In the investigation I have chosen to change one key factor which is concentration gradient. The solutions we will use are 0% salt (distilled water), 1% salt, 2% salt, 5% salt, 10%salt, 15% salt and 20% salt.
All the other factors will need to be controlled in the investigation.
Temperature – will be kept at room temperature
Type of membrane – we will remove the potato skin at the ends of the potato
Time – the investigation will be left for 1 hour
Surface area – the potato will be cut to the same length (4.2cm) and with the same size borer giving the same diameter
pH – we will use water which is approximately pH 7
Mass of potato – the mass of potato used will be 2.4g
Hypothesis/Prediction
Osmosis is defined as the movement of water molecules from a region in which they are highly concentrated to a region in which they are less concentrated. This movement must take place across a selectively permeable membrane such as a cell wall, which lets smaller molecules such as water through but does not allow bigger molecules to pass through. The molecules will continue to pass through until the area in which the molecules are found reaches a state of equilibrium, meaning that the molecules are randomly distributed throughout an object, with no area having a higher or lower concentration than any other.
In this investigation I predict that the lower the concentration of the salt solution in the test tube the larger the mass of the potato will be. This is because the water molecules pass from a high concentration, e.g. in the water, to a low concentration, e.g. in the potato chip. Therefore, the chips in higher water concentrations will have a larger mass than in higher salt concentrations. I also predict that when in distilled water the mass of the potato will increase.
Reliability/Fair Testing
In the investigation I will do each measurement three times allowing me to calculate an average.
If I get three very different measurements I will use the measurement which corresponds best with my previous results.
To ensure the accuracy of my measurements I will use accurate measuring equipment such as a measuring cylinder for the solution and electronic weighing scales for the potato.
Pilot Work
I carried out a preliminary experiment similar to test for a suitable method for the investigation and to see whether the results were what I expected. I investigated the change in mass of potato left in salt solutions of varying concentrations for 1hour. The results are as follows.
Aim
The aim of this experiment is to investigate the effect of varying concentration of salt solution on the amount of osmosis occurring between the solution and a potato chip of a given size.
From the pilot work carried out and the results collected I can determine that the method used was accurate because there were no anomalous results, and therefore it should be able to be used in the experiment. The results also seem to follow my prediction that the lower the concentration of the salt solution in the test tube the larger the mass of the potato will be.
Method
- Take a potato and from it using a borer cut out 7 tubes onto a white tile.
- Make sure all the tubes are 4.2cm long and 2.4g in weight. Remove potato skin from the edge of the tubes.
- Fill 7 test tubes each with 4cm³ of salt solution of different concentrations.
- Place 1 potato chip in each test tube and leave for 1 hour for osmosis to occur.
- After 1 hour remove the chips and remove excess water by rolling them on a paper towel. Excess water would increase the mass and affect the results.
- Record the weight of each potato chip using an electronic balance.
- Then repeat the method twice to give 3 sets of data for more accurate results.
Apparatus to be used
Results
Analysis
The graph shown gives a simple curve of best fit for the mass change of the potato chips over the course of the one hour experiment. The graph is a curve that slopes downwards and does not go through the origin. Because the line is not straight and does not pass through the origin, it means that the loss in mass and concentration of solution are not directly proportional. However, there is a pattern on my graph, and this is, as the concentration of the solution increases, the loss of mass increases. This means that the concentration of the solution and the mass are inversely proportional. The gradient does change in my graph. It gets less steep along the X axis. This is because the potato chip is becoming as flaccid as it possibly can, and so the change in mass of each concentration is becoming closer and closer together. From the line of best fit that has been added in, it can be seen that all of my points were very close to creating a perfectly smooth curve. This shows that my results are fairly reliable. My graph fits in with my prediction of the experiment. I also calculated the percentage change in the mass of the potato. The results in the table below have all been rounded to three significant figures because it is an accurate but manageable figure. Again the results on the percentage change graph show a simple curve sloping downward without going through the average showing that percentage change of the mass and concentration aren’t directly proportional.
Conclusion
For this investigation I predicted that the lower the concentration of the salt solution in the test tube the larger the mass of the potato will be. The results I gathered fit this prediction well. However they don’t fit the prediction that when in distilled water the mass of the potato will increase as well because the mass did increase but only by a maximum of 0.04g my theory was that having a higher concentration of water in the solution would mean that the potato cells would gain mass because more osmosis would occur and the potato cells would therefore have a higher water concentration and would have gained mass. I think that 0.04g of mass gain is not significant enough as to be definitely caused by osmosis occurring within the potato it may have just been an anomalous result. According to the graph I produced to show mass change at 0.2% salt solution no osmosis should occur because this is where the curve of best fit crosses the x axis. This would be because there would be an equal amount of water molecules in all places or an equilibrium meaning that no osmosis would be occurring because they would both have the same concentration of water.
Evaluation
Suitability of procedure and apparatus
The potato borer did not always cut out a perfect cylinder, we would try to select the best cylinders but that was not always possible with the potatoes provided. This means some potatoes would have had a greater surface area and it would also effect their mass slightly. Temperature control was also not very accurate because we just conducted our experiment at room temperature which would have varied between the times we did the experiments
Did the method result in errors?
We attempted to measure everything to the highest degree of accuracy as possible using the equipment we were given but we were measuring the length of the potato with a ruler and we could have possibly misjudged the lengths. Also when measuring the volume of the salt solution we were using pipettes which are not the most accurate piece of equipment
The biggest error
The biggest error that probably occurred in this experiment would be that when we left the experiment set up for 1 hour for osmosis to occur we left it over our lunch break meaning that on one occasion it had nearly 20 minutes extra than we had planned however this doesn’t seem to have caused any visible anomalous results. The errors we encountered would be like this in order of their impact on our work
- Time
- Lack of potatoes that could be used to provide consistently accurate results
- Temperature control
Reliability
As you can see from my results tables and graphs my data seems to be very reliable and accurate to a high degree. There appear to be no anomalous results that could have been caused by errors in my work but there are still things that could be done to improve this experiment.
Possible Improvements
Things that could be done to improve the experiment that I carried out are to carry it out in a controlled environment where the temperature could be kept constant, to be provided with more potatoes that had not been used previously and that were of a reasonable size and to monitor the time more effectively by keeping an eye on the stop clock constantly instead of leaving it over break.
Safety
Safety equipment that we used were aprons to protect our clothes, goggles when producing salt solutions using a Bunsen burner, protecting our eyes. Caution should be exercised when using scalpels to cut the potatoes.
Sources of information
Introduction –
Information on osmosis in roots taken from
Information on osmosis in animal cells taken from
Diagrams of red blood cells
