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In the experiment the water in the milk will diffuse into the potato piece if the water potential is less than the potato. This will increase the mass of the potato. The potato piece can then be weighed to observe any changes in mass. The milk which gives the potato the greatest mass increase will be the one which has the greatest water content. This is an effective way of telling which solution has more water content because the solution which has greater water content will have a lower water potential. Because of this lower water potential there will be a greater difference in water potential between the potato and this will result in a much steeper diffusion gradient. Because of this, the water will diffuse into the cell at a much faster rate and at the end of the set time period the potato mass would be greater than the others because more water diffused into the potato cells within the set time period. Water will be used as a control group because the water potential of water is 0, and this will allow for an accurate comparison and analysis of the other trials, but it will also help in identifying if there is an anomaly. Approximately 5 trials will be done, as this would be the number of trials that could be performed within the time given. This will minimise the effect of anomalies on the overall results.
Risk Assessment:
When using the knife, it will be best to stay in one place. It is also advisable to cut things on a stable, even and solid surface in a non-crowded area. This also applies when using the cork borer. Make sure to be careful when using sharp objects. Lab coats must be worn at all times when in the lab. Nothing must be consumed inside the lab to avoid consumption of harmful chemicals due to contamination. Be careful when handling glass as broken glass can cut you. If any glass is broken, you should clean it up right away with a dust pan and brush. During the course of the experiment, one should be careful with the flask containing water as there will be electrical equipment being used by other students.
Main Hypothesis:
Skimmed milk has more water content than either full-fat or semi-skimmed milk
Null Hypothesis:
Skimmed milk does not have more water content than full-fat or semi-skimmed milk.
Independent Variable:
The type of milk is the independent variable as this will be changed to determine which solution gives the potato the greatest gain in mass.
Dependent Variable:
The change in mass of potato (the rate of osmosis).
Controlled Variable:
- The shape (surface area) of the potato pieces, because the surface affects the rate of osmosis. It is best to keep the surface area the same, to allow for more accurate results.
- The volume of milk, because a larger volume of milk will have more water even though it will be the same ratio of water to milk.
- Temperature because this affects the rate of osmosis.
- Time allowed for each trial, because more time will allow more water to diffuse into the potato.
- Also the potato pieces must come from the same potato because a different potato might have a more permeable membrane which will affect the rate of osmosis.
Method:
- Peel a single potato.
- Using the cork borer, cut out pieces of potato.
- For each strip pulled out by the cork borer, measure out 1cm and cut there.
- Continue doing this until you have 20 potato pieces.
- Measure out a total of 125ml of skimmed, semi-skimmed, full fat milk and water.
- Pour 25ml of each solution into 4 beakers, do this 5 times. You should have a total of 20 beakers.
- Now weigh each potato piece and write the mass down on the beaker you are going to put the potato in.
- Put the potatoes in.
- Have a timer set for 40 minutes.
- Once the 40 minutes are up, weigh each potato.
- Note down the change in mass.
Main Report:
Method:
Peel a single potato and then cut out chunks of it using a cork borer. Measure 1cm along the strip of potato and then cut. Repeat this until you have 20 pieces of potato.
Weigh a potato piece and write down the mass of the potato on a sticky label. Put the potato on top of the label (you wrote the mass on). Write on the label whether it will be in the Skimmed milk, Semi-Skimmed milk, Full Fat milk or Water. Do this for each potato piece. Obtain 20 beakers and then pour 25cm³ of Skimmed milk into 5 beakers each. Put a potato piece into each beaker. Start the stop watch after this is done. Do this for the Semi-Skimmed milk, Full Fat milk and water. Make sure to markdown the time discrepancies between each batch of trials. Wait for 40 minutes, and then remove the potatoes from the first batch of trials. Dry the potatoes and then weigh them, write down the mass on a piece of paper. Put the potato back into the empty beakers, after weighing each one do this so that you do not confuse the results. Repeat this for the other solutions.
Apparatus:
Results:
The Mass Gained By Potato Pieces in Different Solutions:
Anomalies can be recognised by Bold Italics.
Conclusion:
The results of the experiment suggest that skimmed milk has more water content (i.e. is more dilute) than semi-skimmed milk and full fat milk. This supports the initial hypothesis, however a 0.0032 difference is not conclusive and more/different trials would be required to get conclusive results.
Discussion:
The results generally support the initial hypothesis. There is a general trend of the potato’s having a greater mass increase when put in skimmed milk. This indicates that skimmed milk has greater water content when compared to the other milk solutions. Semi-skimmed milk has a greater average gain in mass than full-fat milk. This indicates that semi-skimmed milk is more dilute than full fat milk. This general trend of skimmed milk > semi-skimmed milk > full fat milk (in order of most water content to least) suggest that skimmed milk has extra water added to it to compensate for the fat removed from the milk. This is also supported by the fact that semi-skimmed milk gave the potato pieces a higher average gain in mass than full fat milk. Both milk types which had fat removed gave the potato pieces a higher average gain in mass when compared to full fat milk. This indicates that something is done to semi-skimmed and skimmed milk to compensate for the fat removed. This compensation may very well be extra water added to the solutions.
Evaluation:
The batch of trials for full fat milk and semi-skimmed had a range of 0.02g compared to skimmed milk which had a range of 0.031g and water which had a range of 0.086g. The results for the full fat milk and semi-skimmed can be considered more reliable when compared to the skimmed milk because it had a larger range. The control group water had the largest range due to two anomalies. One of the factors may have been the discrepancies in total surface area between the different potato pieces. There were also problems obtaining enough beakers for the experiment, this forced the experiment to be conducted in different shaped beakers. It took time obtaining enough beakers which forced there to be time discrepancies between each batch; however this was partly compensated for by allowing extra time for the other batches. Another factor to be considered was the varying room temperature. The temperature increased due to an increased number of bodies in the room. This may have affected the experiment as the rate of osmosis is proportional to temperature. One of the limitations of the experiment was the volume of the potato itself. Due to the limiting factor of the potato’s volume, any discrepancy in the surface area between the different potato pieces had to be “overlooked” due to the fact that there was not enough “volume” of potato. Furthermore, another potato could not be used for the experiment because this would have added more factors to the experiment which would have made the results less reliable because the permeability of cell membranes varies from potato to potato. This also meant that the number of trials was also limited due to the limited mass of potato. Another limiting factor was time, the time given was barely enough to carry out five trials. Due to the limit of time, the potato pieces were only allowed 40 minutes to soak in the milk solution rather than the originally planned hour. The experiment could be extended by putting the milk into a flask, where it could be heated to allow the water to evaporate from the solution, and then condensed and collected in a separate flask using a condensation tube. Measuring the volume of water would then allow one to compare the differing volumes of water content for each type of milk.
Appendix:
Range of Results (In order of most accurate to least):
Calculations:
The Water Content Of Different Milks When Compared To Water:
