In the case of sucrose, we use a number of different concentrations which in turn means the water concentration of each sucrose solution is also different. The relative difference of concentration between each of the sucrose solution and potatoes immersed in them should result in different rates of osmosis.
Besides the rate of osmosis, we should also observe osmosis acting in different directions, that is, when the water concentration is higher inside the potatoes, water movement is outwards whereas when the water concentration is weaker inside the potatoes, water would go into the potatoes. When the relative concentrations are similar between the solution and the immersed potatoes, there should be little or no movement.
Figure 1 is a sketch of a graph of how we expect to see the behaviour of the phenomenon taking place as described. The sketch is not scaled except that the origin (x=0) represents pure water. Each point on the curve represents a specific solution concentration, for example, point (1) on the curve represents concentration of solution which has more water than inside the potato whilst (2) on the curve is the solution concentration which has less water than that inside the potato. Where the curve crosses the x-axis, the concentration of the liquid outside the potato is the same as that inside the potato, that is, in equilibrium and therefore the mass of the potato remains the same.
Figure 1
5. METHOD, PREPARATION AND FACTORS INFLUENCING THE OUTCOME
In any experiment, it is important to reduce the number of variables to a minimum, and preferably, to one. The reason for this is because when there are more than one variables, there is no way of knowing which variable has the greater contribution or influence on the outcome of the experiment or whether and outcome is wrongly attributed to a variable when it should be the other. By eliminating all possible variables, standardising them and leaving only just one variable, the results can be compared accurately and the outcome is most likely to be objective and unbiased.
In the case of this experiment using potatoes, there are a number of possible factors influencing or contributing to the outcome. They are : -
Length and width / diameter of the potato tube - the longer the tube, the more area exposed to allow osmosis and the shorter the tube, the less surface area exposed to allow osmosis. For this reason, if tubes of different lengths are allowed, there will no standard. Hence, in order to be able to make objective comparisons, each of the tubes must be the same length. Similarly, the width / diameter must be kept exactly the same for each potato, so that they have the same surface area and volume.
So in accordance with this, all the potato tubes within the same group, were of the same length. Although for different groups the average lengths of the tubes were different. For example, in group one the tubes would all be 1.5cm, while in group 3, the tubes would all be of 2.5cm.
Length of immersed time – osmosis is not an instantaneous process, it takes time for diffusion to work and equalise two solutions of different concentrations. This means that osmosis is time dependent, regardless of the degree of difference in concentrations of the two solutions. It is stated that the greater the difference, the greater the rate of diffusion and hence the concentration gradient, hence, after the diffusion process has taken place for a while, the difference in concentration is reduced, meaning that the rate of diffusion slows down. This means that, just like the exposed surface area of the tubes, if the time each potato is immersed in solution is different, it would not be possible to make an objective comparison. For this reason, immersion time for each potato tube must be the same, if one is immersed for 15 minutes, then the other must be 15 as well.
Amount of solution – the amount of solution in which to immerse the potatoes must be the same. The larger the volume of solution, the longer the reduction in concentration compared to a smaller volume of solution, for the same sized potato.
6. THE EXPERIMENT
The apparatus required for the experiment are as follows : -
- Stop Clock - to time our experiment with
- Cutting tile - to cut the potato on
- Knife - to cut the potato
- Measuring cylinder - to measure the solutions
- Distilled water - part of the experiment
- Sucrose - part of the experiment
- Potatoes – to do the experiment on
- Tissue paper - to dry the potatoes after the osmosis takes place
- Balance - to weigh the potatoes
- Cork borer - to cut out potato cylinders
- Ruler- to measure the potato slices
There was no special setting up of apparatus. The experiment simply consisted of making sure that potatoes were cut into tubes as described before, distilled water and sucrose solutions of different concentrations made up. The potatoes were weighed before the start of the experiment and then at regular intervals. Beakers had exactly the same volume of liquid in them.
7. DATA COLLECTION
After all the apparatus was collected, the items were set up. Using the uniform cork borer, holes were made into the potatoes, and cylinders of potatoes were produced. These were then cut into equal, smaller tubes. This is so that each sample has exactly the same opportunity for osmosis to take place. The tubes were then weighed out, and recorded then placed in small plastic beakers, 5 in each. Each beaker is then filled with solution.
Each of the beakers has a different concentration of sucrose. These were of 0 mol/dm³, (distilled water), 0.2 mol/dm³, 0.4 mol/dm³, 0.6 mol/dm³ and 1 mol/dm³.
The potatoes were left in these beakers for 10 minutes, then dried out on tissue paper, and weighed. Then, it was left for another 10 minutes, dried and weighed, and so on, till the required 40 minutes was over. The reason for this is to discover what happens as the two different concentrate begin to equalise. That is to say, if, at the beginning of the experiment, the difference in the respective concentrations is at their greatest, we expect to see osmosis at its most active whereas, during the third or fourth period of immersion, we expect to see the rate of diffusion slow down.
8 ANALYSIS OF DATA
Table 1 shows the data collected during the experiment.
Table 1
In can be seen in table 1, readings for each potato were recorded individually at each concentration. For example, at 0.2 mol/dm3, readings were 0.60, 0,58, 0.62 and 0.53 respectively. The average of these readings was calculated and in Table 2, the average figure is shown against each concentration.
Table 2
Graphs were constructed using Table 2. These are shown in Graphs 1, 2, 3, 4 and 5 respectively
Graph 1 – distilled water : 0.0 mol/dm3
Graph 1 shows the rate of osmosis with the potato immersed in distilled water. Since distilled water has the highest concentration of water, the graph shows a very raid rise, i.e., diffusion into the potatoes in the first ten minutes. The curve then slops more gently after 10 minutes, peaking at around 15 minutes. From 15 minutes onwards, sign of reverse osmosis is seen to be taking place until about 30 minutes and finally, the osmosis is again in the direction of the potatoes. The period of decline is difficult to explain. To see what is happening, the readings for each individual potato are also shown as individual graphs, namely, 1a, 1b, 1c, 1d and 1e. It can be seen that each individual graphs show the same trend, to a greater or lesser extent but the general pattern is clearly one in which the early period sees a rapid rise in the mass of the potatoes and the again in the later period. This suggests that, as predicted, when the difference in concentration is greatest, the osmosis process is the greatest and most rapid, that is, in the beginning.
It is quite possible that due to the very rapid process in the early period, the rate of mix/dilution inside the potato is not able to match that of the osmosis between the two concentrate, hence, the areas nearest the surface between the two concentrate reaches temporary saturation point. When the concentrate inside the potato has been evenly diluted, the osmosis process continues.
However, to put things in context, it should be noted that whilst the curves show sharp and great movements, the difference is in fact very small, for example, where a dip occurred between 20 and 30 minutes for potato No.2, the fall in weight was only 0.01g. The overall result was an average rise in weight over the 40 minutes.
Graphs 1a – 1e are plots of each individual potato-
Graphs 1a – single potato No.1 in distilled water
Graph 1b – single potato No.2 in distilled water
Graph 1c – single potato No.3 in distilled water
Graph 1d – single potato No.4 in distilled water
Graph 1e – single potato No.5 in distilled water
Graph 2 – concentrate at 0.2 mol/dm3
Graph 2, with potatoes immersed in sucrose concentration at 0.2 m/dm3, shows a curious flat beginning, a gentle rise then a sudden sharp drop followed by an equally dramatic sharp rise. Checking the individual recordings, the result appears consistent; every potato tube shows the same overall pattern hence proving that the results were not due to any recording error. However, the overall trend was an average rise of mass over time.
Graph 3 – concentrate at 0.4 mol/dm3
Graph 3 shows a steep rise in the first half of the experiment then an equally sharp drop in the second. Checking the individual records taken suggests, like that for the experiment using sucrose at 0.2 mol/dm3, this is no freak as each individual potato shows the same overall pattern. The first half can be explained by osmosis in the direction of the potatoes, due to a higher water concentration in the sucrose solution. The second half appears o see the osmosis reversing. The overall result is an average fall in mass over time, suggesting that the actual concentration of water was greater in the potatoes.
Graph 4 – concentrate at 0.6 mol/dm3
Graph 5
Graphs 4 and 5 shows a clear and unambiguous pattern of osmosis from the potatoes towards the sucrose concentrates. This suggests that the water content inside the potato is higher than that in the sucrose concentrate.
Graph 6 – comparison
As a comparison, all the graphs were put together. Here, when the scale of the y-axis is put in perspective, the steep drops and rise seen in the individual graphs are no longer as dramatic. Potatoes in distilled water shows the rise predicted, potatoes in high sucrose concentrations show a drop. This pattern is repeated in graph 2, where the sucrose concentrate is at o.2 mol / dm3, suggesting that the water content is lower inside the potato.
Graph 3 suggests that the relative concentrations between sucrose and that inside the potatoes are not dis-similar.
9. CONCLUSION
I think that my hypothesis was correct as I said “when the water concentration is higher inside the potatoes, water movement is outwards whereas when the water concentration is weaker inside the potatoes, water would go into the potatoes”. And indeed, this was so. The higher the concentration of sucrose in the beaker, more weight the tubes would lose. The lower the concentration of sucrose in the beaker, the more weight the tubes would gain.
The principle of osmosis can be applied to many situations in our everyday life. For example, water purification process, desalination, industrial processes involving liquid chemicals and solutions, in hospitals etc. For example, Foods that are packed in salt or sugar prevent bacterial growth by essentially sucking the water out of the bacterial cells (or, more properly, preventing water from entering the cells) and preventing their growth. This utilise the fact that when starch or other large molecule is on the outside of the cell, the cell loses water faster than it comes in, and the cell shrinks, which might not be too bad except that the cell needs the water for the chemical reactions that take place inside that keep it alive.
