As this experiment is testing the concentration of the metal ions, I think that as the concentration of the calcium ions increases so will the amount of soap solution needed to form a lather. I think this because when the number of calcium molecules increase, the amount of stearate in the soap needed to react with it will also increase.
Also, I predict that as concentration is doubled, so is the amount of soap solution needed to form a lather. The equation for the reaction between stearate and calcium is as follows:
Calcium + Stearate Calcium Stearate
Ca2+ + St- CaSt2
This equation shows that for every calcium ion, two stearate ions are needed for them to react. The table below shows the amount of stearate ions needed to react with the calcium ions:
I expect doubling the amount of calcium ions would double the amount of stearate ions needed to react although but, even though the graph would be a straight line, it wouldn’t go through 0,0 as to form a lather, there must be some soap even if there are no calcium ions at all. Below, there is a graph from a GCSE paper. But, they haven’t taken into account that even when there are no calcium ions, soap is needed to form a lather. The graph would look like:
Variables
In this experiment, the definition of a lather is, frothy white bubbles at the top of the solution that still cover the surface after ten seconds.
Safety
This experiment must be done safely. Firstly, safety goggles must be worn as a barrier against hazardous chemicals. In this experiment, soap is an irritant. The second precaution taken is that whenever a chemical is spilt, it must be wiped up.
Apparatus
The following pieces of apparatus will be used I this experiment:
1 x 50cm3 measuring cylinder
1 x Burette
1 x 100cm3 conical flask
1 x funnel
1 x 250cm3 beaker
1 x clamp and boss
Method
The burette will be fixed to the clamp. The funnel then be put into the top of the burette and the soap solution will be poured into it. The funnel must then be taken out of the top as there will be a drop of soap solution at the bottom of the funnel and to prevent inaccuracies, this must be taken out. A beaker must then be put under the burette and the tap must be opened to fill the capillary. This must be done, so that the initial reading is accurate. Then, the soap solution must be discarded to prevent any contamination. The following values of distilled water and Calcium ion solution will be put into the conical flask:
Distilled water must be used as this experiment is done to test the concentration of calcium ions and if tap water will be used, there would most likely be calcium ions in it already. Firstly the tap on the burette must be opened fully so 2cm3 of soap pours out. Then the conical flask must have a bung put into it and then must be shaken hard for five seconds. If it looks like a lather might be forming, then the amount of soap solution added will be reduced, if not then 2cm3 will be added continually. When it looks like a lather is going to form, then the soap solution will be added drop by drop.
There are different types of lathers, some are for only a short time, and others are for much longer. In this experiment, the lather must be still visible after fifteen seconds. After each measurement is done, the apparatus must be cleaned using distilled water.
Distilled water must be used as this experiment is going to be done to test the concentration of calcium ions and if tap water is used to clean the apparatus, then there will be some calcium ions from the water on the apparatus. This would make the results inaccurate, as the amount of calcium ions wouldn’t be measured properly, as there would be additional ions on the conical flask. This procedure will be repeated so that there will be two measurements for each concentration. This will be done so that an average is formed and it then it will be easy to find out any anomalous results.
Results
Analysis
The graph shows that as the concentration of calcium ions increases, so does the volume of soap needed to form a lather. The relationship is linear. When drawing the line of best fit, the two anomalies weren’t taken into account. If the anomalies were taken into account, then a line of best fit couldn’t be drawn. The graph therefore supports my prediction of, “As the concentration of calcium ions increases, so does the volume of soap needed to form a lather.
For my second prediction of, “As the concentration of calcium ions doubles, so does the volume of soap needed to form a lather.” To prove this, I need to draw the following table:
The value at the intercept is taken away from the volume of soap as the intercept represents the amount of soap needed to form a lather when there is no calcium, only water. This means the actual amount of soap needed to form a lather with each concentration is 1.00cm3 too large on the graph.
5.2 / 2.5= 2.08 10.2 / 5.2=1.96 (2 d.p)
The two numbers above show doubling within experimental error. If the results were very accurate, the numbers would be exactly 2.
The gradient of the line is = =0.52. The intercept of the graph is 1. Therefore the equation for this graph is y=0.52x+1. In this case, y is the volume of soap needed to form a lather and x is the volume of calcium ions. This formula can then be used to find the amount of soap needed to form a lather for any volume of calcium ions. For example, when 10cm3 of calcium ions was used, there was an anomaly. Using the formula, y= 0.52 x 10 + 1 =6.2cm3. This is confirmed by the line of best fit.
The reason why the amount of concentration of calcium ions increases, so does the volume of soap needed to form a lather is because when the number of calcium molecules increase, the amount of stearate in the soap needed to react with it will also increase.
The reason why as the volume of calcium ions doubles, so does the amount of soap solution needed to form a lather is because of the following equation:
Calcium + Stearate Calcium Stearate
Ca2+ + St- CaSt2
This equation shows that for every calcium ion, two stearate ions are needed for them to react. The table below shows the amount of stearate ions needed to react with the calcium ions:
This shows for every calcium ion, two stearate ions are needed for them to react.
Evaluation
Overall, my results were actually quite good. Only two points weren’t on the line of best fit and they weren’t very far away from the line. One as 1cm away from the line and the other was only 0.5cm away from the line. Otherwise, the results were very good. The correlation coefficient was 0.992430314823109 which is very close to 1. The closer the value is to 1, the better the values lie on the line of best fit. If the value is larger than 0.95, then the results are good, so my results are actually excellent.
I believe that my results justify both of the predictions. The volume of soap solution clearly increases as the volume of calcium ions does. Also, for the second prediction, it is very clear that the results double (see 1st table in analysis). 2.08 and 1.96 are both very close to 2 and therefore show doubling within experimental error.
As said before, there were two anomalies. One reason why this could’ve happen was because when the soap was poured into the burette, a lather formed. This meant that the initial and final readings were inaccurate. Also, it was hard to find out when to stop adding soap to the calcium ions. This means, the lather was different each time. This meant that the lather formed was different each time and so, some of the results were too large or too small. Also, the initial and final readings were only accurate to half of the smallest measurement. This means that the results were only accurate to 0.05. To make this more accurate, a burette with more measurements could be used. Lastly, when the solution was shook, it wasn’t the same each time. This could be because my arm got tired and therefore some of the results weren’t shaken enough or sometimes shaken too much. To ensure that the solutions are shaken the same amount, a shaking machine could be used that shakes the solution for the same time and at the same speed.
Instead of this experiment, an EDTA titration experiment could be done. In this experiment, 50cm3 of calcium ions would be put into a Erlenmeyer flask. A few drops of 1M hydrochloric acid would be added so the solution would become acidic. Then, boil the solution for a few minutes. Let the solution cool and add 4 drops of methyl red. Add 1M sodium hydroxide until the red colour disappears. Them, add 1 or 2ml of pH-10 buffer, 3 drops of Eriochrome black T indicator solution, and titrate with standard 0.01M EDTA until one drop causes the solution to turn from red to blue. Titrate slowly. Because the colour changes take place slowly, the solution should be swirled and wait a few seconds between drops as the endpoint is approached. From the amount of EDTA used, the hardness can be told.