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How is Osmosis Affected By the Concentration PLANNING Independent Variable The independent variable in this experiment is the thing that I will be changing in the experiment. This is the range of concentrations that have been obtained by diluting the original 1M solution of Sucrose. This is therefore the only thing that is going to be altered as I am investigating how different percentage of water molecules affects the weight of the the potato thus the rate and direction of osmosis. Dependant Variable The dependant variable will be the mass of the potato as this is the variable I will be measuring.
From this I can see the change in mass(g) and so work out the amount of water that has moved and the rate of the osmosis in a set time. This can then be used to see how the concentration affects osmosis. I Control Variables. The control variables are the set of factors that must be kept the same for every test to make sure that the results are reliable. The first controlled variable is the type of potato. This is because it may have more or less water than another type as well as a different selective membrane and so the results may be altered because of this.
If the potato has more water, for example, the weight of the potato might go up with one type whilst go down with another even if it was in the same solution; therefore the potato type must be controlled throughout the tests. The second control variable is the mass of the potato and the surface area. This may affect the results because if the potato is bigger and so the surface area is greater it will mean the membrane on the outside is bigger an so more areas for the osmosis to occur.
Because of this I will be using a cork borer to make the cylinder sample the same size throughout the experiment and then cut them to the nearest millimetre. Also all samples will be weighed and it will be the difference in mass that is observed as it may be a few hundredths of a gram out, every time, due to human error. Also to stop the weighing being distorted the scales will be wiped clean after every weighing as due to a early experiment we found this altered the results, with excess water. One of the control variables could be the volume of liquid in the test tubes.
However this shouldn’t have any affect because the concentration will be the same throughout whatever the volume. The next control variable is the temperature of the solutions. This is because if the water is hotter the particles will have more energy. This means the random movement of particles will be quicker thus meaning the net movement will also be quicker. The final are which is linked to the temperature is the environment they are kept in. 4) Prediction In this section I will be predicting what I believe will happen in the experiment when it takes place.
I believe that the first solution of 100% water molecules (distilled water 0M) will cause the potato to lose the least mass, and may even get heavier. Whilst the 1M solution which is saturated with sucrose will cause the potato to lose the most mass because there is the biggest concentration gradient. This is because of the process of osmosis. Osmosis is the natural process where water leaves and enters cells via semi-permeable membranes from a dilute solution to a more concentrated solution and the random process continues until the concentration inside and out is in equilibrium.
I have used this knowledge to work out if the potato will gains or lose weight. The reason why the first solution should gain weight is because outside the potato it is 100% water molecules, whilst inside there are less as the potato isn’t 100% water. Because of this over the time it will begin to move across from outside to inside the potato to even out the concentration gradient; because the potato is gaining water molecules it should therefore gain weight. On the other hand the last solution is a low concentration of water molecules and high concentration in sucrose partials.
This means there is probably going to be more water molecules inside the potatoes and so due to the concentration gradient the water molecules for the potato will go into the solution outside so the potato will lose weight. When I have plotted this onto a graph with the other solutions in between I should be able to work out a point of which the water molecules were even and no osmosis took place as the net movement will even itself out, consequently the mass wont change.
Finally the sucrose will play no part in the osmosis, this is because the sugar particles are to big to fit through the semi-permeable membrane and so will stay in the outside solution with only the water molecules passing in and out of the potato. As long as I ensure that my control variables are kept the same throughout the test I will hopefully see this process of osmosis work and show the results I have planned above. To makes sure that this does happen and that the controlled variables are controlled I will be doing a second test to ensure that the results are reliable. pic] | |(Sample of Potato) Change in Length/mm | CONCENTRATION / M |1 |2 |3 |mean | |0. 0 |50 |52 |54 |52 | |0. 2 |50 |49 |50 |50 | |0. 6 |48 |47 |47 |47 | |0. 8 |48 |46 |47 |47 | |1. 0 |46 |45 |44 |45 | | Evaluation From looking at the table the main anomaly is between O. 6M and 0. 8M where there is no change in length of potato, a reason for this may have been due to errors explained below or because the length isn’t a fair reflection on recording how much water has gone into the potato.
With the method I used, an error due to the apparatus was the way of cutting the length of potato into 50mm chips. I used graph paper to measure the length, this is full of errors because there would firstly be a higher error percentage from printed graph paper than using a ruler, also it wasn’t very affective at being able to record and cut the length so there would have been a big human error. A better method would be at using a ruler if I wanted to measure the length. Another error in the apparatus is in the dilution process.
The measuring cylinders will have an error and so the solutions may not have been exactly the correct concentration. Next is the range of concentrations chosen which did not increase evenly and so a whole set of results, sufficient to draw conclusions wasn’t possible. To reduce this problem more results at 0. 1M intervals could be taken. Another error was in choosing to measure the length of the potato, the problem with this is it only takes into account a very small change in size of the potato as it will be enlarged on the whole surface area and doesn’t just grow horizontally.
Also cubes would have been more affective because then the surface area could be calculated and controlled more easily rather than just controlling the length as the surface area is one of my controls. Also because a petri dish doesn’t have much height for the solutions then some of the chips were not totally covered, this also affects the results as no osmosis could take place at that membrane. A similar problem could also take place at the base which could be touching the dish and not the liquid so again no osmosis could take place. To solve this problem I could have used a test tube.