Which Ions Cause Hardness in Water?
Chemistry Investigation
Which Ions Cause Hardness in Water?
Objective of Investigation
The objective of this investigation is to find out which ions out of the following cause hardness in water:
Na+
Cl-
Ca2+
Mg2+
K+
NO3-
SO42-
Fe2+
Prediction
Due to the fact that I have already learnt that hard water is caused by calcium ions and magnesium ions, such as in the case of temporary hard water being caused by calcium hydrogen carbonate, I am certain that the subsequent experiment will echo those facts. It would also be logical to assume that the ions that cause hard water would be linked, or be similar in some way. As they would obviously share the same properties in causing hard water, one would assume that they would share other properties. Apart from causing hard water, calcium and magnesium also share other properties. They are in the same group in the periodic table, and thus their properties are in fact very similar. As they are in the same group, then their ions will both have the same charge, in this case, 2+. This is the similarity that I would like to focus on for the moment, as it ties in with my next point.
Soap, or sodium stearate is used to soften hard water. It does this by means of a chemical reaction. Take a hypothetical sample of hard water. If the salt that is present in the water, and thus causing the hardness is calcium sulphate, the formula for the reaction which occurs when soap is added to the water would be as follows:
CaSO4 + 2C17H35COONa = Ca(C17H35COO)2 + Na2SO4
Calcium Sulphate + Sodium Stearate = Calcium Stearate + Sodium Sulphate
Sodium Stearate (Sodium Octadecanoate) = Soap
Calcium Stearate (Calcium Octadecanoate) = Scum
As this ...
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Soap, or sodium stearate is used to soften hard water. It does this by means of a chemical reaction. Take a hypothetical sample of hard water. If the salt that is present in the water, and thus causing the hardness is calcium sulphate, the formula for the reaction which occurs when soap is added to the water would be as follows:
CaSO4 + 2C17H35COONa = Ca(C17H35COO)2 + Na2SO4
Calcium Sulphate + Sodium Stearate = Calcium Stearate + Sodium Sulphate
Sodium Stearate (Sodium Octadecanoate) = Soap
Calcium Stearate (Calcium Octadecanoate) = Scum
As this is the reaction that softens hard water when it is caused by Calcium Sulphate, then this same formula should work for any other reaction between a hard water causing salt, and soap. Therefore, the ions that cause hard water must have a 2+ charge, as they must fit into the above equation. It is also obvious from looking at the equation that Sodium is not a cause of hard water, as it is an ion present in soap. Also, from work done on another method of softening hard water, ion exchange, I am certain that sodium ions do not cause hard water. On a basic level, ion exchange works by simply exchanging the ions that cause hard water (Calcium for example) for Sodium ions. As Sodium ions do not cause hard water, scum would no longer be formed if soap was added.
As Sodium is not a cause of hard water, and it is a positive ion with a 1+ charge, I would say that all other ions with a 1+ charge are not causes of hard water. Therefore I have so far predicted that ions which cause hard water will have a 2+ charge, and those ions with a 1+ charge are not causes of hard water. That means that I have predicted the effect that 5 of the ions I will be testing will have on the hardness of water. The 3 ions that I have not accounted for are all negatively charged. This means that they will not fit into the equation of the reaction between soap and the hard water causing salt. The principle of this reaction is that the hard water causing ion, a cation with a charge of 2 is replaced by 2 sodium ions, which although are also cations, they each only carry a 1+ charge. The ions literally swap places. So if the hard water causing salt was calcium sulphate, the calcium ion would be replaced by 2 sodium ions, and the calcioum ion would take the place of the sodium. Therefore the products of that reaction would be calcium stearate (scum) and sodium sulphate. The ion causing hard water forms a salt with the stearate ion, which is scum. The stearate ion is a anion (negative ion) with a charge of 1. When it forms soap with sodium it is in the ratio of 1 stearate ion to 1 sodium ion, as they both carry opposite charges to the value of 1. However, when the stearate ion forms scum with a cation that causes hard water, calcium for example, there are 2 stearate ions to one calcium, as calcium, and all hard water causing ions carry a positive charge of 1. Therefore in scum there must be 1 hard water causing ion to 2 stearate ions. Therefore it is obvious that hard water is not caused by anions, of any strength of charge, as they just simply won't form an ionic bond with another anion, in this case stearate.
Thus to summarise my prediction, I predict that out of the ions I was assigned to test, the following will cause hard water:
Mg2+
Ca2+
Fe2+
Basically, all cations with a charge of 2.
Plan
As I can obviously not test each ion for the property of causing hard water individually, ionic compounds consisting of two of the said ions shall have to be used. Obviously as each compound will be ionic, one ion will be a cation, and one will be an anion. As the method that is being used to determine whether or not an ion causes hard water is just simply whether or not soap will lather when added to a sample of a solution of an ionic compound, if the soap didn't lather, and formed a scum, it would be hard to determine which ion was the cause of hard water without prior knowledge. However,
