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answers: Investigating Hard Water

What you should have seen:

So what was happening?

If calcium and magnesium are in water, this makes it hard. If they are not in water, then they can't react with soap to form scum. The scum is called calcium stearate, so if calcium is missing, no calcium stearate can form.

Rain water is a good choice for making up your hard and soft water samples because when water falls as rain it is soft water. The calcium and magnesium are collected after the rain falls on the ground. Water becomes hard once it falls on limestone and chalk rocks.

If magnesium is in the water, instead of an insoluble scum of calcium stearate another insoluble salt is formed. A word equation for this reaction is:

magnesium sulphate + sodium stearate (soap) = magnesium stearate (scum) + sodium sulphate