Which ions cause hardness in water?

Aim: I want to find out what ions cause hardness in water and the reasons as to why certain ions have an effect and others don’t.Background information: Calcium hydrogen carbonate is the main cause of hard water. It forms when rain falls on rocks of limestone and chalk. These are made of calcium carbonate, which is not soluble in water. But rainwater contains carbon dioxide dissolved from the air, which makes it acidic. So it reacts with the rocks to form calcium hydrogen carbonate which is soluble and ends up in our taps.This reaction is:H20(l) + C02(g) + CaC03 – Ca(HCO3)2(aq)Water that contains salts of calcium and magnesium principally as bicarbonates, chlorides, and sulphates. Ferrous iron may also be present; oxidized to the ferric form, it appears as a reddish brown stain on washed fabrics and enamelled surfaces. Water hardness that is caused by calcium bicarbonate is known as temporary, because boiling converts the bicarbonate to the insoluble carbonate; hardness from the other salts is called permanent.Calcium and magnesium ions in hard water react with the higher fatty acids of soap to form an insoluble gelatinous curd, thereby causing a waste of the soap. This objectionable reaction does not take place with modern detergents.(Britannica 2001)Prediction: I already know that hardness in water is caused by calcium and magnesium ions which both have a 2+ charge. I think that this will in some way correspond with the experiment I am doing.I also know that soap can be used to soften hard water, the chemical equation for this is: CaSO4+ 2C17H35COONa=Ca (C17H35COO)2+Na2SO4(Calcium Sulphate+ Sodium Stearate= Calcium Stearate+ Sodium Sulphate)I think that the ions most likely to cause hardness are probably the 2+ ions because the cations seem to be the only thing that is having an affect on the situation.Method: The apparatus I will need to use is minimum, a test-tube, pipette and a measuring cylinder.I have to measure 6 ml of the chosen compound e.g. iron sulphate, to do this I will need to use a pipette to make sure that the measurements are very accurate. I need to measure 2 ml of Sodium Stearate (soap solution), which is to be added. Next I will put the solution in a test tube followed by the soap solution, when both compounds are in the test tube I will shake it so that the two solutions will mix together at a faster rate. Finally I will decide if it has lathered or whether scum has been produced and note this down.To make sure that it is a fair test I will need to make accurate and precise measurements each time. Always wash out the apparatus to make sure there are no traces from the last experiment as this could produce an inaccurate result. To ensure that the experiment is a success I will do each one twice.Table of results:CompoundIonsLather (soft)Scum (Hard)Potassium nitrateK+ NO3-YesIron SulphateFe2+ SO42-Yes