Chemistry C3 Hard Water

Notes on hard water

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Different parts of britian have different chemicals in their tap water due to the rocks that they pass over.

Some of these are soluble and will release ions into the water.

Examples of soluble salts that commonly get into our water supply are magnesium SULPHATE and calcium SULPHATE.

Water containing these salts are referred to as HARD WATER.

If water has no soluble salts in it, it is referred to as SOFT.

Hard water requires MORE SOAP.


Soft water lathers easier with soap.

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Another way hard water is formed.

Carbon dioxide from the atmosphere dissolves in water droplets.

This forms a weak acid.

Acidic raindrops fall on the rocks containing calcium carbonate.

The acid and calcium carbonate react to form calcium carbonate.

Calcium hydrocarbonate is soluble and will dissolve in the water.

The water is collected in a reservoir.

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Water becomes hard because it has come into contact with the CARBONATE rocks, which thus release the HYDROCARBONATE ION.

Water may have spend millions of years underground and has probable come into contact with several rocks.

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Limescale- heating reaction and soap reaction.

When hard water is heated it is broken down into carbon dioxide, water and the carbonate ion.

Calcium ions present in the water will combine with the carbonate ion to form calcium carbonate which is deposited as limescale.

The metal ions in the hard water react with the soap to form a solid precipitate 'scum'.

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Softening water

Using sodium carbonate- make the insoluble calcium carbonate form before it gets to the kettle.

Using ion exchange- replace the ions in the water with ones which don't cause insoluble precipitates.

Descaling- the calcium in calcium carbonate is converted into a soluble salt.

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