A potato.
Test tubes.
A knife to cut the tubes to the same length.
A cutter/ corer to get the potato tubes.
Salt solution.
Water. This will be mixed with the salt solution to create different levels of salt solution.
I am going to take 5 different levels of salt solution. One will be pure distilled water, the next 20% salt solution, then 40%, 60%, 80%, and finally 100% salt solution. I will do 3 tests on each level.
Everything but the concentration of salt solution should be kept the same. This includes the thickness of the potato tubes. It would take longer for the cells at the centre to be affected by the salt solution when the tube is thicker. The duration of the potatoes being in the salt solution should also be kept the same. The potatoes will have been affected by the salt solution for longer and therefore may have a more extreme change. I will also be repeating my experiment twice with each concentration in case one goes wrong.
I will measure the mass of the potato tubes to a hundredth of a gram. An electric scale shall be used for this procedure. The concentration of salt solution shall be measured as accurately as possible by my eye.
My test results went quite well. I found that the potato tube immersed in water increased in mass. This was not the case for the others as they decreased.
Conclusion
From the results and graphs I can see that my hypothesis was half right. I correctly predicted that the potato mass would increase with the distilled water. I also predicted that the mass would get lower as the concentration of salt got higher. However the results kept on going up, down and back up. The osmosis process says that higher water concentration will go to a lower water concentration. However the potato in the lower water concentration in my experiment decreased less that the 25% concentration. I believe that this is because there was so much salt in the solution that it blocked much of the water from passing out of the cell. Then again it is hard to trust the last set of results as they differ so much. Even so the 50% and the 75% concentration solutions decreased less than the 25% concentration. My other theory for this set of results is that the cells will only lose a certain amount before they become flaccid. Therefore the potatoes with less mass will have less to lose. This does not explain why the results kept on going up and down though. One last theory on why this happened is that the salt sank to the bottom of the solution and so the solution seemed more water based.
Evaluation
I thought the experiment was a good way of finding out my aim. It was a pretty fair test with only minor faults. One fault was the mass of the potatoes beforehand. It the experiment was to be as fair as possible they would have all been the same mass and the same shape. I felt that the percentage increase or decrease helped the cause slightly but still left room for improvement. That way you would know each one also had the same water concentration. Another fault was the temperature of the room. I had no idea whether the room changed temperature throughout the experiment. To fix this I could put them in an room where the temperature can be controlled. In my experiment all of the test tubes would changed temperature together so this fault would not have made much of difference to the experiment. I felt that accuracy was easily good enough for my experiment. In fact the level of detail is not able to be seen on my graph. I am not sure whether I had enough observations to draw a valid conclusion as I still am not sure why my results went up and down all the time. I felt I could expand the results into shorter intervals of concentration and make sure the salt does not sink to the bottom.
Apart from one result all of the results were different so in a way nearly all of them did not fit well into a pattern. The only results which really looked out of place were the 100% concentration solutions. None of them looked anywhere near like each other. The theories I gave in the conclusion are the best reasons I can give for these results. As I felt that the salt settling to the bottom of the test tube may have caused the experiment to go wrong I would like to change this so that the salt will not settle to the bottom. This is possible by shaking the test tubes (though your wrists would become very tired!).
by Ross Aylard
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