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What causes the temporary and permanent hardness of water?

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Hint: Many soaps form insoluble scum when dissolved in water. A layer of white salt is observed on the surface of utensils when water dries up. This happens due to the hardness of water. The presence of mineral chlorides makes the water hard and unfit for use.

Complete step by step answer:
Let’s look at the answer of the given question:
First we will define hardness of water:
Hardness of water is defined as the very high content of minerals present in the water. It is formed when water passes through rocks made of limestone, chalk or gypsum.

Next is temporary hardness:
Temporary hardness of water is caused when thermally unstable chemicals like magnesium hydrogen carbonate and calcium hydrogen carbonate are dissolved in water. Temporary hardness is treated by heating the hard water. When the water is heated the calcium and magnesium hydrogen carbonates gets precipitated out and then it can be separated by physical means.

Now, we will define permanent hardness
Permanent hardness of water is caused when chlorides and sulfides of calcium and magnesium are dissolved in water. It is known as permanent hardness because it cannot be removed by boiling. To remove permanent hardness the hard water is treated with washing soda.

Hence, chlorides of Ca and Mg cause permanent hardness while bicarbonates of Ca and Mg cause temporary hardness.

Note: Students can get confused between hard water and heavy water. Heavy water contains deuterium instead of hydrogen atoms.
The hardness of water is generally expressed in parts per million.
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