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)

(Information taken from Complete Chemistry Book-Oxford University.)

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.

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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:

Compound

Ions

Lather (soft)

Scum (Hard)

Potassium nitrate

K+ NO3-

Yes

Iron Sulphate

Fe2+ SO42-

Yes

Magnesium Chloride

Mg2+ 2Cl-

Yes

Calcium Chloride

Ca2+ Cl2

Yes

Sodium Carbonate

Na2+

Yes

Sodium Chloride

Na+ Cl-

Yes

Magnesium Sulphate

Mg2+ SO42-

Yes

Analysis: The results show that my prediction is correct, as only the compounds consisting of cations with a charge of two, caused scum to be formed. Compounds with a cation charge of one were only present in soft water; anions were present in both hard and soft water, which by observation means that they have no affect. So by the evidence claimed only compounds with a cation charge of two produce scum.

Evaluation: The results I obtained were all correct; there were no signs of anomalous results. As there were only two possible outcomes this made it easier to get the right result. I did each experiment twice to make sure that nothing could have gone wrong, to improve the experiment I could have used equipment that can measure the amount of scum or lather produced; also I could look into the effect of temporary hard water.