Fair Test
To make this a fair test I am going to make sure that the volume of the sucrose solution the same at 20ml. I will change the concentration of this 20ml solution. I will keep the size of the potato the same. Also the sort of potato will remain the same. I will control the size of the potato by using the same size of cork borer and I will cut the length to 3cm.
Plan
Below is a diagram of what my experiment will look like. Also used but not shown in the diagram were pipettes, a cork borer, scalpel l, tile, ruler and scales.
Below is a list of the apparatus I used: Test tubes, potato, scalpel, tile, ruler, scales, distilled water, test tube rack, sticky labels, pen and paper.
Before each experiment the potato will be weighed and then the weight recorded. That potato will then be placed in to one of the tubes, which tube the potato is put in will be recorded. Below is a set of instructions as to how I would carry out the experiment.
- Cut and weigh the potatoes (record weight)
- Measure out sugar solutions (3x2M)(3x1.5M)(3x1M)(3x0.5M)(3x0M)
- Label tubes ie. 2M 0.5M etc
- Place potatoes in to the tubes (be sure to record which potato goes in to which tube)
- Give time for osmosis to occur, overnight if possible.
- Remove potatoes from the tubes and then dry and weigh
- Compare the weights of the potatoes before they went in to the tube and after they went in to the tubes.
Safety
For this experiment I made sure all the glassware was away from the edge of the table to avoid knocking them off and smashing them. Also I was careful whilst using the scalpel and made sure I was sitting down when I was using it and I was careful carrying to and from the tray.
Reliability
I have tried to make the results as reliable as possible by using distilled water to eliminate any sort of variation in the water. Also the measuring cylinder was washed and cleaned between measuring to prevent the next mixture being inaccurate. I used two 15ml measuring cylinders rather than larger ones so there was little room for error. Also the same type of potato etc(see fair test).Also I will repeat the experiment three times and take the average measurement in an attempt to get the most reliable and accurate results possible. Also I rolled the potato in a tissue before weighing after it had been taken from the test tubes.
Other Sources
As well as the preliminary work I have also used various biology textbooks from my library and school. They state that osmosis occurs when there are varying concentrations either side of a partially permeable membrane then water will pass through the membrane from a higher concentration to a lower concentration until
quantities either side are equal, thus backing up my prediction and reason.
Preliminary Work
For my preliminary work I used a 3cm strip of potato cut with a size 7 cork borer. I had decided on using concentrations between 2M and 0M (or water). I tested these to see if any complications would arise such as “would it get stuck in the tube or would it overflow with the potato that size”. I left them for 2 days to make sure that anything that was going to happen would happen. When I came back there were no problems - nothing got stuck and both the potato pieces had both changed in size. Also to get some idea of the results I would be getting I did an experiment using 5 different concentrations so as to see if the results would be varied enough or if they were too varied. I used the concentrations 2M 1.5M 1M 0.5M and0M sucrose sol. The results can be seen in the table below.
Modifications
As a direct result of the preliminary work I have learnt to clean the test tubes prior to my experiment as I got a build up of a brown substance in a test tube which may have affected my results. Due to this I repeated this specific concentration again. I also changed the size of the cork borer as the largest potatoes were getting close to being stuck in the tube. Other than this I did not change anything as a result of my preliminary experiment.
Area O-Obtaining Evidence
Observations
We were measuring the change in weight in the piece of potato placed in the solution.
Evidence
I measured 5 different concentrations of solutions to make sure that I got a meaningful set of results. I feel that taking 5 different concentrations was varied enough to get a fair set of results due to my preliminary work. Also testing it three times allowed me to identify any anomalies that may have occurred and consequently take them in to account.
Accuracy
The way I have ensured maximum accuracy is to use 25ml measuring cylinders and made sure not to contaminate all the equipment. Also, and most crucially, I am repeating the experiment 3 times so as to obtain maximum accuracy and then taking an average figure.
Recording Evidence
I think that a clear and concise way would be to put them in a table such as the one shown below:-
SEE TABLE ON NEXT PAGE
Anomalies
There were no anomalies that I could see from the table, although the graph may reveal one or two.
Skill Area A: Analysing Evidence
Can You Identify Any Trends?
Yes I was able to identify certain trends from the evidence. The pieces of evidence which relay my findings are the graphs I have produced. The one entitled “Average Change In Mass Against Concentration”. The graph quite clearly shows the relationship of how increasing the concentration of the sucrose solution forces the potato to lose more mass. This trend can be summarised as: If you increase the concentration of the sucrose solution , the mass of the potato will decrease as well. The line is not a straight line indicating to me that the graph shows no directly proportional or indirectly proportional link. Or it could be that my points are slightly wrong and I have made an error. However, the graph entitled “ Change In Mass Against Concentration” makes me think my results were accurate as the 3 lines are very close together. This indicates to me that I have kept within a good accuracy and consistency level. So from both graphs I am able to find that there is no directly or indirectly proportional link to the loss of mass and sucrose solution, but there is a relationship that as you increase the concentration the amount of mass change increases. I did however notice that in the middle of the graph the line was straight, but it started to tail off towards the extremities of the concentrations used. I feel this was due to the fact that the cell walls were unable to expand or contract anymore, so therefore put up more resistance and prevented the shape from being altered more. I will go in to more detail later in the coursework.
The graph entitled “Average % Difference Against Concentration” is a straight line graph indicating a proportional link between the concentration and mass lost, although the graph does not run at exactly 45°. This is I think due to slight errors or bad measurements that could have happened upon my part. . From the graphs I am able to deduce that the concentration is proportional to the % of mass lost. I have made the following approximation - for every molar you increase the acid by, the mass lost decreases by 25 grams. This is taken from the average % graph.
Recording Of Data
There are various ways of presenting the data I have achieved and each have individual qualities. The first way I have presented them is in a table format, this is useful for obtaining exact numerical values for calculations and so forth. But from the table it is difficult to identify certain trends, so I plotted graphs with the data , so I was able to identify the above trends.
Conclusion
From my investigation I can conclude that it was an accurate investigation, due to the graphs which all fall in line with a certain degree of consistency. My conclusions are that as the concentration increases the amount of weight lost by the potato increases. This statement is backed up by my results. It is easier to see in the graphs. They prove that as the concentration increases, so will the amount of mass lost by the potato. All of this evidence backs up my initial prediction. “Essentially what I am saying is that as the concentration increases, so will the amount of mass lost by the potato. However I did not expect a straight line graph. I thought that the cell wall was so rigid that any sort of large expansion or decrease would not be possible without the cell bursting or caving in. I was unaware that the cell wall would be quite flexible. However nearer the edges of the curved line graph the line starts to flatten out. I think this is because the cell wall was starting to put up a bit of resistance. I would be interested to go further with the concentrations. I anticipated the cell wall making a resistance but I expected it to allow a certain amount of loss or gain of mass. This is backed up by my prediction of “ I feel that nearer the extremes of concentration the results will be more closely grouped.” . This was because of the rigidity of the cell walls that I mentioned.. I predicted that the cell wall would affect the loss of mass much more severely. But in the light of the results I would have to say that the cell wall had a lot less effect than I previously anticipated. All in all I would say that the experiment was well conducted and to a good deal of accuracy.
Skill Area E Evaluating Evidence
Evaluation
I do believe that my results were accurate. The proof for this is in the graph I have drawn entitled “What Affect Changing The Concentration Of Sugar Solution Has On The % Of Weight Loss”. This shows results from all three of my experiments, and as you can see all three are consistently together. This proves that I am consistently accurate with my experiments. There is little chance that they were all wrong as they were done on separate days to each other.
There are however many places and ways that errors could occur. First starting with the concentration. It was possible that the concentration of the sucrose was not already precisely 2molar when I got it. Also there is the large possibility that I measured the quantities wrongly. To make this aspect more accurate I would use a smaller measuring cylinder and be sure to measure from the meniscus of the liquid (the dip in the liquid). This allows for any residue of liquid remaining in the cylinder after pouring. Also a major factor was the cutting . I could have easily left a larger surface area on one than the other. Therefore one would be at an advantage as a larger surface area induces more osmosis to occur. One way to avoid this would be to use complicated machinery to weigh and cut the potato; although beneficial, this wasn’t practical. So just taking extra time and care with the cutting would make it more accurate.
I could have done more work. I could have done the experiment more times to gain a more accurate result . Also I could test more of a range of concentrations - again to obtain a more accurate result. Also I could experiment with exposing the same weight pieces , but with different surface areas. This would be interesting to do. The results were perhaps a bit undetermined here and there. I think this could have been from one, if not more of the above factors.
If I was to repeat the experiment again then I would go to a higher concentration to see what happens in relation to the cell wall. And exactly how much osmosis is able to occur and what would happen to the cell and its wall when it reached the limit. The reason I would do this as I think it would be both interesting and beneficial to see what the capabilities of the cell and its cell walls were. Also if I were to do it again I would test more values of concentration. For example I tested 0,0.5,1,1.5,2. Then I would go in to more values like 0,0.25,0.5,0 .75,1, 1.25.....etc. The reason for this is I feel it would provide me with more accurate results to base my graph and theories on. Also I would like to see how the surface area would affect osmosis. This is because I feel that it would be interesting to know what affect it has and to compare it to the other set of results I obtained.
