An experiment to investigate the effects various ions on a soap solution.

Dependent Variable

A dependent variable is a variable that changes as a result of the independent variable, making it dependent of the independent variable. In this experiment, the dependent variable is whether or not a scum or lather is produced.

Control Variable

A control variable is the thing in the experiment that needs to stay the same, whatever happens. The control variable needs to stay the same in order to make the experiment a fair test. This is to make sure that only one variable is changed at a time. The control variables in the experiment are:

• The amount of soap in the distilled water. This needs to be controlled because if all of the test tubes have large amounts of soap, then all of the solution will eventually form lather therefore making the results extremely inaccurate and produce many anomalous results.
• The concentration and amount of the various compounds used. This, similar to the previous control variable needs to stay the same in all solutions because if it was not the same, there would be more than one independent variable and this means that the experiment would be unfair.
• The amount of time and how vigorously the test tube was shaken. If the vigorousness was changed in each experiment then some solutions would form lather faster/slower than others and there would be more than one independent variable making the experiment unfair.

RESULTS

The compounds that cause hardness in water are as follows:

• Calcium chloride
• Magnesium chloride
• Calcium nitrate
• Magnesium nitrate
• Magnesium sulphate

The list below is to show which ions cause harness and which ions do not.

• I managed to fill the blank spaces in the results table by using a simple method. Take sodium chloride and calcium chloride as an example. Calcium chloride does not form lather; therefore either the calcium ion or the chloride ion causes hardness. However, sodium chloride does not cause hardness hence the chloride ion does not cause hardness, which means the calcium ion must cause hardness. This method was used to conclude which ions do and do not cause hardness.

• Calcium sulphate was not used in this particular experiment because it is insoluble (does not dissolve in water.) However, if calcium sulphate was soluble, it would not form a lather therefore causing hardness in the water. This is because from previous investigation it can be seen that the calcium ions cause hardness.