An investigation of the effect of concentration of a chosen ion on the ''Degree of Hardness'' of water.

Authors Avatar

An investigation of the effect of concentration of a chosen ion on the ‘‘Degree of Hardness’’ of water

Preliminary Work

I have used a previous experiment that I carried out, to help inform my plan. It is called Investigation the ions, which cause hardness of water and it, will be included in my appendix.

In this experiment there were 14 different substances dissolved in distilled water. 4cm³ of each one was put in a separate test tube. 30 drops of soap were then added to the solution. A bung was placed in the test tube, and the test tube was shaken. This shaking was done until either a scum or lather formed.

This experiment has helped to inform my plan because it showed which metal ions I could have used. I told me that both Magnesium and Calcium ions caused a scum to form in hard water and that I could use either in my investigation. Because of this I chose calcium, as it was readily available to me.

At the beginning of my coursework I will take different volumes of Calcium chloride solution and add enough soap to form a later for 10 seconds. These volumes will be 20cm³, 30cm³ and 40cm³.

This will allow be to see what is the best overall concentration to use in my experiment. Because if one needs more that 50cm³ (more than a burette can hold) it cannot be used as it is immeasurable. And if another volume gives a low result is cannot be used. This is because this will need the highest amount of soap and if it gives very small results they will all be very low and too close together. And if it is immeasurable then it is no use.


I predict that as the concentration of calcium ions increase; the amount of soap needed to form a lather for 10 seconds will also need to be increased.

Also I predict that if I double the number of calcium ions, I will also need to double the number of stearate (soap) ions.

E.g. Ca  &  2St

     2Ca  &  4St

     4Ca  &  8St

Each time both amounts are doubled. If this was put into a graph of volume of soap used against concentration of calcium, it would be a straight line through the origin. However I predict the line to start slightly higher on the y-axis.


Degree of hardness- this is an amount of mineral (either Ca or Mg Bicarbonates or Ca and Mg Sulphates or Chlorides) in water. The greater the amount of mineral in the water, the larger the degree of hardness.

Join now!

When soap is added to calcium the equation is:

Ca + 2St → CaSt

This equation shows that all the calcium ions need to react to form a lather. If we then increase the number of calcium ions. More soap will be needed to react with the increased amount.

The reaction is a precipitate reaction. This precipitate is insoluble. This also shows why my hypothesis should be true.

The equation for this experiment is:

2NaSt + Ca → CaSt +2Na

In my hypothesis I stated that the graph would have a line of best fit, which starts slightly ...

This is a preview of the whole essay