Chem 3 - Water - Hard Water

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Hard Water

Most of us in the developed world take it for granted that we have fresh, clean water piped to our homes. The water which comes out of our taps in different areas of the country may look very similar, but there are some very big chemical differences in it. These differences become obvious when we get washed.

When we wash ourselves with soap, the water in some areas of the country forms a really rich, thick lather easily. But in other parts of the country the water doesn't behave in this way. It is quite difficult to get the bubbles that the soap adverts promise us. This is because the water is hard.

Hard water not only makes it difficult to wash ourselves, it also makes it difficult to clean the bath or sink when we have finished. This is because hard water contains dissolved compounds which react with the soap to form scum. The scum floats on the water and sticks to the bath.

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Hard Water 2

Most hard water contains dissolved Calcium and Magnesium compounds. These dissolve when streams and rivers run over or through rocks containing Calcium and/or Magnesium compounds.

Limestone is an example of such rock. It contains Calcium Carbonate. As raindrops fall through the air, Carbon Dioxide dissolves in them. This dissolved Carbon Dioxide makes rain slightly acidic, even without polutants like oxides of Sulfur and Nitrogen. The water in streams and rivers is therefore slightly acidic too. This means that compounds such as Calcium Carbonate react and the products formed dissolve in water.

The dissolved minerals are carried into the reservoirs and on into our domestic water supply. It is the dissolved Calcium and Magnesium ions that react with soap to form scum.

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Using hard water is expensive because we need to use much more soap. Before soap ever gets anywhere near dirt it first reacts with the dissolved Calcium and Magnesium ions in the water, forming salts called stearates (the chemical name for scum). It is only when all of the Calcium and Magnesium ions have reacted with the soap that a lather can begin to form.

Sodium stearate (Soap) + Ca2+ and Mg2+ ions --> Calcium Stearate, Magnesium Stearate (Scum) + Na+ ions soluble in water.

As well as forming scum with soap, hard water often leads to scale forming in pipes, immersion heaters and other parts of our hot water systems. Pipes can eventually blcok up. The same scale forms in our kettles, 'furring up' the heating elements and making them much less efficient, because scale is a poor conductor of heat.

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But hard water isn't all bad news. The same dissolved compounds which are bad for our water pipes seem to be good for our health. Calcium ions in drinking water help in the development of strong bones and teeth. There is also evidence which suggests that hard water helps to reduce the incidence of heart disease of people who drink it.

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