VARIABLES:
In this experiment there should only be two changing variables:
1: Different molarities of sugar solution.
2: Changing mass of the potato samples.
To keep these two variables the only variables in the experiment I must:
1: Keep the potato samples the same length (4cm) and diameter. This is because if one potato sample is 3cm long and one is 5cm long then the 5cm long sample will have a larger surface area.
2: Use the same potato. This is because many factors due to the potato may affect the experiment. For example the age, species can cause the potato to have different water potentials.
3: Accurate amount of sugar solution: To make the amount of solution placed in the test tube as accurate as possible a syringe will be used to measure out the exact amount needed.
5: Contamination: As each test tube is filled up with the different molar solutions the syringe which would measure the amount of solution placed in the test tube may become contaminated with different molarities. To stop this the beaker and syringe must be washed every time they are used or changed.
7:Temperature: The temperature may affect the reliability of the experiment for example at extreme temperatures the cells of the potato may die and 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
PRELIMINARY WORK:
In this experiment because I must use a variety of molar sugar solutions to select the range I did a pilot test before the real experiment. I have selected 0 (distilled water), 0.2, 0.4, 0.6, 0.8, and 1.0, as this is a wide range and will be appropriate for my experiment.
METHOD:
Here is a summary of our method:
- Put on safety goggles
- Collect all the equipment
- To make the different solutions I have to use 0 (distilled water) solution and 1.0 sucrose solution to make the correct concentration. Fig 1:a shows how I do this:
Fig 1:a
- Place the potato on the white tile and using the potato borers cut 6 tubes out of the potato. Make sure to press down onto the white tile, to sure maximum safety.
Using borer
Potato on white tile Potato tubes
- Cut the potato tubes to 4 cm using the scalpel on the white tile so that all are now the same size.
- Place the potato tubes on filter paper.
- Weigh the potato tubes separately; take a note of the start mass.
- Place potato tubes on to white tile.
- Using the preliminary work, work out how much sucrose solution and distilled water is required to make up 20ml of 0.2, 0.4, 0.6, 0.8 molar solutions.
- Use separate syringes to get required amount of sucrose and distilled water for 0.2, 0.4, 0.6, 0.8 molar solutions and put it in separate labelled boiling tubes.
Distilled water syringe 1.0 Sucrose solution syringe
- Place one potato tube into each concentration solution and then start the stopwatch.
- Leave for 20 minutes.
Stopwatch
- After 20 minutes is up, take out the sucrose solution and remove all surface fluid from each potato tube using filter paper, making the potato have minimum contact with the hands.
- Weigh each potato tube and make a note of the end mass.
- Repeat twice if results are still unreliable repeat once again.
RISK ASSESMENT
To make the experiment safe the equipment that was sharp i.e. Scalpels and core borers were used carefully on a cutting tile. Also in case of anything getting into the eyes goggles were worn at all times.
RESULTS
Below there is a result table of all my results from this I have created two graphs one to show the molarity of the potato using the % change in mass and the second to show the relationship between molarity and solute potential of sucrose solution where I will be using fig 1.b for reference.
Fig 1:b
Look at graphs 1 and 2
CONCLUSION
Graph 1 show the % change in mass of the potato compared with the concentration. As you can see from my line of best fit (the red line) it is approximately 0.64 M where the line crosses the x-axis this shows where the potato neither lost nor gained mass. I have one anomalous result this is where the concentration was 0, I got this result because the concentration I used was distilled water and therefore the potato lost a lot of mass because of this result being anomalous I have not included it in my line of best fit.
In my prediction I stated in a solution of high concentration the potato would lose water and mass and gain water and mass in solutions of lower concentration. This was certainly the case for my experiment. As you can see from my graph 1 at points 0.0M, 0.2M, 0.4M, 0.6M the % change in mass is all positive which shows it has gained mass and at points 0.8M and 1.0 the % is negative which shows the potato loss mass. Also in my prediction I was very close to stating that the molarity where the potato would neither gain nor lose mass was 0.64 as my actual prediction was 0.6 The scientific explanation for why this happened is explained in my interpretation.
Graph 2 shows the relationship between molarity and solute potential of sucrose solution. From my experiment I can conclude that the estimated solute potential of my potato is -1950kPa. The equilibrium is reached at 0.64M. To work out the solute potential I had to create a graph using the data in Fig 1.b.and from this I worked out that the solute potential of the potato was -1950kPa.
INTERPRETATION
From my conclusion I worked out that the solute potential of my potato was -1950kPa I got this result by finding the state of equilibrium in my potato this is when there is no osmosis occurring and when there is the same amount of water molecules in both solutions, which therefore stops the net movement of water molecules.
Looking back at my prediction I stated that in a solution of high concentration the potato would lose mass and gain mass in solutions of lower concentration from results you can see they support my prediction. Fig 1.c shows this concept in osmosis terms.
Fig: 1.c
In my experiment at the concentrations 0.0, 0.2, 0.4, 0.6 the potato gained mass because the water molecules moved from a high concentration being the solution, to an area of low concentration being the potato cell. At the concentrations 0.8 and 1.0 the opposite happened and the potato lost mass due to the water molecules moving from a high concentration, which is the potato cell to an area of lower concentration being the solution.
EVALUATION
In general my experiment and method went really well as I received the results I needed and therefore enabled me to analyse them and work out the final solute potential of the potato but as always there are some errors that could have been reduced and some procedures that could have been changed to stop these errors from occurring.
Firstly I would like to look at my anomalous result, it occurred possibly because the solution was distilled water and this was extremely concentrated compared to the other solutions and resulted in a dramatic increase in mass of the potato tube. For this reason I did not included it in my working out of the solute potential.
Human error could have been reduced by taken more accurate measurements in cutting the potatoes. I used a 20cm ruler with millimetre divisions this means I could have only got an accuracy of 0.5mm. This could be improved by using a Vernier Scale tool of accuracy 0.05mm so the accuracy would improve.
One disadvantage of the method I used was that it was easy to contaminate the sucrose and distilled water because as each test tube is filled up with the different molar solutions the syringe, which would measure the amount of solution, placed in the test tube may become contaminated with different molarities. To stop this the beaker and syringe must be washed every time they are used.
Also when drying off the potato it was difficult to remove al the excess solution off the potatoes without pressing to hard and removing the fluid from inside the potatoes. This would have resulted in an inaccurate increases in mass therefore the percentage change in mass would also be too high. This could be changed by using a rolling technique of the filter paper to remove the excess fluid instead of just pressing on the potato tubes or checking under a microscope to see the effects on individual cells of the potato
I also had no way of telling how concentrated the sucrose solution of 1M was. This would also result in the solutions I used to be unreliable because if the Molarity of for example 1.0 sucrose solution was too high the potato would lose extra water making the weight loss increase causing the results to be erratic. I could have made my own solution, or use a simple titration to find out the Molarity.
Out of all the errors that could have occurred from above the one I believe would have caused the greatest impact was the contamination of the solutions and unreliability of the measurements in cutting the potatoes.
Increasing the number of results taken so instead of taking results from 7 different molarity levels 10 molarity levels could be taken. This would make the lines of best fit on the graphs more reliable, therefore the end results would be more reliable. Furthermore repeating the whole experiment and taking the average to make the results as reliable as possible, also ignoring any clearly anomalous results.
The results I got were matching my prediction therefore they are good. My experiment was reliable but if I were to do it again to make it more reliable I would make time to repeat it and use the changes I have mentioned above.
