C3 AQA - Softening Hard Water Revision Notes

Concise revision notes on softening hard water.

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  • Created on: 14-06-11 13:00
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Water ­ Key Notes CHEMISTRY UNIT 3
KEY FACT: Burning fuels in engines produces other substances and pollutants including particulates ­
these are tiny particles of unburnt hydrocarbons. These travel into the upper atmosphere, reflecting
sunlight back into space, causing GLOBAL DIMMING.
Water Purification
1. Water enters treatment works, and passes through screens, which catch large objects.
2. Settlement tank ­ sand and soil settle out.
3. Water passed through special filter made of fine sand to remove remaining particles.
4. Small amount of chlorine added to kill any bacteria in water.
5. pH of water checked and corrected so it is neutral. It is then stored in tanks and reservoirs
to be pumped to homes, schools, factories, etc.
Softening Hard Water
METHOD 1: WASHING SODA
1. Add Sodium carbonate (washing soda).
2. This precipitates out calcium and magnesium ions as insoluble carbonates.
3. Once these ions are no longer in solution, they can't react with soap, and so water is soft.
Ca (2+) + CO3 (2-) -> CaCO3
METHOD 2: ION-EXCHANGE COLUMN
1. Pass water through columns.
2. Columns contain sodium ions, which are exchanged for calcium and magnesium ions in
harder water when it passes through the column.
3. Once all of the sodium ions have been exchanged the resin is washed with salt solution to
exchange ions for sodium.

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