Investigating Hardness in Water

I want to find out which ions cause hardness in water. For instance, would Na+ or Cl- ions cause hardness, or would they have no effect.

The ions that I will use are:

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From preliminary testing I have learnt that Calcium and Magnesium ions both cause hardness in water, I believe that I could also carry out an experiment to prove this true. Due to what I have learnt I believe that there may be some connection between Calcium ions, Magnesium ions and the ions which cause hardness in water. Due to the fact that they both cause hardness in water they most likely have similar properties, this leads you to think that there is probably some other connection between them. This is reinforced by the fact that they are in the same group in the periodic table of elements. Due to the fact that they are in the same group they are both going to have the same charge, they both have a 2+ charge. I can also be certain that sodium ions do not cause hard water because in an ion exchange experiment I carried out before hand. This experiment works by replacing an ion that causes hard water with sodium ions. This means that if soap were added now a scum would no longer be formed, this would prove the water to be soft.

This means that sodium ions do not cause hard water and as they have a 1+ charge I would expect that all other ions with a 1+ charge would not cause hard water. This means that I have predicted the effect of five of the ions I will be testing. The other three have I have not accounted for have a negative charge. This means that they do not fit into the equation of the reaction between soap and the hard water causing salt. This means that ions with a negative charge will not have an effect and will not cause hard water.

To sum up, cations with a 2+ charge will cause hard water, cations with 1+ charge will not cause hard water and neither will anions with a 1- or 2- charge. This means that the ions I expect to cause hard water are Ca2+, Mg2+ and Fe2+.


Firstly I will measure out 6ml of the one compound (i.e., Sodium Chloride, Magnesium Sulphate) in a measuring cylinder then I will put it into a test tube. Once I have done this I will measure out 2ml of soap solution and add it to the compound. I will then shake the test tube to allow the compound and the soap solution to mix. Then I examine what the resulting solution looks like. From preliminary testing I have done, I know that if a scum is present the water is hard where as if a lather appears the water is soft. I will repeat the process seven times for the different compounds I will be using. As I cannot test each ion individually I will put them into compounds for ease of testing (e.g. Magnesium Chloride.) I will use Sodium Chloride, Calcium Chloride, Magnesium Chloride, Potassium Nitrate, Sodium Sulphate, Iron (II) Sulphate and Magnesium Sulphate. To ensure fair testing I will accurately measure the correct amount of 6ml compound, and 2ml soap solution. I will also ensure that I shake the test tube for approximately the same time for each compound as this will mean they all have roughly the same time to mix. If I do not make sure my tests are done accurately I may get false results. I will also need to ensure safety doing these experiments, as there are no immediate dangers although the compounds could be dangerous if bought into contact with eyes, so goggles should be worn.

Apparatus Used:

Test tube, compounds, soap solution, measuring cylinder (to measure compound), pipette (to measure soap solution) and beakers.


Test tubes, some show lather in top.

Obtaining & observing evidence:


I did not need to make many changes to my method because the experiment was not particularly difficult and I planned carefully beforehand to make sure that I did not encounter any problems. Due to preliminary work I knew rough quantities of what to add, this insured I got accurate results as the table below shows. Although the equipment used was not very accurate it did not really need to be as this was a simple experiment although, all measurements were carried out accurately and fairly. Sometimes it was slightly difficult to determine whether a scum was present or not as the water went a bit of a dull cloudy colour although in the ones that did produce scum it could be clearly seen that there were small bits floating around in the solution and it has turned quite cloudy.



Ions present

Lather (soft water)

Scum (hard water)

Sodium Chloride

Na+ Cl-



Calcium Chloride

Ca2+ 2Cl-



Magnesium Chloride

Mg2+ 2Cl-



Potassium Nitrate

K+ NO3-



Sodium Sulphate

2Na+ SO42-



Iron (II) Sulphate

Fe2+ SO42-



Magnesium Sulphate

Mg2+ SO42-



Analysing evidence:


The table in the results section shows that any compound with a 2+ ion in (i.e., Magnesium Chloride, Magnesium Sulphate, Iron Sulphate and Calcium Chloride) caused hardness in water. Relating back to what I was saying in the prediction, about the effect of the anions being cancelled out this shows how the compounds with the 1+ charge ions are related to soft water where as the 2+ charge ions cause hardness in water. This proves my prediction correct and is clearly explained in scientific reasoning earlier in the planning stage. This means that I would expect any other ion with a charge of 2+ to be related to the cause of hard water.

Ions associated with hard water Ions not associated with hard water

Mg2+ Na+

Fe2+ Cl-

Ca2+ K+



This is exactly how I predicted it to be for reasons explained above. However, these 2+ ions only cause temporary hardness and can be removed by boiling, which also sterilizes the water.

These results show my testing to be reasonably accurate and it has produced a good range of results.

Evaluating evidence:


The method I used to obtain my results was a safe and fair procedure. The results I obtained were accurate considering the simple methods I used to obtain them. I did not obtain any anomalous results and my other results seem accurate because they all follow a trend. I am especially pleased with the accuracy of my results due to the fact that quite a lot of the experiment was based on judgement. I had to judge whether a scum or lather was present by observing the solution. It would not have made a difference if I had used more accurate equipment because my results I obtained were accurate enough. However, if I were to have used more accurate equipment it may have been easier to judge whether a lather or a scum was present and therefore would decrease the chance for error, meaning an overall improvement in the experiment.

The amount of time I allowed the compound and the soap solution to mix also varied very slightly and this could have caused inaccuracies in the experiment. However, I do not think these inaccuracies would be sufficient enough to alter the final outcome of the experiment. This does not discard the fact that using a timer would have made it a fairer test. I could also have increased the number of ions I investigated as this would have broadened my range of results and would either, back up my prediction further or proved me wrong, although I see this as very unlikely due to the fact I have lots of scientific reasons and this experiment which fully back up my prediction.

My results clearly show the data in an appropriate manner and all the results are sufficient enough to draw a clear conclusion, which supports my prediction made in the planning stage. I am pleased with the outcome of the experiment and feel that my prediction was not only backed up by lots of scientific facts but was also supported fully by the experiment, and proved to be correct.