I am going to make sure my test is fair by:
- Keeping the shape and size of each potato cylinder as constant as possible, if this was not done then the amount of water being able to be taken in by the cylinder would be different for each
- Using potatoes from the same batch if more than one potato is required, which will help my results to be as constant and as accurate as possible.
- Making sure the temperature is the same for all of my cylinders, because the warmer the temperature the faster osmosis takes place. I will do this by keeping all of my cylinders in the same place.
- Keeping all of the test tubes in the fridge over night, because it will prevent bacterial activity
- Using the same amount of liquids for each test tube to help my results to be constant and accurate.
- Making sure that I blot all of the potato cylinders equally to get as much excess water off each as is possible
Shape and size are especially important because Osmosis will start at the surface of the potato and the bigger the surface area the faster Osmosis will take place. This will mean that all the cylinders of potato have to be as identical as possible, for example;
Potato cylinder 1; dimensions are 2x2x2= a volume of 8cm
Surface area = 6x2x2= 24cm
Potato cylinder 2; dimensions; 8x1x1= volume of 8cm
Surface area = 4x8x1x2x1x1=34cm
So as you can see, even if there is the same volume of potato the surface area may be different which could effect the outcome of the experiment a great deal.
Apparatus
Cork Borer- I will use a cork borer because it is easy and relatively safe to use, it also standardises the shape and size of each cylinder
Pipette- I will use a pipette because it enables me to obtain an accurate volume. I will also have to pipettes, one for making relatively large measurements and the other for smaller measurements so my volumes are as accurate as possible
Needle- I will use a needle because it enables me to retrieve the potato cylinder from the inside of the cork borer without damaging it in any way
Knife- I will use a knife because I will need to slice some potato from the bottom of it, to enable me to have a steady cutting platform
Potato- I will use a potato for my experiment because potatoes are cheap and easy to use and their cells are pretty much identical to each other
Sucrose Solution- I will use a sucrose solution because it is inert (not absorbed or used by the potato cells in any way).
I will also use test tubes to keep the potato and solution inside and a test tube rack to keep them all in.
Prediction
My prediction is that the cylinders that are exposed to the least amount of sucrose are going to gain more mass and the cylinders that are exposed to the least amount of sucrose are going to lose mass. Also, somewhere within my sucrose solution percentage range, there will be a concentration in which no mass is gained or lost which is the aim of the experiment.
Basically, my prediction is that the greater the concentration of the solution, the greater the loss of mass and the weaker the concentration of the solution, the greater the gain of mass.
Preliminary work
Before carrying out the experiment, I am going to make sure that I know exactly how to use each piece of apparatus such as the pipettes. I shall practice sucking in and blowing out the water from a test tube into another helping me to gain experience in obtaining the volume of water and solution required.
Making the solution
To make the solutions I require for my experiment (bearing in mind that the solutions will be made before the rest of the experiment is carried out) I divide by the ml of sucrose by the total volume and times by 20. This will be based on 30ml of sucrose in the boiling tube. If I apply this rule to some concentrations going up to 20% (the maximum concentration) I will come up with a table like this:
I have decided that it is best to use 5 test tubes for my experiment (ten potato cylinders) because I feel that this will give me an adequately accurate result whilst narrowing down the chances of error.
Results
I intend the represent my results in a table of raw data, including:
- The concentration of each solution
- The mass of each cylinder and their average weight
- The cylinders’ final mass and average weight
- The mass gained or lost
- The percentage gained or lost
Observations
After leaving the solutions in the fridge over night, removing the potato cylinders from the test tubes, drying each of them as much as possible and weighing them I came up with these results:
To obtain these results I had to find the average mass of the two cylinders in each test tube, which was simple because they always weighed the same, I had to record their average mass after they had been in the solution and I also had to record the amount gained or, lost by the cylinder. This was all straight forward but I then had to use the following calculation to find the percentage gained or lost by the potato cylinder in order to complete my graph.
% Gained/Lost = amount gained/lost divided by average initial mass
As you can see my results show quite an obvious pattern. As I had predicted, the results show that the greater the concentration of sucrose the greater the loss of mass and the weaker the concentration the greater the gain of mass. Using this graph I am able to find out the concentration in which there is no gain or loss of mass which is the aim of the investigation. This means that the water potential in the potato cylinder is equal to that of the surrounding sucrose solution.
Using this percentage graph I can estimate that the sucrose concentration in which there is no gain or loss is about 9%. I can tell this because my line of best fit goes through the 0% mark, which tells me that at this point there has been no gain or loss in mass.
Analysis
I think that my experiment was quite successful although there probably were some faults made during it being carried out. This would have been due to human error. Small and seemingly insignificant things like not drying each potato cylinder and weighing them without due care and attention may have made a lot of difference in the outcome of the results. I also feel as if I could have done tasks more accurately- like in the weighing and cutting of the potato cylinder. Other than these things I was generally happy with the method of the experiment and how it was carried out.
Evaluation
I was very happy with my results, as they appeared to fit perfectly with my initial prediction. I predicted that as the concentration of the sucrose increased, the potato cylinder would lose mass and as the concentration decreased, the potato cylinder would gain mass. This was evident in my results as it clearly shows this pattern. I was also pleased because I attained no anomalous results that may have caused problems and may have brought my standard of performing the experiment into question.