6) If an unbroken lather has not formed across the water’s surface for ten seconds, we will continue to add soap solution, another 0.2 cm3.
7) Instruction 4 and 5 will be repeated and if an unbroken lather has not formed across the water’s surface for ten seconds, we will repeat instruction 6. We will continue to do these instructions until an unbroken lather forms across the water’s surface for ten seconds.
8) When instruction 7 is completed, we will wash out the conical flask (and wash the bung). Instructions 1 and 2 will be repeated, but at instruction 3, we will add 0.5 extra grams of sodium carbonate (so for the 2nd repetition of the experiment we will add 1 gram of sodium carbonate). All the instructions up to 7) will be carried out if necessary.
9) For the other repetitions, all that varies is that we add 0.5 extra grams of sodium carbonate than we did in the repetition before (e.g. – for the third repetition 1.5 grams of sodium carbonate will be added). So we will repeat instruction 8 (adding the extra 0.5 grams per repetition), so we attain five results. We will be varying the sodium carbonate from 0.5 grams to 2.5 grams.
10) As a control we shall place 25 cm3 of distilled water into the conical flask (but not add sodium carbonate). Then we will do instructions 4-7.
FAIR TEST:
To make a fair test only one part of the experiment can be changed. In our experiment this is how much sodium carbonate is added. The volume of water per repetition is always the same, 25 cm3. The amount of soap solution released will be 0.2 cm3 at a time. The conical flask will be shaken for ten seconds at the same “strength”. An unbroken lather must form across the water’s surface for ten seconds or more, but no less time. The water temperature should be within a few degrees of room temperature. If these were not kept constant, they could alter the results (e.g. – a higher water temperature will increase the rate of reaction, meaning a lather is formed quicker).
PREDICTION:
As more sodium carbonate is added, less soap solution is needed to form an unbroken lather across the water’s surface for ten seconds. This is because the water becomes softer.
CO3-2 (S) + Ca2+ (aq) === CaCO3 (S)
Carbonate ions from Calcium ions from Precipitate (removed
sodium carbonate tap water (which from the water)
makes it hard)
The water becomes softer as more carbonate ions are added and more calcium ions are removed.
For a lather to form the calcium ions must be removed. When the soap solution is added if there are any calcium ions remaining, “scum” will form.
Ca2+ (aq) + 2St-(aq) === CaSt2 (S)
Calcium ions Stearate ions An insoluble
remaining (from soap solution) solid called
from tap water “Scum”
Once all the calcium ions have been removed as a scum, lather will form.
From our preliminary work, we have seen that 1 gram of sodium carbonate softens the water significantly, but not completely so we have chosen measures around this amount. Our class average shows that when adding 1 gram of sodium carbonate it takes (30.3 divided by 8) 3.8 cm3 of soap solution to form an unbroken lather across the water’s surface for ten seconds. However my personals results gave a mean average of 2.5 cm3 of soap solution to form an unbroken lather across the water’s surface for ten seconds. This is probably just an anomaly though.
Once we collect our results our graph should look similar to this:
Volume of soap
Needed for lather
Amount of sodium carbonate
This is because when very little sodium carbonate is added to the water, a higher volume is needed for the lather, than with much more sodium carbonate. There could be a levelling off bit (a saturation point), where adding more sodium carbonate won’t make a difference to the volume of soap needed for lather.
