An investigation of the effect of concentration of a chosen ion on the ‘‘Degree of Hardness’’ of water
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.
This is a preview of the whole essay
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 above the y-axis. This is because even if there is no concentration of Calcium. So it is just distilled water. Soap will still be needed to form a lather. As there are more calcium ions in solution, more stearate ions are needed to form calcium stearate.
Soap is a cleaning agent. The most common type of soap is sodium stearate. Soap has a long hydrocarbon chain, this allows it to wash grease and a polar head that is water-soluble. It has a hydrophilic head but a hydrophobic tail.
Hydrophilic means it is drawn to water or wants to be in contact with water and hydrophobic means it doesn’t want to come into contact with water. This allows the soap to wash most things. When soap is mixed with hard water the ions react to form a precipitate, which is also known as a scum. In my experiment I want to form a lather, so more soap will be needed then there is calcium.
The independent variable is the concentration of calcium ions. Adding distilled water, to form other concentrations, can change this.
The dependant variable is the amount of soap needed to form a lather (in cm³). A lather is one that covers the surface of the solution for a minimum of 10 seconds.
Key variables that need to be controlled are the amount of shaking used. Shaking one result more than another could cause a lather to form quicker, than if it was shaken the same amount as the rest.
The temperature in which the experiment takes place needs to be controlled. Because it may have on the solubility and therefore make my results less accurate.
The total volume needs to be controlled to make it a fair test. This will be decided by doing preliminary work.
Concentration of the soap must be kept the same. Using the same soap solution for all my results can control this.
Safety glasses(-This was chosen for safety, and to stop any solution from entering and harming the eye.)
Conical flask(-This was chosen over a beaker, because a bung could be placed in the top of it.)
Burette(-This was chosen for accuracy as it accurate to ± 0.1cm³.)
Measuring cylinders of various sizes(- Various sizes of measuring cylinders were used to ensure the greatest accuracy. Small volumes can be measured in a small measuring cylinder. Because when large volume are measured in the a small cylinder, the cylinder is filled more than once. This causes any inaccuracies to double or treble etc. This makes it better to use a large measuring cylinder for large volumes.)
Distilled water(-This is pure water, and can therefore be used to clean apparatus and mix to form concentrations without contaminating them.)
Bung(-This stops solution from escaping out of the conical flask. )
0.01M Calcium chloride solution
Safety glasses were put on. An empty conical flask was taken and filled with 20cm³ of 0.01M calcium chloride solution. The calcium chloride solution was measured in a measuring cylinder, which can contain a total volume of 20cm³. The 20cm³ was measured to the bottom of the meniscus in the measuring cylinder. This gives more accuracy. This was then poured into a conical flask. A conical flask was used because a bung can be put into it. A burette was then clamped to a clamp stand. It was then filled with soap solution. The tap was turned to allow to some soap solution to run through. This got rid of any trapped air. A burette was used because it gave results accurate to ± 0.1cm³, which gives greater accuracy. The reading on the burette was then measured to the bottom of the meniscus. The conical flask was placed under the burette and the tap was opened until a slight lather formed on the surface of the calcium chloride solution. A bung was then placed on the conical flask, and it was shaken 5 times. The amount of shaking was controlled, so the conical flask was only shaken 5 times, every time it needed to be shaken. This gave greater accuracy. If a lather lasted for 30 seconds or more, the amount of soap used was worked out and recorded. If a lather didn’t form, or it didn’t last for more than 30 seconds, more soap was added drop by drop, and then it was shook 5 times. This was repeated until a lather that lasted for 30 seconds was formed. This was then repeated to give another result for this concentration of calcium chloride solution. This can then used to work out an average, which gives more accuracy.
A new concentration of calcium chloride solution was made. The original solution was made by dissolving calcium chloride ions in distilled water to form a solution of 0.01M. 2cm³ of this solution was measured in a measuring cylinder of total volume 5cm³. 18cm³ of distilled water was then added to form a solution with a concentration of 0.001M. The directions above were then followed to get two results of how much soap was needed to form a lather for 30 seconds or more.
This was repeated for concentrations of 0.002M, 0.003M, 0.004M, 0.005M, 0.006M, 0.007M, 0.008M and 0.009M.
The concentrations were made using the general formula:
Concentration= volume of calcium chloride solution x initial concentration
total volume of final solution
After taking these results I have chosen 40cm³ to use as my total volume for reasons given in my plan.
Results in italics indicate anomalies and will not be included in the graph.
From the results we can see that as the concentration of calcium ions increase, more soap is needed to form a lather for 10 seconds.
The graph shows that there is a linear relationship between concentration of calcium ions, and the amount of soap needed to form a lather for 30 seconds.
See graph sheet
I will be taking readings of the line of best fit.
Difference of soap used between 0.004M and 0.003M
35.8cm³ - 31.9cm³ = 3.9cm³
Difference of soap used between 0.005M and 0.004M
39.8cm³ - 35.8cm³ = 4cm³
This shows that there is linear relationship between concentration of calcium ions, and the amount of soap needed to form a lather for 30 seconds, and also that the amount of soap needed increases by nearly the same amount every time.
The results and graph show that concentration does have an affect on the volume of soap needed. The graph shows a linear relationship but one that does not start from the origin. This is because if there is no calcium chloride and the concentration is 0.00M (distilled water) than soap will still be needed to form a lather. This is why a line of best fit starts above the origin because no calcium ions does not mean no sodium stearate ions. As the concentration of calcium chloride increases there are more calcium ions in the solution. So more sodium stearate ions are needed to form calcium stearate.
The graph also shows a degree of proportionality between the volume of soap and concentration. The working above shows that there is nearly the same amount of decrease and increase either way of the 0.004M concentration value.
The prediction is well supported because all the point lies on or very close to the line of best fit. And the line of best fit is how I had described it in my plan.
The accuracy of the method was good but could be improved.
As shown in the results table, anomalies occurred and these have been in italics in the results table. Other than these anomalous results, the other data obtained is good and useful.
A way of improving my experiment would be to use a stopwatch to time the 10 seconds that the lather has to form for. Counting the seconds in my head is not very accurate and this is a way of improving it.
Different apparatus could also be used such as a burette marked to a higher degree of accuracy, and a volumetric pipette. A machine could also be used to control the shaking because when I did it, it was not very accurate. The machine could be set a set number of shakes, which would all be the same. This would improve the accuracy of the experiment.
The burette is accurate to ± 0.1cm³. The smallest volume I measured in the burette was 24.7cm³.
Therefore percentage error is
0.1 x 100 =0.4%
The measuring cylinder is accurate to ± 0.5cm³. The smallest volume I measured in the measuring cylinder was 4cm³.
Therefore percentage error is
0.5 x 100 =12.5%
Therefore total percentage error is 12.9%
I have added error bars to my graph. All the points no fit on the line of best fit. This shows that if I had improved the accuracy of the measuring cylinder I would improve my results greatly.
The anomalies that I got could have been caused by this inaccuracy in the measuring cylinder.