Osmosis Diagram:
Preliminary Experiment:
For my preliminary experiment, I used a potato cork to remove _______cylinders of potato from the whole potato. I then cut these ______ cylinders into 6 individual potato discs. We had 9 different concentrations of the salt solution and did each concentration 3 times for fair testing therefore we used 27 x 6 potato discs as we put 6 discs in each test tube.
We used 5mls of salt solution- this was the amount needed to cover the six potato discs. The first thing we needed to observe was how different concentrations affected the weight of the potato. We therefore needed to observe each end of the spectrum. We tried 0.2 molar concentrations (the lowest) and 2 molar (the highest.)
We then left each test for 15 minutes, but found after that time no reaction had taken place or at least hardly any reaction we could record, so we decided to extend the time to one hour. Bearing in mind we should really leave them to stand for at least 12 hours for the osmosis to occur, due to time constraints we could not afford to do this.
From doing our preliminary test we observed the basic pattern and procedure the experiment would undertake and therefore needed to make some changed for the actual experiment.
- Extend the time from 15 minutes to 60 minutes
- Extend the range of concentrations from 0.2M-2.0M to 0.05M-0.5M
Hypothesis:
I predict that as concentration increases, the weight of the potato disc will decrease. This is because the higher the concentration of salt in a solution, the lower the concentration of water there is. When the potato discs are put into the solution, it will, by osmosis lose some of its water, and the water will diffuse into the solution of salt, causing the potato discs to lose water, thus decreasing in weight.
However, if a potato disc is placed into a solution of 0 molar concentrations (i.e. Distilled water) it should gain weight. This is because the solution has more water potential (its molecules’ ability to move) than the potato disc, and so water moves from a region of high water potential (the solution) to a region of low water potential (the potato discs.)
Method:
After having done a preliminary test, we knew which conditions we needed to use in order to obtain the correct sort of results. We set up 27 test tubes, because in order to save time, we intended on doing repeats at the same time as the original experiments were going on.
We set up all the test tubes at the same time in order to save time and effort, and we labelled them to save confusion. Into each test tube, we poured 15mls of salt solution, and added six of the potato discs, each weighing around 1 gram.
In order to make it as much of a fair test as possible, we added the potato chips very quickly, to make sure one chip didn’t have a longer amount of time in the solution than another. We then left each one for 60 minutes.
After 60 minutes, we emptied each test tube, trapping the potato discs and carefully dabbed them with a paper towel making sure not to squeeze the water out. We then weighed the discs and recorded the results. If we had of left excess water on the discs, the weights would not be accurate.
Results:
Analysis of Graph:
‘Average change in weight’ is quite confusing but it does follow a trend. As the concentration of salt increases we can clearly see that the average mass decreases, except for the last value which is distilled water. There are bumps in the trend though, such as at 0.2M. This could be due to excess of water being weighed or just simply misreading the scales.
Conclusion:
I can conclude that the movement of water through a selectively permeable membrane is affected by concentration of salt solution, or by any other solution. The higher the concentration of salt solution, the more the potato disc will lose its weight.
Because water molecules have kinetic energy, they are constantly moving randomly from one place or another. The greater the concentration of water molecules in a solution, the greater the total kinetic energy, and the higher its water potential. This means, that as the concentration of salt increases in a solution, the concentration of water decreases, lessening the solution’s water potential, and lessening its ability to move between solutions by osmosis. Relating that idea to this experiment: as the concentration of salt in each solution increases, the water in that solution is less able to move to the potato, causing water from the potato to move to the solution, decreasing its weight.
Evaluation:
Accuracy in this experiment wasn’t too much of an issue as because we weighed the discs before and after the experiment it did not matter that they were different weights to begin with. However different width and length could have had an influence on our results, and if we wished to expand on our investigation and make it a more accurate one, we could include these variables into the practical.
When weighing the discs after 60 minutes in solutions, it was inevitable that they would have excess water on them, and so we dabbed them with paper towel to remove it. However, in doing this we could have removed water that was not excess, and altered our results.
As for anomalies I found one in my results, it wasn’t too far from the general trend but never the less it was still vaguely opposed to it. It occurred at the concentration of 0.2M. I used a table and graph of average change in mass to overcome anomaly results, however if one or two results are inaccurate it still alters the mean. I predict this is the reason for this anomaly.
To extend the investigation further I could investigate the effects of temperature on osmosis, or surface area. This would give me a very clear understanding and knowledge of osmosis in potatoes.
