Variables:
There are a variety of factors that could affect that accuracy and even outcome of the data I collect and so I must consider each variable and how and why I need to control it.
One of the most blatant of these is the size of the potato pieces. This needs to be measured as accurately as possible with a sensitive scale so as to record the mass losses and gains in each segment. Also the type of potato I use should be the same because different types of potatoes have different concentrations of water in them and so some will have higher or lower concentrations of water in them and so if there is a variation in concentration then the rate of osmosis will vary and measurement will not necessarily be in proportion. Another important factor is that the potato must be completely submerged in the solution in order for it to have total contact with the solution and allow complete osmosis.
Another factor that must be kept constant is the volume of solution in each beaker that surrounds the potato segments. This need to remain constant because I am going to be controlling the concentration of water particles using moles rather than in a volume and so if I was to vary the volumes of solutions then the number of water particles wouldn’t be in direct proportion with each other in the varieties of concentrations and so my data would be imprecise.
In order for the potato segment to survive in order to carry out the process of osmosis for the time needed they will need light and heat. This means that both these factors need to stay constant through my experiment because, for example, if the temperature of one solution is hotter then the particles of both salt and water will have more kinetic energy and move faster. This means that the rate of osmosis will be faster and so more water will diffuse in the time period given. So the rate of osmosis in all beakers needs to be kept as similar as possible and so these factor need to be kept the same.
Small things such as dirty equipment and broken/dysfunctional equipment could still have effects on the accuracy of my data and so I also need to take these things into account as well. Also all data that I record should be measures to 2 decimal places in order to be precise as possible and not loose much information through inaccurate readings.
Preliminary Test:
In my preliminary tests I was trying to find out;
- The time I should leave my potato segments in the solution for
- The size and type of my potato segments
- The range of data (in moles) I will use
- The volume of solution I need to use
In my pre-tests I took a range of 3 different concentrations of solution: (distilled) water, 0.5moles and 1moles. I decided that 20minutes was a suitable time to leave my potato segments and from the result I got I think that it is enough time in order to show a clear relationship between the concentration of the solution and the mass lost of gained. I used a corer to cut out the segments of my potatoes which had a diameter of 12mm which was ample. I found that one mole of salt solution is equivalent to 100ml and this was my chosen volume for each concentration of the solution so that I can control the different concentrations. I controlled these concentrations by mixing the solution with the distilled water in the relevant ratios. For example, in order to get 0.5mols of salt solution in 100ml of liquid I had to add both substances to a ratio of 1:1 so that 50ml was water and 50ml water salt solution (half and half)
My results were as follows:
These results coincide with my prediction nicely. The show that the solution with the lowest concentration of water (1mol) had a negative percentage mass gain which was the lowest out of the three and the solution with the highest concentration of water (distilled water) had the greatest percentage mass increase. Slightly unexpected was the fact that 0.5mol had a negative mass gain which means that the isotonic point (the point at which the concentrations between the potato and the solution are at an equilibrium and so no osmosis appears no occur) is 0mol < isotonic point < 5mol. I will be able to find the point reasonably accurately when I have my final results and have plotted them on the graph. I will be able to find this because it will be the point at which my line of best fits meets the x-axis.
Method:
Apparatus:
- Stop clock
- Cutting tile – to cut the potato on (and not the desk or work top)
- Corer (12mm diameter)
- 2 x 25ml measuring cylinders (one for distilled water and the other for solution)
- 6 x 100ml beakers
- Distilled water
- Salt Solution
- Potatoes
- Scales (measuring to 2d.p)
- Knife
Diagram:
Method:
Firstly I will core out my pieces of potato using a 12mm diameter corer. Then using a knife on a cutting tile (rather than cutting on table or work tops which are less clean) I will cut my potatoes piece so they are almost exactly similar in size. I will do this so that the masses of the potato pieces are very similar because if each of them is completely different sizes then the water concentration in some potato will b more because of the size and so the rate of osmosis will be different and so not accurate. I will then weigh each of the six potato pieces before they have been placed in the beakers and record this data. There will not be a control for this experiment because the masses of the potato pieces will vary (meaning there is no set mass) and so it is not necessary to have a control because the vary in masses will give a variation of data.
Before this I will have measured out accurately, using two 25ml measuring cylinders (one for each liquid so that the ratios between the two are as accurate as possible as this can affect the data), six beakers with six different concentrations of salt solutions (measured in moles). My range of data will be 0mol, 0.2mol, 0.4mol, 0.6mol, 0.8mol and 1mol and I have explained my reasons for this previously in my preliminary test evaluation. For the 0mol I will simply measure out 100ml of distilled water. For the 0.2mol I will measure 20ml or salt solution and 80ml of distilled water. For the 0.4mol I will measure 40ml of salt solution and 60ml of distilled water. For the 0.6mol I will measure 60ml of salt solution and 40ml of distilled water. For the 0.8mol I will measure 80ml of salt solution and 20ml of distilled water. Finally for the 1mol I will simply measure out 100ml of salt solution.
After these have been done and the potato pieces have been cut and weighed I will place the potato pieces in the solutions; making sure that they are all properly submerged so the total osmosis is allowed to exist. I will start the stop clock as soon as all pieces are in their beakers and time for twenty minutes. When I put the potatoes in the beakers I will seem aid in order to be able to place each piece at almost exactly the same time so that none of them can have an advantage of time. During the twenty minutes I will not disturb the potatoes at all. Stirring them could provoke a change in the rate of osmosis which would affect the data and moving them around may mean that the atmosphere around them could change (for example the temperature).
After twenty minutes I will remove the potato pieces, again, at almost exactly the same time so as not to give any piece a time advantage. I will then weigh each piece recording the present mass, the mass gained (be it positive or negative) and the percentage mass gain. After this I will repeat the whole experiment in exactly the same way twice; recording the data in the same way so that when the experiment is complete I can work out the average percentage mass gain for it different concentration of solution.
Results:
My results for my experiment were as follows.
Data Analysis:
My data shows a clear relationship between the concentration of the solutions and the amount of mass gain (or lost). The extend to which my data relates to my prediction and preliminary data is strong as all aspects link together for example; the relationship that is shown clearly coincides with my prediction too, my curve of best fit is a steadily sloping negative correlation from an average percentage mass increase of 6.66% at a concentration of 0 moles (100ml of the distilled water) to a average percentage mass decrease of 9.79% (or an increase of -9.79%) at a concentration of 1 mole (100ml of the salt solution) which shows that I was correct in predicting that as the concentration of the said solution increases the amount of mass gained through osmosis decreases which was shown in my prediction and predicted graph. My data doesn’t support my prediction to its maximum capability but this was because the accuracy of my data was restricted by both equipment and control and I also had an anomaly which affects how strongly my results support my hypothesis. The rate at which the masses of the potato pieces are decreasing is, however, not constant showing that the data may not have exact proportional relationships.
From my data I can see that, in the range of concentrations I had, the potato pieces were most turgid in the 0mol solution because this had the greatest concentration of water particles and so a lot more of these were transferred into the potato. I know that these piece we the most turgid because they gained the most weight.
From my graph I can see that an estimate for the concentration of solution for the isotonic point is 0.266mol. This can only be an accurate estimate of the point at which osmosis appears not to occur because the concentrations between the potato and the solution are at equilibrium. As well as this being the isotonic point it also means that 0.266mol of salt solution has a concentration of 0.844mol of water and so this is an estimate of the concentration of water particles in the type of potato that I used in my experiment.
In the results I obtained, once averaged, I observed that I have obtained an anomaly; this is shown on my graph by a O around the point. Analysing this anomaly I can conclude that this could have come from a variety of factors. The point is the average percentage mass gain for a concentration of 0.2mol of slat solution. This average was obtained from a range of three different pieces of data (shown in my results table) and from this I can see that in the first and second repeat of the experiment the figures are reasonably close and accurate in comparisons to the statistic I obtained in the first experiment. The would have lead to the average being off balance and inaccurate and so I cannot take this into account when making conclusions about my results. This anomaly could have occurs through carelessness in the reading of my results. I could have not recorded the potatoes original mass correctly, or similarly I could not have recorder the mass of the potato after 20 minutes of osmosis correctly either. I am certain that the solution that it was in was accurately measured so that the concentration of 0.2mol was that said concentration. I could however have been a fault in the potato piece. The concentration of water, I am sure, is not entirely constant throughout the entire potato and so the piece that I used for the said concentration could have had a slightly higher concentration of water in it and so causing the rate of osmosis to be less. Whether this difference in water particle concentration would have been large enough to have affected my data that dramatically is slightly doubtful because nowhere else in my data does this sort of thing reoccur but nonetheless I will take this into account also.
Conclusion:
From my data I can conclude that the data that I have obtained shows the negative relationship between my independent data (the mass loss or gain) and my dependant data (the concentration of the salt solution). The concentration in which the potato was most turgid after 20 minutes was 0mol of salt solution, or 1mol of distilled water. This was the highest concentration of water particles that I used in my experiment and the potato pieces that were submerged in it had the most amount of water particles transferred into them and so gained the most mass at an average of a 6.66% increase. The potato pieces that gained the least amount of mass were those in the 1mol of salt solution, with an average increase of -9.79% because at this point the concentration of water particles in the potato pieces was greater than that in the solution and so the movement of water was from the potato to the solution.
I have noticed that the average increase in mass for the potatoes in 0mol is has a smaller mass difference than that of the average decrease in mass for the potatoes in 1mol. I can conclude that this is because the isotonic point, which is 0.266 mol of salt solution, has a water concentration which is closer to the concentration of water particles in 0mol of salt solution that the water particle concentration in 1mol of salt solution. This means that the difference of mass when potatoes are in 1mol of salt solution is greater because there is a bigger difference between the water concentration in the potato and the water concentration in the solution and so that rate of osmosis is greater and more water is diffused in the given time.
I can say that my data has successfully supported my conclusion because they it has shown the behaviour that I explained in my prediction and this is confirmed by the appearance of my predicted graph matching that of my actual graph. From this all I have made the final conclusion that as the concentration of water particles decreases in a given solution the mass gained by the potato pieces decreases.
Evaluation:
I think, overall, the data that I obtained was accurate enough to support my conclusion and to reflect correctly on the prediction that I previously made. I was careful to record accurate results and tried to make sure that all reading I made and recorded were not affect my careless errors. Although in the end my experiment was a success this doesn’t mean that it was without fault and could not be improved.
All the repeats that I performed were similar to the original statistics that I obtained except for the data I obtained for the potato pieces in 0.2mol solution. The original statistic that I recorded was a 2.05% increase in mass but then this was then followed by a 4.32% increase and a 3.88% increase which were both a lot closer together and more similar to the occurring pattern in the rest of my data. Because of this anomaly the average percentage increase that I obtained was inaccurate. In order to improve on this, if I could repeat the experiment, I would repeat the 0.2mol experiment in order to replace my anomaly so that my average would have more of a relationship with the rest of the data and strengthen the support for my conclusion. I have already explain in my data analysis that I think my anomaly could have come from an imbalance in the water concentration throughout the potato or that it could have been due to careless and inaccurate recording of data. I cannot seem to think of other factors that could have affected this statistic because the atmosphere in which I performed my experiment was constant throughout; the time that each potato piece was left in the solution was equal and each piece was fully submerged; the ratio of water to salt solution was accurate and the size of the mass of the potato originally was not dissimilar to any of the other pieces.
If I could repeat my experiment in order to improve on it I would probably make sure each potato was of identical mass. This means that the amount of water particles in each potato cylinder would be almost exactly the same and so the rate of osmosis in each potato should have a stronger proportional relationship. I would also obtain a larger range of data such as 0mol-1mol with intervals of 0.1mol rather than 0.2mol. This means that my conclusion would be more supported and I could make a more precise estimate of the isotonic point, which although accurate in my previous experiment could be improved. The equipment that I used in my experiment was pretty basic and had no usage of any technical equipment. If I had been able to have access to technology that could possible record the actually concentration on water in both the solution and potato core then this would have given me further evidence to support my prediction and would also be more precise. And although the environment that I perform my experiment in was reasonably controlled it would have been even more appropriate to perform the experiment in a far more controlled environment in which the aspects that could affect my experiment could be both measured and kept constant. This are the factors that I think are most relevant to improving on my investigation other than simply to be more cautious and observant when performing them.
