Equipment List – (and why I’ve chosen it)
Timer – To measure length of time potato samples are in salt solution
McCartney Bottles (10) – To hold salt solution and potato pieces
Potatoes – Cells inside potato used for water to move in/out of by osmosis
Labels – To write amounts of salt solutions on, and original mass of potato pieces to put on containers.
Corer – To take potato cores of the same diameter
Salt solution (Sodium Chloride) – To act as the variable
Electric scales – to measure the weight of the potato samples
Distilled water – To act as the control
Scalpel – To make the cores the same length and to cut off any part of the potato skin
Syringe – To take accurate measurements of salt solution
Paper towels – To dry the potato cores before weighing
Pen, Pencil, Ruler, Paper (plus graph paper) – To produce graphs and diagram
Ceramic tile – to cut potato on.
Micrometer – To measure diameters of potato cores.
Preliminary Work
To refine my method I will perform a preliminary test. I will take some simple measurements and then improve on the method to try and gain more accurate and varied results.
First of all I will take core samples of apples and measure out the different concentrations of salt solutions into five containers. I will label them and place the potato cores next to the containers. During this procedure I will have implemented the fair test conditions on the previous page.
I will place the cores in the containers and start the timer, after the set time (3 hours – 180mins) I will take them out and weigh them. The new results will be put into a table along with the original masses of the cores.
The table below shows the concentrations of water and salt solution in 5 concentrations.
The table below is where I will put the results for my preliminary work.
Conclusion after preliminary experiment
From the preliminary experiment, I can conclude the following:
- My prediction was correct, as the concentration of salt solution increased, the diameter of potato decreased (the % change in diameter went from positive numbers to negative numbers). At 0.1% salt solution the % change is 60, but at 2.0 the change is –170%.
- I think that the difference in between the numbers for the change in diameter are not very big, only having 5 different concentrations means that a trend is only vague and is not easily recognisable. The results are not very accurate and therefore there is a large margin of error.
- In the main experiment I will take 10 readings so that my results will show more distinguishable trends.
- I found out that getting cores from the potatoes using the corers provided is quite difficult as they frequently get stuck in the corer and have to be forced out using another one. I consequently found out that washing the corers after every core of potato makes them much more easier to use. I will therefore do this in the main experiment.
Method
- First I will gather all the equipment I will need for the experiment.
- Then I will get the potato cores (quartered because of preliminary experiment) and lay them by the empty containers – making sure they have the same diameter and length.
- I will measure the correct amounts of distilled water and salt solution into the containers and label them with their contents.
- The potato samples will then be weighed, putting the results on the labels and table.
- After placing the samples in the containers of salt solution and distilled water, I will start the timer and leave all samples for 130mins.
- When the time is up, the samples will be quickly dried and weighed using the towels and scales. The results will be put in the table.
- I will calculate the mass difference and then the % change in mass.
- With the % change results, I will plot a graph of % change against concentration of salt solution.
- I will then analyse and evaluate the whole experiment.
I will repeat the experiment 3 times in order to get a more varied set of results, which will be more useful in showing the results to be accurate. I will leave the potato cores for 180 minutes because I believe that leaving them for any shorter period would mean that osmosis would not have happened fully and therefore I would get inconclusive results with a large margin of error.
My range of concentration will be from 0% salt solution to 1.8% salt solution.
Safety
Even though this is a fairly safe experiment, safety precautions must be taken in order to prevent physical damage to the body.
Be cautious when using the scalpels or corers as there are sharp metal edges exposed.
In case of any breakages of the glass beakers, the teacher must be informed straight away and the glass cleaned away as quickly as possible.
Syringes must be used safely and only used inside measuring containers to extract or deposit solutions.
The table below shows the concentrations of water and salt solution in 10 concentrations.
Results
For 1st experiment
For 2nd experiment
For 3nd experiment
Average of results
Analysis
My variable was the concentration of salt solution. As I increased the concentration of salt solution, the diameter of the potato generally decreased.
As you can see from all 4 of my graphs, they show the same trend. The higher the concentration of salt solution, the diameter of potato decreased, the lower the % change of diameter.
Each graph has a line of best fit showing the effect of different concentrations of salt solution on potato cores after a 180-minute period.
Osmosis is a case of diffusion in which only water molecules are concerned. It is the movement of water molecules through a partially permeable membrane (in this case, the potato cells) from an area of high concentration to low concentration. Bigger molecules cannot move through the partially permeable membrane (such as starch). So when the concentration of salt solution is lower e.g. at 0 and 0.2, there is a larger concentration of water outside of the potato, therefore the net movement of water is into the potato, increasing its size and increasing the diameter. But when there is a higher concentration of salt solution, there is more water in the cells of the potato than in the surrounding solution so the net movement of water is out of the potato. Therefore water leaves the potato and enters the solution to ‘balance’ the salt solution.
Plant cells have a strong cell wall that protects them. When water moves by osmosis into the plant cells, they swell up and become turgid. When many cells become turgid, they push out on each other and the whole structure of the cells become more rigid and will increase in size (turgor pressure). This is why the potato cores placed in a low concentration of salt solution produced a positive % change in diameter of the potato.
However, when water moves out of plant cells, they become flaccid, the cells become thiner and have shrunk. The contents of the cell (cell membrane etc) have actually pulled away from the cell wall. When this happens in the potato cores because of the increased concentration of salt solution surrounding the potato cores, the cells within the potato became more flaccid and therefore shrunk – decreasing the overall size and diameter of the cores.
This conclusion supports my prediction; my prediction stated the following:
‘Osmosis is the movement of water molecules through a partially permeable membrane (in this case, the potato cells) from an area of high concentration to low concentration. Bigger molecules cannot move through the partially permeable membrane (such as starch).’
This is what I stated in the conclusion, which shows that my prediction was correct and also that my results were conclusive.
Evaluation
The results for all experiments agree with my prediction; I said that:
“I predict that as the concentration of salt solution increases, the potato core diameter will decrease. Therefore, the % change in diameter will initially increase and then will decrease with a minus percentage.”
This is what happened in the results, for the first experiment, the results from a low salt concentration were: 0.2=65, 0.4=10, 1.2= -100 and 1.6= -140.
I think that my results are relatively accurate but I can account for how they may not be accurate.
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The 1st set of raw data was collected by using the same potato, but the 2nd and 3rd were collected from a different potato. Because not every potato is the same and has the same properties, this might have affected my results by making less of a fair test and a larger margin of error.
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For the 1st experiment, there was one potato core per McCartney bottle. But for the 2nd and 3rd experiments I had two cores per bottle so the result taking speed was doubled. I think that because of this, the results would have been less accurate.
- Even after washing all equipment along with my hands, there could have been salts and grease on the surfaces prior to cutting the cores, this would have affected cores placed in low salt concentrations – by making less of an increase of diameter.
- I was unaware of how long the samples would take to cut and then start the experiment, during the main experiment, the cores were left standing for around 5 to 10 minutes, this may have affected the results as water would have evaporated from the potato, reducing its size after the initial measurement.
I think that my evidence is good enough to support my conclusion; I think that I was able to produce fairly accurate results under the circumstances (with limited time and in a lab with limited space). Also, because of the safety measures that I used (like washing my hands and all equipment before use) – I was able to decrease the margin of error and therefore increase the accuracy of the results.
I know that my method must have been a fairly good one because of the accuracy of the results and how they agree with my conclusion. The prediction agrees with the results as well as the conclusion, so I know that the results must be fairly accurate.
Apart from the general margin of error that meant that none of my graphs were completely similar, there was one very erroneous result taken on the 3rd experiment (circled on the graph). I think that this could have happened because all cores were not completely cylindrical, the diameter varied at different points along the sample. This would have meant that readings might have been taken from different points along the core before and after the experiment – giving inaccurate results.
Because of the nature of the thing I was trying to measure (potato cores) and the difficulty of being completely accurate, it has led me to believe that maybe measuring another factor that would have changed as a result of different salt concentrations like the mass of potato cores. This would have given me more accurate results, as there wouldn’t have been any problem with measuring by hand at different points. Also, digital scales would have preformed all measuring, which would have been far more accurate.
To reduce error on measuring the diameters of the potato, I could have taken at least 3 diameter readings from each core and found the average diameter size to use in my results, but this would have been too time consuming under the circumstances.