Practical investigations
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I'm doing an investigation based on saponification and detergents. My experiment must include determining the best water to use: tap water, rain or sea. In the investigation I decided to collect 3 test-tubes containing the three samples of water and to each I added 2 cm3 of the soap detergent.
After vigorously shaking the tubes, I measured the amount of lather produced with a ruler. The water that produced the most lather is said to be the best type. Is this experiment accurate?
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Igloo writes ...
Yes, it’s a reasonably good way of comparing the hardness of the three types of water, providing you shook each test-tube:
1 for the same amount of time,
2 equally vigorously.
It’s unlikely that you were capable of replicating exactly the same degree of shaking for each experiment, so your results are not as accurate as you would want them to be.
In addition, the lather - which contains tiny air bubbles - tends to collapse after time has elapsed, so it’s important to compare the heights after shaking has stopped for the same period of time afterwards - perhaps 30 seconds. Did you do this?
A more accurate technique would be to measure out a known volume of your water sample into a small conical flask and then to titrate with the liquid detergent, shaking the flask after each addition of detergent. You would need to decide what you would accept as the end-point (when a 'permanent' lather appears) in each case – perhaps the maintenance of some sort of lather for a minimum of, for example, 10 or 20 seconds.
Risk assessment
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updated: 11 February 2008
