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answers: Investigating temporary and permanent hard water

What you should have seen:

Water sample Before heating After heating

 

calcium hydrogencarbonate

 

 hardly any lather formed, greyish scum appeared on surface of water lots of lather and bubbles that appeared really quickly

 

calcium sulphate

 

 hardly any lather formed, greyish scum appeared on surface of water no change in the amount of lather

So what was happening?

Boiling makes temporary hard water become soft water. Temporary hard water contains the compound calcium hydrogencarbonate. When temporary hard water is heated it breaks down to form insoluble calcium carbonate. This is the white scale, or fur you see on the inside of kettles. Using heat to break down a substance is called thermal decomposition.

All the calcium hydrogencarbonate has been removed from the water, so it is now soft water. A word equation for the thermal decomposition of temporary hard water is:

calcium hydrogencarbonate = calcium carbonate + carbon dioxide + water

Permanent hard water contains calcium sulphate which is not broken down by heat. It stays in the water, so the water stays hard.

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