The Variables and Accuracy
There are several variables that we have to keep the same to ensure a fair test, these are to keep the temperature the same throught the experiment; the chip length the same; the amount of the time all the chips are in the different concentrations and the amount of solution. Also we will use the same scales when weighing before and after so that we will get the same difference in mass when we weigh, the scales will be accurate to one hunderenth of a gram. We will keep the temperature the same because reactions happen faster when the temperature is higher (it creates energy) and we want all the reactions happening at the same speed. We will keep the chip length the same because if the size of the chip changes, then the surface area:volume ratio will change. If the chips were in the solutions different lengths of time then one chip will have more time for osmosis to occur than another chip. We will keep the same amount of solution in each boiling tube because it would be unfair for one chip to only be half covered in solution and the others to be fully covered. To be accurate, we will record the mass change in percentage and to calculate this we will use the equation: %change = (change in mass/original mass) x 100
Apparatus
Method
We are going to use potatoes to conduct our experiment because they are still living and have cell membranes. We will use 0.2, 0.3, 0.4, 0.5 and 0.6 Molar solutions of distilled water and glucose. We will measure the different Molar concentrations out by using a pippette, a measuring tube and a beaker. We will use the distilled water to make sure that it is a fair test and that the only thing dissolved in the water is glucose. We will cut five potato chips to excatly the same length, measure and weigh them, and drop them into the solution at exactly the same time. After 30 minutes we will take out the chips, dry any excess liquid off them, and reweigh and measure them.
Analysation and Conclusion
We then agreed on a narrower range of concentrations to further test the potato chips on to try and find the concentration of the cell sap. We decided on using 0.24, 0.26, 0.28, 0.32, 0.34 and 0.36. We didn’t use 0.3 because we used that in the previous experiment and now know that the solution of the cell sap isn’t that.
The concentration of the cell sap was found to be 0.34 Molar.
The graph for the second experiment shows that the concentration of the cell sap is 0.34 Molar, this agrees with my prediction of the concentration being about 0.3 Molar. The graphs for both the first and the second experiments show a similar result which shows that they are accurate. Neither of the lines pass through the origin which mean that the percentage gain and the loss in mass are not proportional. But, there is a pattern in the opposite direction going in an inversely proportional way which means that as the concentration of the solution increases, the percentage change in mass decreases. My graph shows that there was an overall decrease in mass during the experiment.
Evaluation
My results were quite accurate but it was hard because the Molar solutions were hard to measure out as they were so tiny also we did the two experiments on different days so the teperature in the lab could have changed which would mean that one of them would work faster than the other. We didn’t have any anomalous results which was good. Next time I did this experiment, I would change the length of time the potato chips were in the solution - I would work quicker at the beginning of the lesson to try and give them about 45 minutes in the solution. The results we got were enough to support a firm conclusion because we did a wide range and then we did a very narrow range which was more accurate.
