Variables
The independent variable is the concentration of sucrose solution in which the potato cylinder is placed in (0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 mol ml-3). The dependent variable is the percentage change of mass (grams) of the potato cylinders before and after the experiment and the percentage change of the length of the potato cylinder after being submerged for a day in the sucrose solution.
The factors controlled in this experiment are the ones that are the same from one sucrose solution to another and hence, the variables, other than the concentration of the solution, that may alter the rate of osmosis. The controlled variables will be the surface area of the potato chip – It will reduce the rate of osmosis, if a larger potato chip is used. [One must first measure the sizes using a ruler (±0.5 mm) as well as using a cork borer to maintain the same diameter for each potato chip]; age of potato – osmosis may be more efficient in newer potatoes than older ones and the species / variety of potato will be kept the same for all experiments as the rate of osmosis is likely to differ if a different species / variety of potato cells are used. Moreover, I will control the temperature, because if the temperature is warmer for one experiment, this will speed up the process of osmosis. [I’ll use room temperature to maintain the same set temperature for each experiment performed and use a thermometer to monitor this.]
Apparatus
- Cork borer
- 1 Potato – so it’ll be the same age and species
- White tile - Do all cutting on a white tile; therefore I will not damage anything.
- 2 Measuring cylinders
- Paper towels - to clear up any spillages quickly and to place the potato chips on after taking them out of the sucrose solutions
- Top pan balance (±0.01g)
- Ruler (±0.5mm)
- Stopwatch
Method
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From the 1.0M sucrose solution prepare 20cm3 of each of the following concentrations and put them into a series of labelled test tubes: 0.7M, 0.6M, 0.5M, 0.4M, 0.2M, 0.1M and 0.0M.
- Using a cork borer (ensure you have the same size one throughout) cut potato chips 40mm long, making sure the ends are cut square and they will fit in to a test tube. Ensure all chips weigh at least 0.5g.
- Blot each chip dry and weigh accurately to 2 decimal places. Measure the length of each chip to the nearest half mm.
- Place one chip in each solution.
- After a day remove the chips, blot dry and re-measure the length and mass. Carefully record this data for each relevant chip and solution.
- Calculate the percentage change of mass using the formula below:
- Calculate the percentage change of length using the formula below:
- Calculate class averages and standard deviations for the length and mass values. Plot graphs of average percentage mass changes against sucrose concentration. Add the standard deviation values as error bars to your plotted points.
Qualitative Results
The potato chips fizzed small air bubbles when placed in the less concentrated solutions (0.0, 0.1 and 0.2). In addition, I observed that the potato chips floated in the 0.7 mol ml-3 solution and in the 0.6 mol ml-3 solution, and half floated in the 0.5 mol ml-3 solution. Another thing that I noticed was that potato chip became more dull when place is distilled water hence slightly changing its colour.
Conclusion
Evaluation and Improvements:
The experiment required more than one piece of potato; 2 potatoes were needed to be used and hence that meant that I accumulated data from two different sources of sucrose and each piece of potato might have had a different amount of nutrition, hence affecting the amount of sugar in each, so that might have affected the results of the experiments that were conducted.
As well as measuring the mass changes I should have measured the volume changes. I could have done this by using the diameter and length of the chip and seeing how this changed after being in the solution. This may have been a better indicator as to what the water potential is. By examining the point in which there is no volume change, due to incipient plasmolysis.
Instead of doing all the repeats in the same boiling tube. I should have made a fresh solution for each chip in a separate boiling tube. This would have reduced the chance of human error. So if the dilution was done incorrectly it wouldn’t affect all the repeats as it did in my experiment. Also I wouldn’t have to use thread to identify which potato is which as they are in a separate boiling tube. Thread could have affected my results as it damages the surface cells and also disallows osmosis to occur in the small region it covers.
After taking the potato chips out of their sucrose solution, the potato were not thoroughly dried before they were placed on the top pan balance, hence the final mass may have been much higher than the actual final mass, hence it may have led to inaccuracies in the final percentage change results. To improve this, I should have used a fair drying method to remove the water outside the surface of the potato. If I allowed them all to dry for five minutes and then measured the mass, maybe my results would have been more accurate. Other methods such as using a hairdryer etc. would incur more human error.
I could have used a narrower range of solute concentration to find the point of no mass change and length more precisely for example 0.1M, 0.125M, 0.15M, 0.175M, 0.2M, 0.225M, 0.25M, 0.275M, 0.3M, 0.325M, 0.35M, 0.375M, 0.4M.
Moreover, I carried out all of the experiments assuming that that the room temperature remained constant and that each experiment was carried out at the same temperature. I should have implemented a way of controlling the temperature. Using a water bath could have done this.
As soon as I placed the potato into the sucrose solution, I covered all of the test tubes, with paper towel in order to prevent any water to be lost via evaporation, as this would make the solution more concentrated. However, the paper towel could have caused humidity, which could have affected the results. So instead of using cling film I could have used light proof sheets, which would have also stopped variations in light intensity affecting the results.
by
abuelgasim (student)
