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The temperature of the solution should be kept the same at room temperature. This is because high temperature causes the particles to vibrate more and therefore moves faster. This will cause the water particles to bombard the surface of the membrane more often and their chance of going through one of these ‘hole’ is increased. So more particles will go through and the overall speed of the process is increased.
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The amount of time the potato is left in the test tube should be the same because particles are moving through the membrane all the time as long as there is a concentration difference between the two sides. More particles will go through if the potato chip is left for longer and fewer particles will go through in a shorter time.
Before the experiment starts, for each potato chip, I will measure its:
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Diameter with a ruler
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Length with a ruler
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Mass with a scale
I will also calculate the volume of each chip by using the formula:
Volume = π × radius² × height
V = πr²h
Radius = Diameter ÷ 2
Three measurements will be taken from each potato, the reading will be repeated to give an average result, which is more accurate. After the experiment, the length, the mass and the diameter of the potato chips will be measured again. The difference between the original value and the experiment result will be calculated. The data will then be collected to draw up tables and graphs.
Prediction:
Osmosis is the movement of water molecules from a high concentration of water to a lower concentration of water molecules through a practically permeable membrane. The process will continue until the concentration of water is equal in both sides.
The potato chip, like cell membrane, has tiny invisible holes that allow water particles, which are small, to pass through, but not sugar molecules, which are lager than the water molecule.
By using the theory I predict that:
- In the most dilute solution (pure water), the potato chip will gain mass and its volume should increase. This is because the concentration of sugar is higher in the potato than it is in the solution. So therefore, there is a higher concentration of water in the solution. Water molecules should move form the dilute part (the solution) into the more concentrated part (the potato) due to osmosis. The number of water molecules in the potato increases so the potato becomes heavier and its size increase
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On the other hand, in the most concentrated solution (1.0Molar), the situation is reversed. The solution contains high percentage of sugar and very few water molecules. The potato chip is identical to all the others but in comparison with the solution, the percentage of sugar molecules is lower. So the water will escape from the dilute part (the potato chip) into the more concentrated part (the solution). It makes the potato chip shrink (loss mass and volume).
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In some solution, the volume and mass of the potato chip may stay the same because the concentration of water is identical in both sides.
Line of best fit
Conclusion:
The experiment proved that my prediction was right.
In the most concentrated solution (1M), the potato chips lost mass by an average of 0.230g. The length decreased by 4.667mm on average and caused the volume to decrease too by 91.431mm³.
In the most dilute solution (0M), the potato chips gained mass by an average of 0.123g. The length increased by 2.333mm on average and caused the volume to increase too by 46.014mm³.
The graphs show a clear pattern. The values on the Y-axis decreases as the value of X increase, which means that the value of Y is inversely proportional to the value of X. On the graph, the value of Y represents the difference in mass/volume, and the value of X represents the concentration of the solution. It shows that the more concentrated the solution, the less the mass/volume of the potato chip; and the less concentrated the solution, the more the mass/volume of the potato chip. In other words, as we increase the concentration of the solution, more water will escape due to osmosis, so the volume and mass of the potato will decrease as a result. On the other hand, if we decrease the concentration of the solution (make it more dilute), more water will enter the potato chip and its volume/mass will increase.
On the graph, the point where the line crosses the horizontal axis is called the isotonic point. At that point, there are no overall movements of water molecules because the concentration of water is the same on both sides. On graph A, the isotonic point is at 0.2M, but on graph B, the isotonic point is slightly different, at 0.24M. By taking the middle value, I estimate that the isotonic point is 0.22M. In other words, the concentration of the potato chip is about the same as 0.22M solutions. So, I estimate that a potato contains 78% water and 22% sugar (starch).
The experiment demonstrates that osmosis is a special case of diffusion of water molecules. The potato chips have tiny invisible holes to allow small water molecules to diffuse through but not sugar molecules, which are lager.
Osmosis can be explained by using the kinetic theory of matter. Molecules of water and sugar are constantly moving in the liquid. They keep bombarding the surface of the semi-permeable membrane. Occasionally, a water molecule passes through one of the tiny holes from one side to the other. If the concentration of water is equal in both sides, equal number of water will pass through so there is no overall change. But, if the concentration of water is different (for example, pure water on one side and sugar solution on the other side), more water will flow from the pure water to the sugar solution. This is because in the sugar solution, large sugar molecules impede the movement of water molecules and block their passage through the holes so less water molecules flow out than in. So the overall effect is that water flows through the membrane into the solution.
There were some ‘funny’ results that did not fit in the pattern. For example, in graph A, the difference in mass is slightly lower than the line of best fit. These funny results occurred because:
- The potatoes were not dried properly, causing differences in mass.
- The concentration of certain solution can be slightly higher/lower than expected because too much/not enough water were put into the solution.
- The data was not measured correctly due to:
- The inaccuracy of the instruments used.
- Measuring errors.
- Differences in temperature (very rare). The test tube that is close to a heater become hot, and the rate of osmosis is affected by the increase in temperature because hot particles move faster.
Evaluation:
In conclusion, the experiment was a success. It proved that osmosis is a special case of diffusion of water molecules. It was certainly a good experiment to illustrate osmosis. Although there are few slightly ‘funny’ results, the experiment was fair and the data collected were accurate. But, following improvements can certainly be done:
- Use more accurate measuring instrument.
The only instrument we had for measuring length was a ruler. It was OK for measuring the length but it is useless for measuring the diameter of the potato chip. The diameter of some potato chip certainly changed after the experiment, but the differences were so tiny (about 0.01mm), we could not measure it with a ruler. It will be much better if we had a micrometer for measuring the diameter and the result will be more accurate.
- The time.
The potato chips were not cut at the same time; the potato chips were not put into the test tube at the same time; and the results were not collected at the same time. Although the time gap was only about a minute, it may have some effect on the data we collected. Firstly, the potato chip may become dry and loss mass if it was left for too long, or it may gain/loss more mass if it’s left longer in the solution and etc.
- Repeat measurements more and use a wider range of solutions.
We used six solutions of different concentration (0M, 0.2M, 0.4M, and etc) and put three potato chips in each test tube. It should be better if we used 11 solutions of different concentration, (0M, 0.1M, 0.2M, etc) and put 5 potato chips in each test tube. This can give us a wider range of data and the results should be more accurate.
During the experiment, I also noticed that when identical potato chips are put into solutions of different concentration, they all float at different depth. This is because the concentration of sugar in the solution affects its density. The solution is denser if it contains more sugar. So, if a potato chip is put into a very concentrated solution, it should float at the top because the potato chip itself is less dense than the solution.
In conclusion, osmosis is very important in living organisms. Cell membrane contains solutions of various substances in water. These substances must be kept at reasonable concentration all the time for the cell to function properly. So, water must be able to get in and out of the cell. This happens by osmosis and the whole process is called osmo-regulation.
END
Roy He
Thursday, 03 May 2007
