Investigating Osmosis.

Our third preliminary experiment used visking tubing as a semi-permeable membrane. We filled the tubing up with a dilute starch solution (high water concentration) and put it in a beaker full of a strong starch solution (low water concentration). The visking tubing had tiny holes which only allowed the small water molecules through. After ten minutes, some of the water molecules had left the tubing and moved into the beaker by osmosis. This equalised the water concentrations both inside and outside the visking tubing. The tubing lost weight because water had left it.

Doing these preliminary experiments found us a suitable range of water concentrations and proved our theory of osmosis was true.

Method

I will use this equipment:

I will use a size 6 cork borer and pressing on a tile, I will cut out 15 cylinders of potato. I will cut these to a length of 3cm using the ruler and knife. Using the measuring cylinder I will measure the following combinations of solution:

20cm3 of water 0cm3 of salt 0% salt solution

15cm3 of water 5cm3 of salt 25% salt solution

10cm3 of water 10cm3 of salt 50% salt solution

5cm3 of water 15cm3 of salt 75% salt solution

0cm3 of water 20cm3 of salt 100% salt solution

I will make 3 of each concentration combination and pour them into the 15 test tubes. I will then measure the mass of each potato cylinder on a balance. Then I will drop a cylinder into each test tube. Using a stopwatch I will leave the test tubes alone for 15 minutes. After this time I will carefully blot the potato cylinders to remove excess water and weigh them again, recording my results.

Safety measures:

* to prevent salt getting in eyes -----> always wear goggles

* to prevent cutting your hands -----> cut the potatoes on a tile

Fair testing procedures:

* use the same technique for blotting the potatoes each time

* use the same species of potato all the time

* repeat the whole experiment again to get more reliable results if the first set of results are wrong

* use the same width cork borer each time

* keep the temperature of the water consistent

* same volume of solution (20ml)

* same time left for

Results

My first set of results did not seem accurate as when I drew them as a graph, the line of best fit was a zigzag (I was looking for a curve). I decided to repeat the whole experiment to just to check. The second set of results were great and made a curve on a graph. The first and second sets of results are below:

* as you can see, I have done my calculations and measurements to 2 decimal places

Conclusion

I have found from my repeated results, that my prediction was correct. My line of best fit is a steady curve. This agrees with my prediction. As the salt concentration rose the potato lost weight. Alternatively, as the water concentration increased the potato gained weight. This shows that osmosis happened.

For example in the case of 100% salt solution:

1. there was more salt (low water concentration) outside the potato than in it. There was more water in the potato (high water concentration).

2. water left the potato and passed through the semi-permeable potato cell membranes.

3. the levels of water concentration were then equal both inside and outside the potato

4. the weight of the potato decreased when water left it.

For example, when we had an increasing concentration of water, the low water concentration solution diffused into the potato, this balanced the water concentrations inside and outside the potato.

* this is all proven correct because in 100% salt solution there was an average weight loss of 9.27%

Evaluation

I think my first investigation went badly but it could have been due to a number of reasons. My second experiment went much better. All of my results now support my conclusion. The anomalous result has been corrected after repeating it. I think this anomalous result could have occurred because of a few factors:

. the potatoes may have been damaged and thus might have varying surface areas. a larger surface area means that the experiment is unfair as more water may have entered/left the potato.

2. the potatoes may have been blotted differently. inadequate blotting may not have removed all the excess water, making the weights increase. Too much pressure may have forced all the water out of the potato.

3. the solutions may have been measured incorrectly

4. different balances may not be as reliable or accurate. the same balance should be used throughout the experiment.

I think my second set of results are very accurate.

If I was to redo the experiment I would use more accurate equipment such as measuring cylinders that measure to 0.01cm3 or burettes for measuring the solutions. These would improve the accuracy of my results. I would find a better way of cutting the potato so that the slices are not damaged in the process and are all uniform length and surface area. Once again this would give more accurate results. I would give one person the job of doing all the blotting, and he/she could perhaps just roll the potato cylinder once along a paper towel each time. This would make the experiment more consistent and fair.

I believe with these new arrangements, my experiment would be a much fairer test.

To get more information on osmosis, I could try using another vegetable, perhaps a courgette. I could check whether courgettes take in as much water as potatoes.

xxx