Q1. A student produces several cylinders of potato, of exactly the same length and diameter. Some of these potato cylinders are placed in a beaker containing pure water whilst others are placed in beakers containing solutions of sugar at various concentrations. These beakers are left overnight and the following morning the student takes each cylinder and measures it to see if there has been any change in length.



The student notices that the potato cylinders that had been in the water seemed to have expanded the most, and those cylinders that had been in the sugar solution had expanded least, and in some cases had contracted in length.


Using your knowledge of Osmosis and including reference to the definition of osmosis in your answer, explain the students observations.


A1. In the case of the potatoes that had been in water, the student would conclude that the concentration of water inside the potato chip/cylinder was less than the concentration of the water outside (as there was nothing but water outside!). As a result of this water would pass by osmosis from the region of high concentration to the region of low concentration, into the potato cylinder and would therefore cause it to swell/increase in length.


In the case of the potato cylinders that had been in the different concentrations of sugar solution, the student would conclude that as the sugar solution concentration increased this would lead to a decrease in the concentration of available water in their solutions (in other words the more sugar you have, the less water you have!).


As the sugar concentration increased therefore, the availability of water outside would decrease, the difference between the concentrations of water in the potato cylinder and outside (in solution) would increase as the sugar concentration rose. The lower sugar solutions would have a higher concentration of water and therefore it is likely that water would pass into the potato by osmosis, however as the sugar solution got stronger we would reach a point where they would be more water in the potato than outside. The potato would then lose water by osmosis to the surroundings and would shrivel up/shrink.


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