Fair Test
Below are the variables and non-variables of the experiment:
In order for the results to be reliable and accurate the variable need to kept the same. I will make sure that the results are accurate and reliable by:
- Using the same volume of sucrose solution. If there is inefficient sucrose solution to cover the potato then not the whole of the potato pieces will osmosise.
- Using the same balance to weigh the potato as different balances can have slightly different measurements.
- Using the same ruler to measure the potato as different rulers may have slightly different measurement.
- Doing the experiment in room temperature as changes in temperature can affect the rate of reaction
- Using the same type of potato as different potatoes may have different water potential.
- Running the experiment for the same amount of time for all potatoes. This is because time the time the experiment runs for can effect the mass gain or loss.
- Using the same size and shape potato as different surface can effect the rate of reaction
- Using the same length potatoes as different size potato will gain or lose different amounts of mass.
Preliminary Work
Prior to the initial experiment, a quick experiment was done to determine what equipment, solution molarity, solution volume, and potato sizes to use. In this experiment I used 2 cm long potatoes, 50 ml of sucrose solution. I ran the experiment for 40 minutes
Below are the results obtained from the experiment.
It is clear that the results obtained aren’t that representative this could be because the experiment wasn’t ran for long enough or the range of molarity wasn’t big enough. Considering the information gathered from the preliminary work I am going to do the following for the initial experiment:
- Use a wide range of moralities; 0.2, 0.4, 0.6, 0.8 and 1. This will ensure that I get accurate and representative results.
- Use 2 cm long potato pieces, as they are easy to handle.
- Run the experiment for 60 minutes. This will give enough time for osmosis to take place and help get accurate results.
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Use 100 cm3 measuring cylinder. This will help make sure that measurements are accurate.
- Use tissue papers to lightly dry potato pieces. This is because they could carry excess water after been fished out.
- Use 100 ml beakers to hold the solution and potato, as fishing potato pieces out of a test tube is a bit difficult and can cause inaccuracies.
Apparatus
For this experiment the following equipment will be used:
Safety Precautions
This experiment involves using sharp scalpel to cut the potatoes. Take care while using the scalpel or you could cut yourself. Cork Borer are also sharp, care needs to be taken when they are used.
Plan
- Take a potato that’s healthy and hard.
- Drill a hole into the potato using 10mm cork corer. Take the drilled piece of potato and cut it to a length of 2 cm using the ruler. Cut another 4 potato pieces the same way.
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Take five beakers and label the 1, 0.8, 0.6, 0.4 and 0.2. Now measure 50 cm3 of 1 molar sucrose solution using a 100 cm3 measuring cylinder and place the solution into the corresponding beaker. Repeat this with the rest of the four molar solutions left.
- Now place the potato pieces into each of the molar solutions and start stopwatch.
- Run the experiment for 60 minutes.
- Then fish out all the potato pieces with a tong and lightly roll them on the tissue paper to remove excess water.
- Then weigh each potato piece on an electronic weighing scale and record the change in mass (if any).
- Repeat the experiment again to get reliable results.
Results
Average Results
Analysis
According to the results the cell sap in the potato have a molarity just over 0.2. After drawing the line of best fit on the graph it is evident that the cell sap has a molarity of 0.235. The graph shows a negative coloration. This means that as the molarity of the solution increases the mass of the potato piece decreases. In a hypotonic solution the potato loses mass and in a hypertonic solution it gains mass.
The cell sap has a molarity of 0.235. The reason for this is that, it doesn’t gain mass when placed in an isotonic solution (has the same molarity as the potato). There isn’t a gain in mass because water moves in and out of the potato both ways in equal amount through the process of osmosis. Water constantly moves both into the potato and out of the potato to maintain equilibrium in an isotonic solution. There are also no changes in the size of the potato in an isotonic solution as no additional turgid pressure is applied.
As the graph shows, when a potato piece is placed in a hypertonic solution (with a molarity of 1 on the graph for instance) then there is no equilibrium between the two as the concentration of water in the potato cell sap is higher then the solution outside. Osmosis takes place in order to bring about equilibrium between the two. Water moves out of the potato cell sap and into the solution outside, as this happens the potato piece loses water and therefore mass (in the case of a 1 molar solution the lost is – 0.55 gm). Because the potato piece is very small compared to the solution outside, it will never be able to eject enough water into the solution outside to bring about equilibrium. So therefore it will keep losing mass and the potato piece will become more and more concentrated until the cells become completely plasmolysed and die. In a hypertonic solution the potato piece will shrink as plasmolysis takes place. This means that the cells content will break away from the cell wall due to the lack of water. When this happens to all the cells in the potato, there is a reduction in size. The diagram below shows what happens to cells when they plasmolyse:
The graph shows that on the other hand potato piece will gain mass when placed in a hypotonic solution (0 molar solution for instance). Again when the potato piece is placed in the hypotonic solution there is no equilibrium. Water moves into the potato cells sap through osmosis, as the concentration of water in potato cells is lower then the solution outside. This gain in water results in a gain in mass (in the case 0 molar solution then gain is 0.35 gm). Water moving into the cells causes them to swell because the vacuole pushes against the cellulose cell wall (turgor pressure). Pressure applied due to gain of water through osmosis is also referred to as osmotic pressure. When turgor pressure is applied to all the cells in the potato there is an increase in size.
The results completely match the prediction. As suggested by the prediction, the results reflect the fact that potato pieces gain mass in a hypotonic solution. This is because water moves into the cell due to osmosis as the solution outside has a high water concentration. The increased amount of water also caused the cells to swell and become turgid. As the prediction suggested the potato piece lose mass in a hypertonic solution. This is because water moves out of the potato pieces and into the solution due to osmosis. The decrease of water in the cells causes them to become flaccid and shrink. As the prediction suggested, there is no gain or loss in mass when a potato piece is placed in an isotonic solution, as water moves both ways in equal amount.
Evaluation
The results were relatively reliable as they showed a pattern, i.e. the weight decreased in all hypertonic solutions and increased in hypotonic solutions. As evident through the results and the graph, there was an anomaly for the 0.2 molar solution both times the experiment was done. The results weren’t 100% reliable because there were a few inconsistencies during the experiment. Below is the list of inconsistencies and ways in which they could prevented next time the experiment is carried out:
The results matched the prediction completely as regards to the gain and loss in mass in different types of solutions. But the results don’t show a definitive molarity of the potato piece. The results only suggest that the molarity is above 0.2. The evidence obtained is sufficient enough to match the prediction regarding weight loss and gain. But there isn’t enough evidence to show what the real molarity of the potato pieces is.
There are various ways in which the experiment could have been extended to get reliable, accurate and definitive results. Firstly, the experiments could be repeated several more time to get accurate figures. The moralities of the solutions could have been narrowed down further to between 0.2 and 0.3. To get accurate results a wide range of moralities between 0.2 and 0.3 could be tested, i.e. 0.205, 0.201, 0.215, 0.220, etc. Also in addition to measuring the mass the length and diameter of the potato pieces could be measured to look for any changes in different moralities of sucrose solution.
