- For the 0.2 molar sucrose solution I think the result will be similar to the distilled water. The concentration gradients are very similar therefore the mass won’t vary a large amount. I predict that the weight of the potato will stay above 1gram producing a rise in mass.
- I think the 0.4 molar solution will decrease in weight by a fractional quantity, as the concentration gradients between the 0.4 molar solution and the 0.2 molar solution are similar. This is why I predict the mass will only be decreases by a small amount.
- The 0.6 molar solution, I think, will decrease in weight. The concentration is higher in the potato, consequently causing water molecules to transfer from the potato into the solution surrounding it.
- I predict that the 0.8 molar solution will decrease in weight. The gap between the concentration gradients of 0.8 molar solution and potato tissue is large. I presume potato has a higher concentration therefore resulting in water molecules transferred from the potato into the solution. This concludes a lower weight of the potato.
Preliminary test:
We did a preliminary test to test the time it took for the potato to experience osmosis at its maximum gain or minimum loss. We put the potato in the solution and checked it every 2 minutes. The preliminary was vital to ensure I had sufficient results. I stopped testing the potato every 2 minutes when I reached 15 minutes. At this point the potato was at its maximum for water gain or water loss, I could tell this when I weighed it. We used distilled water and 1 molar solution to carry out our preliminary test.
We used these results to decide how long to leave the potatoes in the solution in the main experiment. We knew that this would be correct, as these two solutions are the maximum and minimum in the range of solutions that we used. There fore all the other solutions will take less time as they have a smaller concentration gradient so osmosis will complete quicker.
Diagram
This is a diagram of the equipment I will be using and how I will set out my experiment:
Apparatus
The apparatus I will use is the following:
- 5 test tubes
- 1 potato
- A cork borer
- Paper towels
- Test tubes holder
- 5 sticky labels
- 20cm³ of 0.2,0.4,0.6,0.8 molar solutions and 20cm³ of distilled water
- Stopwatch
- 5 bungs
- Digital weighing scales correct to 2 decimal places
- Scalpel
- Measuring cylinder
- Tile
Method
I plan to carry out this experiment by using all the safety issues and fair testing procedures to give me the most reliable and most accurate set of results.
The method I will use to carry out my experiment is shown below:
- Excavate the potato into 1.00-gram cylindrical shapes using a cork borer. Having used a scalpel, I will remove the skin of the potato. This will be done on a tile to prevent any unnecessary mess.
- Blot the potato using tissue paper to eliminate any excess water from the potato.
- 5 test tubes will be put into the test tubes holder and 20cm³ of sugar solutions and water will be measured into each of them.
- Label the test tubes to ensure no confusion of the solutions
- Add the 1-gram potato segments to the solutions
- Bungs will be used to cover the test tubes; this prevents any evaporation.
- Start the timer for 15 minutes;15 minutes was found as a good time from the preliminary test
- After 15 minutes I will remove the potato from the test tubes and blot the excess solutions and water off
- Weigh the potato and check the weight to see if it shows any increase or decrease of osmosis with in the mass of the potato
- Record my results in a table
Fair testing
In this experiment on osmosis there should only be two changing variables:
1: Different molarities of sugar solution,
2: Change in mass of the potato samples.
To keep these two variables in the experiment I must:
- Keep the potato samples at 1 gram before they go in to the solution. If any are above 1 gram then the surface area will be bigger causing osmosis to happen at a quicker rate and more cells to lose or gain water
- Make sure I remove the skin of the potato. The skin if different from the tissue of the potato
- Use the same potato. This is because many factors due to the potato may affect the experiment. For example, the age and sizes might of the potato might be different which means one potato might have more water in them then the another
- Stop the evaporation from any of the solutions. If evaporation causes the potato tissue to be out of water then the potato wouldn’t have as much surface area as any of the others. I will use a bung to stop evaporation
- Temperature: The temperature may affect the reliability of the experiment. For example, at extreme temperatures the cells of the potato may die, at less extreme temperatures the experiment may be speeded up. To keep this from happening, all the test tubes will be kept in the same place and at the same time of the 15-minute experiment
- An accurate amount of the solutions. More bathing solution could affect the rate of osmosis, I will use a measuring cylinder the measure the solutions
- The beaker and measuring cylinder must be washed every time. If it is not washed it could cause the solution to become contaminated with different molarities
- Averages will be taken out of the results to make the experiment as accurate as possible
Obtaining Evidence
When I am obtaining the evidence for my results I will take all these aspects into consideration:
Safety aspects
I will carry out the experiment safely and as accurately as possible. I will wear safety glasses at all times to prevent any substances entering my eye. I will be very careful so that no substances are swallowed or enter my body through any cuts or grazes.
Accuracy
To ensure my experiment is as accurate as possible I will measure the solutions very accurately and I will get a partner to check it for me. I will do the same when measuring the potato. I will repeat the experiment three times to make sure my average is consists of a good range of results. The solutions go up by 0.2 every time so our results will not be too similar. A good range of results will be produced. If I find any anonymous results I will record them but then I will repeat the same test to see if I get a result which follows some sort of pattern.
The weighing scales I will be using are correct to 2 decimal places. I think 2 decimal places are accurate enough for this experiment. It is accurate and it is also easy to convert our results to averages.
All the results I will use I will measure to 2 decimal places. This keeps the experiment accurate and corresponding. I will also record the units I will use in my result’s table.
Results
This is our table of results:
From my results I can see a trend going down the list of solutions. From 0.2 molar solution to 0.8 molar solution the weight decreases. This shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. I will draw a table to show the change in mass of the potato slices:
A table to show the change in mass of the potato slices
I will draw a graph from my averages of results to the table of results:
I have drawn a frequency curve onto my graph and from that I can see the weight decrease as the solutions increase.
At point A (low water concentrations), it shows that the cell is not decreasing in size. This is because the cell is becoming plasmolysed and no more water can leave the cell.
At point B (high water concentration), there is no indication that the cell is rapidly increasing further in size. This is because the cell is becoming fully turgid and no more water can enter.
Conclusion
In conclusion to my investigation I now know that the evidence I obtained supports the prediction I made. It shows that if the concentration of sucrose solution is higher than the solution in the cells of the potato, water will be transferred out of the potato cells by osmosis through a partially permeable membrane.
My investigation was successful. My results agreed with the predictions I made; as the molar solution increases the weight decreases due to the potato cell becoming turgid at a low molarity and plasmolysed at a high molarity.
The graphs and the results show that:
- Osmosis actually took place in the experiment
- As the molars increased, the percentage of the weight difference decreased
This experiment helped me find out that osmosis occurs between two liquid substances with a partially impermeable membrane, and that the higher the water concentration is the larger increase in grams (mass).
During my investigation as I demonstrated in my introduction the potato cells changed like this:
Diagram 1. Diagram 2. Diagram 3.
The plasmolysed cell – diagram 1 – this cell is present in the investigation when the potato cells were put into a solution with a high molarity, no more water can leave the cell, this is shown by my graph (page 8, Point A).
The fully turgid cell – diagram 3 - is also present in my investigation. My results and graph prove that as the potato cells are put into a solution with a low molarity then the turgidity of the cell is at its highest point, therefore no more water can enter the potato cell. This results in no increase in size (mass), which is demonstrated on my graph (page 8, point B).
From this scientific knowledge entwined with my in my investigation I can be confident that my prediction are reliable and my results support the predictions I made.
Evaluation
I believe we gained accurate and sufficient enough results, shown on our graph and tables that concludes the experiment, and to prove our hypothesis. My final results were very reliable, due to the precautions I took to make this a fair test.
Looking at my graph of average results (page 8) I can see no anomalous points; this tells me that the measuring and suitability of the procedure I did was very accurate. However, looking closely at my result table (page 8), I have found an anomalous reading from the table. It is situated on my ‘second attempt’ column. I have plotted a graph in order to identify and prove the anomalous result:
From the graph I have circled the anomaly. It is the 0.4 molar solution reading that seems out of sequence with the curve of best fit.
Due to this anomalous result looking back at the overall experiment I have thought of a number of improvements to give me better and accurate results.
Firstly the concentrations were measured using a measuring cylinder and could be made more accurate using titration. Also when the potato was dried to remove surface liquid it was not necessarily done the same on each potato, a more accurate and uniform way of drying would improve the accuracy further.
The evidence I obtained is very reliable as you can see from my graph of average (page 8). I collected a big range of evidence therefore I can back up my conclusion with the evidence I obtained.
As I have found only one anomalous point in my results I can confidently state that the evidence I have collected is sufficient to support the conclusions I have made. However the anomalous point that I encountered makes the results less reliable but as I did three attempts of each molar concentration and then took averages the anomalous point is not noticeable in my final averages (see graph page 8) the graph is systematic and in a stabile shape.
This critical account of the evidence concludes in reliable data collected, giving me a reliable conclusion as the results, overall, support the conclusions that I have made.
Further work that could have been extended onto this investigation to provide additional relevant evidence would be to test different molar solutions closer than the solution I tested. For example, - 0.20, 0.21, 0.22,0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30. This would produce much more accurate results. I would carry out the investigation in exactly the same uniform pattern that has been used in this experiment, but I would change the molar variables. Together with the information that I have achieved in this investigation would produce very precise results because there is more information that is collected.
Other variables in the experiment could be changed for example instead of changing the mass of the potato the species of the potato could be changed. For example I could use a new potato and a really old potato.