Apparatus
For this experiment, I will need;
Corer- to cut potato pieces
Tile- to cut potato on as to not damage work surfaces
Scalpel- to cut potato to length
Boiling tubes- to contain potato and solution
Scales- to weigh potato
Ruler- to measure potato
Test-tube rack- to keep tubes upright
Beakers- to transfer solution into boiling tubes
Distilled water- to dilute solution
Potato
Sugar solution 1M
Method
A range of sucrose sugar solutions will be prepared with concentrations 0 molar, 0.2 molar, 0.5 molar, 0.6 molar, 0.8 molar and 1 molar. This will be done by adding varying amounts of distilled water to varying amounts of sucrose solution. Sections of potato will be cut using a scalpel and will be measured using a ruler. This part of the preparation must be done very accurately as a change in the surface area may allow more or less osmosis to occur. The mass of each chip will be measured as well so that more results can be obtained. I will place one piece of potato in each tube, ad leave them for 24 hours. Then the potato pieces will be removed, the excess liquid removed using paper towels and then will be re-weighed.
My measurements of the potato should be accurate; the length can be measured to the nearest 0.5mm, and the weight can be measured to the nearest 0.1g.
Fair Testing
To create a fair test certain aspects of the experiment will have to be kept the same whilst one key variable is changed. I have chosen to vary the concentration of the sugar solution. This will give me a varied set of results from which I hope to make a decent conclusion. Carrying out all the tests on one day will keep the temperature constant. I have to get the measurements and the weights of the solutions and the potatoes as exact, and as accurate as possible. I will try and get the measurements of the potatoes as accurate as possible for every single potato, evenly cutting the potato pieces, and making a record of the length to the nearest half a millimetre. The potato also must be completely covered in the solution to ensure even coverage. I will keep the water potential of the potato the same by using the same type of potato, which have been treated in the same way, e.g. have all been cut without being washed and peeled. I am also going to use the same balance to weigh my potato chips. This is because the measurements may slightly vary between scales.
Safety
I will be using a sharp knife and this is the only unsafe part of my apparatus; I will take care when handling and using it.
Prediction
For my main experiment, I predict that as the amount of H2O increases in the solution, the higher the mass, length and width of the potato. This is because water moves from a higher to lower water concentration across the semi-permeable membrane during osmosis; ie
DILUTE STRONG
(lots of H2O) (little H2O)
As the amount of H2O lowers, the potato will gain less mass, or lose mass; get shorter and have a smaller diameter.
As the amount gets higher, the potato will gain mass, length and have a larger diameter.
I predict that if the potato cells are not fully turgid at the start of my experiment, they should absorb water through the osmosis process; causing an increase in weight, length and diameter. When the sugar solution is stronger than the cell sap of the potato cells, the chips should shed water through osmosis. The cells should lose their turgor; the potato will become lighter, shorter and less rigid. During osmosis, the cells try to reach equilibrium; when there is a higher concentration on one side of the semi-permeable membrane than the other. Only water is able to pass through easily as sugar molecules are too large; they take a great length of time to break down into a diffusible size.
Pure water has the highest possible water potential because water molecules will flow from it to any other aqueous solution, no matter how dilute.
Preliminary
To see if my experiment will work, I will carry out a preliminary experiment. I will carry out the same experiment as described above; I will only use 3 concentrations as it is a preliminary to ensure my experiment will be successful, and see if I should change anything.
Results of preliminary
From my preliminary results, I can see that the weights have not changed much; this could be because the potatoes did not have enough time in the solutions to allow much solution in/out of its semi-permeable membrane. It has, however, proved my previous theory of osmosis as the 1m solution caused the potato to lose mass, the 0M solution caused the potato to gain mass, and the 0.5 molar caused the potato to stay at a similar weight. Due to this, I will extend the time I leave the potato in the solution from 3 to 24 hours, to ensure my experiment has worked, and there is sufficient time for osmosis to occur.
Analysis
I used a line graph to show the comparison between the dimensions and weight with the molarity of the solution. The graph depicts the results of the experiment. It shows the average change in weight in the six pieces of potato in a range of molarity in solution- sucrose. The general overview of the graph is a strong, negative corellation; as the molarity increases, the weight change decreases. As the concentration increases, the more water cells will diffuse through the semi-permeable membrane, adding more weight to the cell and increasing its’ turgor. I did not encounter many anomalies because I was very careful when cutting, measuring and weighing the potato and the solution. The one I did encounter (circled) was probably due to human error. The line of best fit is used to find the point of equilibrium. This is the point where the number of water particles is equal on each side of the partially permeable membrane. The line of best fit demonstrates that all the points follow in a downward direction. The graph proves that when the sugar concentration in water increases, the potato will lose weight (water) in a bid to equalize the number of free water molecules, as the sugar molecules are not free to move.
Evaluation
My experiment was very successful in my opinion. I obtained a many accurate results from which I was able to create informative graphs. I chose a suitable amount of time to leave my potatoes; the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. If I was to repeat the experiment I might increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the chips. The range of concentrations was adequate but I could have created more concentrations if I repeated the experiment so that I would have more varied results.
The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by length, width and diameter, it could have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I could use a machine to cut the potato as it would ensure that all potatoes would be exactly the same weight and dimensions. I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. I could have used a burette to ensure that I have an accurate amount of liquid in each test tube. I could also weigh each chip on a more accurate scale.
I encountered one anomalous result; his may have been caused by human error. When I removed the chips from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the weight. If the experiment was repeated I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same amount of time. Overall, I think that my experiment was successful and I was very pleased with the comparison of my results with my initial prediction.
