an albatross (believed to bring good luck). The ship was in the middle of a vast ocean full of water that the sailors could not drink because it was seriously contaminated with minerals.

News You Can Use

  • Over 70% of the earth’s surface is covered by water. However, only 2.55% of that water is fresh, available to humans for drinking and other uses. About 70% of the fresh water is unavailable since it is frozen in ice caps, in soil moisture, or inaccessible underground. Less than 1% of the fresh water on the earth is available for human consumption.
  • The major cation in sea water (not surprisingly) is sodium, present at a concentration of 10.6 grams/liter. Other cations include magnesium (1.3 g/L), calcium (0.4 g/L) and potassium (0.4 g/L).
  • A major modern approach to purifying sea water for use in drinking, watering crops, and other applications is reverse osmosis. In this process, sea water is forced under pressure through a semipermeable membrane. The membrane pore is large enough to allow water molecules to pass through while retaining the ions.
  • Credit: Official U.S. Navy Page
    Source: http://www.flickr.com/photos/usnavy/9250286164/
    License: CC BY-NC 3.0

    Reverse osmosis systems can be deployed to disaster areas to provide fresh drinking water [Figure2]


  • Watch a video about reverse osmosis at the link below:


Show What You Know

Use the links below to learn more about reverse osmosis. The answer the following questions.

  1. List the four components of a reverse osmosis plant.
  2. What are the major costs associated with reverse osmosis?
  3. What is a semipermeable membrane?
  4. How does ionic charge affect osmosis efficiency?
  5. If you have a system operating at 80% recovery rate, how many gallons will go to waste when 100 gallons of sea water are treated?


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