Analysis
In pot 1, the potato chips increased in mass (4.96% of original mass). This was because the concentration of water in the potato chips was lower than in the distilled water. The distilled water moved into the potato cells to try and balance out the concentrations of water. This happened through osmosis. There was diffusion both ways, but mostly into the potato cells so that the cells became turgid. The cell wall of the potato cells stopped the cells from bursting.
In pot 2, the potato chips reduced quite a lot in mass (9.91% of original mass). This was because the concentration of water in the potato chips was higher than in the salt solution. The water from the potato chips moved out of the potato cells through osmosis and into the salt solution to try and balance out the difference between the two concentrations. This meant that there was diffusion both ways, but mostly out of the potato chips. The cell membrane of the potato chips shrivelled and detached slightly, meaning that the cells became flaccid.
In pot 3, the potato chips also reduced in mass quite a lot, but not as much as in pot 2. This was because the concentration of water was higher in the potato cells than in the water and salt solution, so the water diffused out of the cells into the solution to try and balance out the concentrations of water. Not as much water was lost from the potato cells as in pot 2 due to the concentration of the salt solution being lower. This meant that not as much water was needed to balance out the concentrations of water in the salt solution and the potato cells. The loss of water meant that the cells of the potato chips became slightly flaccid, but not fully.
Aim
I am going to investigate the effects of different concentrations of salt solution on water diffusion in and out of potato cells.
Prediction
The basic graph expected from the results of this experiment would be as follows…
By looking at my preliminary work and using the knowledge of osmosis that I already have, I believe that I can make an accurate and well-informed prediction. When potato chips are placed into a weak salt solution, they will increase in mass because free water molecules will pass through the semi-permeable membrane into the potato chips, which have a stronger concentration of salt solution inside them. The potato cells will become turgid.
If potato chips are placed into a strong salt solution, the water molecules will move out of the potato cells into the surrounding solution. This is because the potato cells have more free water molecules than the salt solution. Therefore, the potato chips will decrease in mass and become flaccid. For example, when the concentration of the salt solution is 70%, the potato will lose around a fifth of its original weight.
When potato chips are placed into a solution of equal strength, I would not expect there to be a change in mass as the concentrations are equal and therefore the same amount of water molecules will move out of the potato cells as into the cells.
I will be able to find out the concentration of salt solution inside the potato from my graph of results by looking at the concentration of salt solution at 0% change in mass on the y-axis. This will be where the line crosses the x-axis.
Apparatus List
- 128ml saturated salt solution
- 182ml distilled water
- Potato chips
- Knife
- Balance (accurate to 1d.p.)
- 10cm syringe (accurate to 2cm )
- Stop clock (accurate to 1d.p.)
I have chosen to use a syringe to measure the amounts of saturated salt solution and water, instead of a measuring cylinder, which I used in my preliminary work. Using a syringe would be a more accurate measurement and would therefore give me more accuracy in my results. It would also make the test fairer.
Chosen measurements and concentrations
I have chosen to change the concentrations that the potato chips are and measure the change in mass because I believe that this will most effectively show me the affects of osmosis on plant cells. I have chosen to use 40ml of salt solution and water in each pot as in my preliminary work I found that this is enough to cover the potato chips fully, and the concentrations are not difficult to work out and measure. I will be using salt concentrations of 0, 10, 20, 30, 40, 50, 70 and 100%. I have chosen to use these as the concentration of salt in the potato cells will be below 50% as potatoes are not salty foods. My preliminary work also showed that the concentration of salt in the cells was below 50%, as at this concentration the potato chips decreased in mass by 8.88%.
Safety
To ensure that my experiment is safe, I will wear goggles throughout and tie my hair up. I will also wash my hands before and after the experiment.
Fair test
I will ensure as far as possible that my experiment is a fair test by ensuring that I only use one variable. I will keep the temperature that the pots are kept in the same; they will be left in the same area of the room. I will also make sure that the three potato chips that are in the same pot are of equal masses. The type of potato that I use will be the same. I will also measure quantities as accurately as possible by using a syringe. I will leave all the potato chips in the solution for the same amount of time, and use equal volumes of solution each time. I will also pat dry the chips before weighing them at the start and end so that the mass does not include solution on the surface of the potato, and only the solution in the cells.
Method
- Using the syringe, measure out the appropriate quantities of saturated salt solution and water, and put them in the correct pots. This information is shown in the table of measurements above.
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Cut the potato into 24 cube-shaped chips using the knife, ensuring that there are three for each pot. The three chips that are to be used for the same pot should be of the same shape and surface area, or as close as possible to the same.
- Pat dry the potato chips evenly. This should be done to the same extent for each potato chip to make the test fair.
- Record the collective mass of the three potato chips that will be in the same pot.
- Add the potato chips to the correct pots simultaneously, and begin the stop clock immediately.
- Leave the potato chips in the pots for 60 minutes.
- After 60 minutes stop the stop clock, remove the chips from their pots, pat dry once more and record the masses again. This should be done as quickly as possible to make the test fairer.
- By calculating the percentage difference in mass, you can work out how much water has passed in or out of the potato chips and how the chips are affected by different concentrations of salt solution.
Results
(See graph of results included on next page)
Conclusion
From my investigation, I have achieved my aim and have found out the effects of different concentrations of salt solution on water diffusion in and out of potato cells. When potato cells are put into a weak salt solution, they increase in mass. This is because the salt concentration inside the potato cells is higher than that in the surrounding solution, meaning that water molecules diffuse out of the weak salt solution into the potato cells across a semi-permeable membrane to even out the concentrations of salt. When potato cells are placed in a strong salt solution, they decrease in mass. This is because the salt concentration inside the potato cells is higher then the salt concentration of the surrounding solution. Water molecules then diffuse out of the potato cells and into the surrounding solution to try and equal out the concentrations of salt and water. This is the process of osmosis.
When potato is put into a salt solution of equal concentration, the mass of the potato stays the same, as equal amounts of water molecules move into and out of the potato cells. This is because the concentrations are equal. This can be proved by looking at my graph if results, as it shows that the higher the concentration of salt solution, the greater the decrease in mass.
The results of my investigation support my prediction fully. I said “When potato chips are placed into a weak salt solution, they will increase in mass because free water molecules will pass through the semi-permeable membrane into the potato chips, which have a stronger concentration of salt solution inside them. The potato cells will become turgid.” For example, when the concentration of salt solution was 0% (pure water), the potato gained 9.36% of its starting mass.
I also said “If potato chips are placed into a strong salt solution, the water molecules will move out of the potato cells into the surrounding solution. This is because the potato cells have more free water molecules than the salt solution. Therefore, the potato chips will decrease in mass and become flaccid. For example, when the concentration of the salt solution is 70%, the potato will lose around a fifth of its original mass.” My results proved this, for example when the potato was placed in a salt solution with a concentration of 70% the potato lost 21.36% of its original mass. This is just over a fifth of its original mass. Therefore, water molecules diffused out of the potato cells into the surrounding solution to try and equal out the concentrations. The potato became flaccid as it had lost water.
Overall, my prediction was fully correct as I predicted exactly what happened.
Evaluation
My graph showed no anomalies. This was because my experiment was a fair test and was accurate and reliable. I didn’t find the procedure too difficult to perform, however I did experience a few problems. It was difficult to accurately cut the potato cubes so that they were of equal shapes and masses. The surface areas were also not exactly the same. This would only affect my results very slightly, if at all, but is something that I could possibly improve in future by measuring the dimensions of the potato chips, or using a potato cutter that cuts the chips equally. It was also quite hard to dry and reweigh the potatoes at the same time, so there was around a two-minute gap between the first re-weighing and the last. Again, this may not have affected my results very much, or at all, but in future I could use more balances and more people so that all the chips are removed from the solutions, dried and re-weighed simultaneously. This would improve the accuracy of my results very slightly. I felt that the balance I used was accurate enough, and that the quantities were measured accurately. However in future I could improve this accuracy by using balances accurate to three decimal places and a digital syringe to ensure that my quantities are exact. Therefore, my results are reliable and allow me to draw an accurate and reliable conclusion. In the future, I could extend my investigation by using different concentrations of salt solution, for example ranging from 0-10% salt solution. I could also use more concentrations in my experiment, which would give me more results. This would then also give me a more accurate line of best fit, and give me a more accurate measure of the salt concentration inside the potato cells. Therefore, the reliability and accuracy of my results and conclusions would be improved.
