When a potato is placed in a hypertonic solution (high solute, low water concentration), the potato will have a high concentration of water compared to the sugar solution. As a result water leaves by osmosis – the water molecules have moved from a region of high concentration (the cell sap) to a region of low concentration (the surrounding). This causes the potato to lose mass. If the potato loses too much water due to the amount of solute in the solution being high, the cell content shrinks or becomes flaccid. This process is known as plasmolysis.
Equilibrium occurs in an isotonic solution – the amount of water going into the cell is equal to the amount of water leaving the cell. This means that the osmotic pressure on one side of the membrane is equal to the pressure on the other side. Therefore the mass of the potato does not change. From my data, equilibrium is achieved when the concentration of sugar is at approximately 1.5cm³.
Evaluation
As part of the experiment, I have got results for ten different readings ranging from the potatoes being placed in pure water (50cm³ water), to a concentrated sugar solution (50cm³ sugar solution). This is a good range which allows me to be more confident in my conclusion. I can improve this by having more readings within this range (e.g. twenty readings instead of ten), or by increasing the range (e.g. 100cm³ water to 100cm³ sugar solution).
My ranges do not provide confidence to my conclusion:
Although the ranges for the nine readings above (the table excludes the potato placed in pure water) are not huge, it lacks confidence. For the readings 20cm³, 30cm³, 35cm³, 40cm³, 45cm³ and 50cm³, the ranges are small, making me confident in my results. In contrast, the readings for 10cm³, 15cm³ and 25cm³ are too big, questioning the confidence in my results.
Whilst carrying out the experiment, I did not find any difficulties. However reliability of the equipment to carry out the experiment at times certainly did cause a problem. One example of equipment which caused complications was the measuring cylinder used measure a certain amount of sugar solution/water. The measuring cylinder was only able to measure to the nearest cm³ which is not accurate enough for its purpose. Nevertheless, the mass balance used, measures to 2 decimal places which is very accurate for its purpose.
My results show two anomalous results which are marked on one of my graphs. The sources of error in my experiment that could explain the reason for that particular unreliable result include that not all the potato pieces were the same length to start with. This could have been solved by measuring each potato piece with a ruler and cutting all the potatoes to the same length. Similarly stated above, the measuring cylinder was not accurate to measure volumes of solutions with. This only measures to the nearest centimetre. In the future I will use a burette to measure volume. A burette is accurate to the nearest 0.1cm³. Another major problem might have been the drying time of each potato piece. The time that each potato was dried varied. To prevent this problem, how long the potato pieces are dried for should be timed. Other potential problems may have caused this anomalous result which could have been improved to make the results more reliable.
The reading for each concentration is close to my line of best fit (with the exception of the anomalous result). This makes my results reliable because the readings are close to the line of best fit. However my other graph shows that there is not a real difference between the results. The ranges of the results overlap slightly. This shows that the concentration of solute did not have a great effect on the percentage mass change on the potatoes (excluding the potato which was placed in pure water). My range bars therefore make me less confident in my conclusion that the concentration of a solute has an effect on the mass of potato tissue.
There were no real hazards to my experiment. No goggles were needed necessarily to protect the eyes. I had to take care with the glass beakers, keeping them away from the edge so they don’t fall and shatter. This can easily be avoided so apart from that there were no minor or major hazards that I had to tackle.
Similar Experiment
Below is an experiment similar to the one I completed. In contrast to my experiment, the solute that was used was salt instead of sugar.
In this investigation, the following variables were controlled:
- Type of potato
- Size of potato
- Temperature of water
- Type of solute (salt)
The independent variable in this investigation was:
The results of the experiment are seen in the table below:
Like the experiment I completed, when the potato was placed in a hypotonic solution (0%, 25% salt solution), water enters the cell sap by osmosis due to it having a low concentration of water compared to the water in its surroundings. This causes the potato to gain in mass, which can be seen in the table above.
When the potato was placed in a hypertonic solution (50%, 75%, 100% salt solution), water leaves the cell sap by osmosis due to it having a high concentration of water compared to the sugar solution. This causes the potato to lose mass. The results from this experiment clearly back up the data I received from completing my experiment.
