Diagram:
Apparatus:
- Distilled water.
- Sugar solution in these strengths (measured in molar concentrations) 0.1, 0.2, 0.3, 0.4, 0.5
- Potato and a cork borer
- 6 plastic Petri dishes
- Ruler
- Tile and Knife
- Clingfilm (to prevent evaporation)
- Sticky Labels
Preliminary method:
When doing this experiment, I intend to…
- Take a potato and using the cork borer, cut out twelve potato cylinders
- Then cut these cylinders into the same size of preferably 4cm in length on a tile using a knife
- Take two cylinders and allocate them to each plastic dish and each group of potato cylinders will subsequently weighed on a scale. The average weight of the cylinders will then be determined using the formula
Total weight / no. Of potato cylinders
- Take the six plastic dishes and label them with the strength of the solution that they are to hold
- Add the same volume of 40ml of the different strengths of solutions into their intended dishes, totally immersing the cylinders
- Cover each of the dishes with cling film to prevent evaporation and place them away from direct sunlight.
- Then weigh the potatoes and measure them at predetermined intervals over the next day. The potato cylinders will be removed from their dish and dried with a paper towel and then measured. From their measurements the average weight and length of each cylinder will be found using the previously mentioned formula.
Fair Testing:
To make sure the experiment is reliable the following things should be considered…
- The same balance and ruler should be used every time to prevent inaccuracy
- The dishes should be left in the same place each time so the same atmosphere conditions are kept
- The same amount of solution should be used in each dish
- Each dish must be completely sealed over with cling film or evaporation will occur, causing the strength of the solution to increase therefore altering the results
- Do not make a disturbance around the scale as you are measuring the cylinders, as it will cause the scales to show an inaccurate weight
- Do not press hard on the potato cylinders when drying them
Method:
The preliminary method used for the preliminary experiments worked well and so was applied and used in the actual investigation. A potato was taken and twelve potato cylinders were produced using a cork borer, which were then cut with a knife to the same size of four centimetres in length on a tile. Two cylinders were placed into each of the six plastic dishes and were weighed on weighing scales. Forty millilitres of each of the different solutions were added to the dishes, immersing the cylinders and the six dishes were labelled with the strength of solution that they held. The dishes were covered in cling film to prevent the occurrence of evaporation and were placed in a cool dark area. The potatoes were then weighed and measured after twenty-four hours, after being dried with a paper towel. The fair testing and safety factors were taken into consideration in addition.
Diagram:
The apparatus was set up as shown below and how the potato cylinders were left for 24 hours…
Results:
My results were recorded in a table to make them clear and easy to understand. The results that were obtained from the experiment are shown below.
The average percentage change in mass will be calculated by using the formula
Change in mass / original mass x 100
I think that my results are reliable and accurate enough to draw a dependable conclusion as they prove my predictions and hypothesis to be correct. The results shown in the table below are reliable as they prove what was initially believed about osmosis, as written in my area of background knowledge.
Analysis of Graph:
Please see attached graph.
It is clearly shown on the attached graph that as the concentration of the solution was increased, the greater the average percentage change in mass. The graph shows a predominant negative correlation, that as the concentration was increased, the percentage change in mass went from positive to negative, in a steep decline. It shows that with the weakest solution, the distilled water, there was a positive increase in the change in mass, whereas where the molarity of the solutions was extended, the percentage change in mass decreased into negative percentages, in regular intervals
Both the results obtained from my experiment and the graph prove my hypothesis that the potato cylinders in water would increase in both size and mass, making them turgid. In addition, they prove my hypothesis that the potato cylinders in a stronger solution than water, a concentrated sugar solution, would decrease in both size and mass, causing them to become flaccid.
Conclusion:
From my previous experimental work, I know that osmosis is the transit of molecules of water from a weak solution to a strong solution through a partially permeable membrane. The reason that these molecules pass from a weak solution to a strong solution because a weak solution has a high solvent concentration and these molecules are seeking a high solute concentration. When a certain material, potatoes for example, as used in my experiment, is placed in a solution weaker than the solution inside it, this material will grow in size and mass, becoming turgid. When this specified material is placed in a solution stronger than the solution inside of it, it will decrease in size and mass, becoming flaccid.
As we can see from the results, Osmosis has obviously taken place in this experiment. For example, take the first dish which contained distilled water; the potato cylinders in this dish increased in mass by nearly 18%. This observation means that water molecules from the distilled water (the distilled water represents a relatively weak solution in this experiment) have passed through the partially permeable membrane to the relatively stronger solution within the potato itself. The fact that these potato cylinders have grown in mass proves their turgidity.
In the fifth dish which contained a concentrated sucrose solution of five molars, the potato cylinders in this dish shrunk significantly in mass by almost – 27%. This means that the solution initially inside of the potato, in relation to the water, seeped through the partially permeable membrane to the much stronger solution outside of the potato. As it is obvious that the potato cylinders have decreased in mass, their flaccidity is clearly shown.
My results turned out as I predicted that they would, as I predicted that potato cylinders placed in a concentrated sucrose solution would decrease in mass and size, becoming flaccid. I predicted that potato cylinders placed in distilled water would become turgid, increasing in size and weight. After looking at the results table its clearly noticeable that these predictions are correct because as shown on the table, the cylinders in water became larger and as concentration of the sucrose solution increased, the greater the decrease of the potato cylinders.
To furtherly ensure that my results were displayed clearly and accurately I plotted my results on a graph. From this graph, you can approximate the strength of the solution inside of the potato cells by determining at what point the graph crosses the X-axis, as this represents no change in the potato’s mass. I estimate that the line of best fit will cross the X-axis between 0.1 and 0.2 Molar. This however cannot be a totally accurate estimate although there is a strong correlation on the graph and all of my results have so far adamantly supported my predictions.
Evaluation:
In conclusion to this experiment, I think that it was a relatively successful experiment as it displays a good presentation of osmosis and supports my initial predictions, which were drawn on the basis of my background knowledge of osmosis. I expected that the mass percentage would decrease as the molarity of the solutions got more concentrated and the graph displays this well with the majority of the points being negative and a representation of a strong correlation.
There aren’t any anomalous results, as all of the results seem to be in relation to one another, wavering only slightly. However, although there are no obvious anomalous results, between the points of 0.3 and 0.4 molars, the line of best fit goes through both of these points, suggesting that the potato cylinders did not change very much in mass between these two strengths of solution. This reason is likely, but it is more likely that this occurred due to an insignificant inaccuracy somewhere within the experiment.
Although I managed to follow all of the points I made about how important fair testing was, the point that was difficult to measure was “Do not press the potato cylinder when drying it” and so I cannot be completely sure whether I managed to do this and could have consequently disturbed the results in someway as the cylinders could have been damaged. Furtherly, the points at which this occurred on were when the potato was placed in a higher molarity, where the potato had become much more soft and was therefore, in consequence, much more easy to damage.
If I were to repeat this experiment, I would do specific things differently to improve the reliability and accuracy of my results. To begin with, one thing that may have aided the experiment was to have more concise molars, for example, smaller recesses between molars, as in my experiment, there were distinctly larger ones. This would improve the results by providing a better reading. Another aspect of the experiment, which could be changed so to improve it, would be to have a greater number of potato cylinders designated to each dish to give a more exact average.
To hopefully extend this experiment by researching more into osmosis, I would perhaps use different plant cells, such as radish, for the cylinders to see if the same results occur and if not why? To develop the results I have achieved in this experiment I could do the same experiment again with more concise molars, as this would help me determine the exact strength of the solution inside of the potato.
If I were to repeat this experiment and therefore improve it, the method used would be loosely based upon the original, which was used in this experiment. Two potatoes would be taken and twenty-four potato cylinders would be cut out, double the amount used before to ensure a greater reliability of the results. These cylinders would then al be cut to same size of four centimetres, using a knife on a tile. There would be two sets of dishes for each of the solutions and so two cylinders would go in each dish. Prior to being added to the solutions, the cylinders would be weighed using weighing scales, and their weight would be noted. In dissimilarity to the method used before, more concise molars would be used, for example, 0.05, 0.1, 0.15, 0.2 etc… as well as water, instead of 0.1, 0.2, 0.3, etc, as used before as it would increase the chances of establishing the strength of the solution inside of the potatoes. Then, forty milimetres of each of the solutions would be added to the dishes, dousing all of the cylinders, and then the dishes would be labelled with their solution. Cling film would then be used to cover the dishes to prevent evaporation and would then be placed in a cupboard, a cool and dark area. After about 24 hours, the potatoes would then be weighed and measured and dried very carefully with a paper towel. The differences in both weight and length would be compared and then noted down.
If I wanted to further this, I could also use this method with radishes as well as potatoes, and observe the differences between the two plants. By putting the above method into practice, this would help gain a greater understanding, accuracy and reliability of the results acquired from the experiment testing osmosis.