When the concentration of water is the same on both sides of the membrane, the movement of water will be the same in both directions. At this point, the net exchange of water is zero and the system is in equilibrium.
This information is relevant to the experiment we are doing because we are investigating osmosis and how different molar concentrations affect the weights of pieces of potato due to osmosis taking place. It explains what we will expect to happen with strong concentrations of sucrose and weak concentrations of sucrose and explains why I have predicted what I stated in my prediction. Osmosis is complete when the concentration is balanced.
Method
Equipment needed:
14 test tubes (for each molar concentration of sucrose and an extra 7 for repeats of the experiment for an accurate test).
Test tube rack
2molar, 3molar and 4molar concentrations of sucrose solution so that we can produce the concentrations that we require. We received this information from a preliminary experiment to find the more precise concentrations of sucrose solutions that we will be testing with.
Measuring cylinder to measure out the amounts of sucrose solutions that we need.
Potato borer and knife to obtain our small pieces of potato that we will need.
Ruler to measure each potato and to check that each piece is of the same length.
Electric weighing scales.
A potato.
Kitchen towel to place the pieces of potato on after the experiment has finished. This is so we will not make any mess.
As said in the prediction, we will be investigating the effect of different concentrations of sucrose on the weights of pieces of potato after osmosis has taken place and how much sucrose is in a potato. We will do this by finding the concentration that will have the least affect on the potato, the one that will have the least change in weight. If there is no change in weight then we can see that osmosis has not taken place and so this must mean that the amounts of sucrose and water are virtually the same. We carried out a preliminary experiment to find out the solution of sucrose that will produce the most accurate results. For this experiment we used 0 molar, 1 molar, 2 molar, 3 molar, 4 molar, 5 molar, 6 molar, 7 molar and 8 molar concentrations of sucrose. We found out after the experiment that the solutions that we will obtain the best results from will be between 2m and 4m concentrations. Therefore we will conduct this experiment using more precise solutions (0.225m, 0.250m, 0.275m, 0.300m, 0.325m, 0.350m and 0.375m) to find the solution that will produce the most accurate results.
Measure the same amount of sucrose out each time, 15cm³, using the measuring cylinder, making sure that we are measuring as accurately as we can so the experiment will be fair. Rinse the measuring cylinder out after measuring each concentration out so that there will be no remains of sucrose left in the cylinder that will affect the next solution being measured out in any way, i.e. different molars mixing together or different amounts of solution each time. This would produce an unfair and inaccurate set of results. We need to produce reliable results each time so we can draw a correct and accurate graph and identify trends in the graph and our results. We can therefore come to a firm conclusion using our results obtained. So, replicates are essential for all measurements, i.e. getting the same answer at least 2 times out of 3 after repeating each measurement. To do this, we must measure out the correct amounts of solution carefully each time, taking care to be as accurate and as careful as we can. We need to produce precise results e.g. 1.10g instead of 1g because this way the experiment is more accurate and we will not make any mistakes with rounding up numbers wrong, which will results in an incorrect answer and may produce an anomalous result on the graph.
We will repeat each concentration four times, with two pieces of potato in each test tube and then with another set of test tubes to ensure an accurate test that we can draw a firm conclusion from and that will fit in with the background information.
To make this experiment safe we will act sensibly in the laboratory and we will not run around. The test tubes will not be placed near the end of the bench in case they fall off and break and cause any injuries. They will be placed in the test tubes racks so that they will not roll of the end of the bench or will not tip over and spill everywhere.
The results graph will be plotted using the average change in mass after the experiment.
We will need to have a suitable number and range of observations so that there are enough values in the important region that we will need to be investigating in i.e. the range of concentrations that we obtained from out preliminary work. It is essential that we obtain enough results so that we can plot a graph and achieve a firm conclusion from them.
To make it a fair test only one variable can be changed, this is the concentration of the sugar solution.
Other factors that may affect the experiment:
Temperature- if the temperature is high then this may affect the experiment and may speed up osmosis. To control this we can make sure that the test tubes aren’t left near any open windows or near any heaters.
Surface area of the potato- we will need to ensure that the surface area of each potato that we will cut out will have the same surface area. We will therefore need to take care and be as accurate as we can whilst measuring each potato and make sure that each piece of potato is the same length (2cm). We will also need to use the same potato borer each time as some will produce thicker pieces of potato and if we use different sizes of potato each time then the experiment will not be fair. Cutting the ends off some pieces of potato may have an effect on the weight of the potato therefore we will be careful not to cut off too much.
Weight of the potato- this is similar to surface area so it is another reason why we will have to be careful when boring and measuring our pieces of potato. Also, when boring out pieces of potato, make sure that enough space is left between each hole. If we bore out pieces too close together then we may not bore out the same size piece of potato each time.
The time that the potato is left for- this will be three days.
What to do
Bore out 28 pieces of potato, each the same size and measure each piece using a ruler to make sure that they are all 2cm long. Using the knife, cut every other piece of potato at an angle so it is easy to distinguish each piece from one another.
Using the 2m, 3m and 4m concentrations of sucrose, measure out carefully to produce the correct concentrations needed, which are 0.225m, 0.250m, 0.275m, 0.300m, 0.325m, 0.350m and 0.375m. For 0.225m, measure out 5cm³ of 3m and 10cm³ of 2m solution. For 0.250m, measure out 7.5cm³ of 2m and 7.5cm³ of 3m. For 0.275m measure out 5cm³ of 2m and 10 cm³ of 3m. For 3m measure out 15cm³ of 3m concentration. For 0.325m measure out 10cm³ of 3m and 5cm³ of 4m. For 0.350m measure out 7.5cm³ of 3m and 7.5cm³ of 4m. For 0.375m measure out 5cm³ of 3m and 10cm³ of 4m.
Weigh each potato and record the weights using the electric scales. We are using the electric scales because they are accurate and so it will be easier to record accurate measurements of the weights of the potatoes each time.
Place two pieces of potato, one cut and one uncut into each test tube. We place two pieces in each test tube because this way we are repeating the experiment to make the test accurate.
Repeat this all over again for the other set of test tubes.
Place the test tubes in the test tube racks and leave for two days away from any window or heater so that the temperature will not affect the experiment.
After two days, pour the solutions down the sink and record the weights of each potato in each test tube using the electric scales. Record the uncut piece of potato and the cut piece of potato in each test tube separately and record each weight.
Work out the change in mass from before and after and work out the average change in mass. Use these results to plot a graph.
Results
Test tube A:
*= cut piece of potato
Test tube B:
*= cut piece of potato
Please see separate results graph.
~With thanks to Lucy Zecchin for these results. ~
Conclusion
From the results we achieved we can see that the amount of sucrose in a potato must lie somewhere between 0.350m and 0.375m as these two concentrations ended up with the least change in weight. This means that the rate of osmosis must have slowed down and so the concentrations will have more or less evened out at this point. We can see that there is no particular trend in the results, but when the concentration of the sucrose solution is high, the average weights of the potato pieces are lower than the weights when the concentrations of the sucrose are low. This must mean that the amount of sucrose in a potato is fairly high.
The graph produced shows negative correlation. This means that the higher the concentration, the lower the weight of potato. This fits in with my prediction because I predicted that the higher the concentration, the lighter the weight of the potato.
Evaluation
As I have had to borrow another person’s results, it can be said that my set of results achieved was not sufficient enough to draw a conclusion or produce a graph from. However, this is not because we made any major mistakes during our experiment, it is because my partner and I discovered that we did not have any readings of the weights of the potato beforehand. This is because we lost them so it shows that we were not quite responsible enough with this experiment. However, the actual test we did went well, we worked sensibly and when we did have the recordings of our weights before the experiment, we could see that the procedure was adequate for what we were investigating. Our results achieved were similar to the ones recorded here from another person’s test, and these results are adequate enough to draw a conclusion from and draw a graph from.
Although the graph looks a bit scattered, it is still quite easy to draw lines of best fit. This means that the experiment does have a trend, so the graph and results must be accurate.
It is not easy to say whether the evidence is correct or not, and it is not easy to say whether the replicates of the measurements were correct. Cutting the ends of the pieces of potato may have affected the weight of the potato if we cut too much off. Therefore this may have had an effect on the weights of some of the pieces of potato, making this a slightly unfair test.
To improve our experiment if we were to do it again, we could take more care measuring out the solutions using the measuring cylinder and we did not have enough time to rinse out the measuring cylinder after measuring out each molar solution. This may have had an affect on our results, due to some excess sucrose left in the measuring cylinder after use. Next time we could also make sure that we rinse out the measuring cylinder after each use so that we will not have any excess sucrose solution left in that will affect the next measurement. Our results were also left by a window, so the temperature could have been affected if the window was open or the weather outside was hot. To improve this experiment next time we can also make sure that we leave the test tubes in the test tube racks nowhere near anything that will affect the temperature i.e. a heater or a window.
The conclusion may only be valid only within a certain range, if we were to use even more precise concentrations of sucrose solution then we can eventually find out the precise concentration that provides the correct answer. The concentrations used were slightly too vague as I had to say that the answer must lie between two amounts. However, if we were to improve the experiment by narrowing down the concentrations we will use even more then this will be time-consuming and very tedious. Therefore, it will not be a good idea to try and improve the experiment in this way.
To extend this investigation we can also experiment changing the following variables:
- Temperature of the experiment
- Surface area of the pieces of potato
- Weight of the potato
- How long the experiment is left for.