Constants
These are the variables that I chose not to use; therefore it is crucial that they remain constant. This is to make the investigation a fair test.
- volume of potato – 4cm³
- length of potato -4cm
- Temperature of the solution – room temperature
- Volume of the solution -25mls
- Type and size of potato -
- Time in solution – 15mins
Apparatus
- 5 boiling tubes
- Some distilled water
- Some Sucrose solution
- A thermometer
- A time device (watch, slop clock, clock, etc.)
- A set of balances
- Potatoes
- Boiling tube rack
- Knife to cut potato with
Diagram
Extra Apparatus
- Knife to cut potato with
- Sharp wire-like prongs to hold down any potato samples that float
- Weighing scales to measure
- Tissues to help keep sucrose spillages under control and for removing surface moisture from potato samples after being removed from the solution.
I have chosen ‘concentration of sucrose solution’ as my input variable. And ‘mass of potato’ as my output variable.
To check that this will provide good results and runs smoothly I have set up a preliminary experiment using the method above.
Here are the results obtained:
These results proved good and fitted a pattern that I would expect according to my background science.
I will keep the input variable (concentration of sucrose solution) and I will keep
The method was efficient; however there were a few things I need to add:
- Use Skewers to hold the potato down as the potato floats in some of the boiling tubes.
- Alter time to 15 minutes to allow osmosis to take place fully
Prediction
Background Science of Osmosis
Osmosis is a special case of diffusion. Biochemical processes in living cells always take place in a solution. The solvent (dissolving fluid) and solute (particles dissolved in the solvent) are the two parts of a solution.
In living organisms: the solvent is water and the solution is called an aqueous solution.
Living cells are separated from their surroundings by the partially permeable cell surface membrane. The contents of the cell, the cytoplasm, is one aqueous solution and the surroundings of the cell (e.g. pond water) are also an aqueous solution. If the two solutions do not have identical concentrations of various substances then the molecules may move from one solution to the other by diffusion, if the cell membrane is partially permeable.
Here is my prediction of what I think will happen:
In the first solution which is entirely distilled water; the potato will gain mass. And in the others the potato chips will lose mass. This means that on the graph there will be an equilibrium. I could maybe try and find this equilibrium concentration to check that my results have worked.
My graph should look something like this:
Observation
My first set of results
My set of repeats
Because these results matched each other very well and complimented each other, I felt that a third set of results would be unnecessary.
However, I plotted a rough graph to clear the possibility of both sets of results being incorrect.
The rough graph seemed to follow the pattern I predicted so all was fine.
I know that my results are accurate also because I kept all of the variables that weren’t used constant.
There don’t seem to be any anomalies; this experiment has worked out perfectly.
Output Variable = percentage change in mass:
This is a table of the percentage changes.
Percentage Change = change in mass / original mass
Precautions
I have carried out a number of precautions in order to make this a safe investigation:
- Use the knifes with ultimate care
- Use small beakers to distribute
Analysis
I have discovered that there is a link between my input variable (concentration of sucrose solution) and my output variable (percentage change in mass of potato samples): As the concentration of the sucrose solution increases – so does the percentage loss in mass.
Anomalies:
I have no anomalies in my results, this is because I kept all of my other variables constant and the equipment was reliable and accurate enough for this investigation.
Also, I was extremely careful and precise at measuring my potato samples. Their surface areas were as identical to each other as possible, using a knife.
Pattern:
In the first run (25mls of distilled water with no sucrose solution) the mass increased by 3.62%. This is because the potato absorbed the water surrounding it, and therefore increased in mass.
In the second run (0.5M of sucrose solution) the potato lost mass by 1.46%, this is because water from the potato passed into the solution through osmosis and it therefore lost mass. The reason the water from the potato sample transferred itself across (diffused) into the solution is because of the basic theory of osmosis – water from an area of higher concentration passes into an area of lower concentration.
In the following runs, the concentration of sucrose increased even more and therefore the concentration of distilled water was even lower. This is why even more water diffused from the potato into the area of increasingly lower concentration.
The general shape of the graph is:
The gradient of the graph slows down at the end. I constructed the table below to try and discover why the gradient does this.
Although the mass decreases by a larger percentage as the concentration of sucrose is raised, the difference between these average changes in mass becomes smaller. This is shown in the table above.
This is the reason the gradient slows down at the end of the graph.
General conclusion
This experiment has worked smoothly and efficiently. As the concentration of sucrose solution surrounding a sample of potato tissue increases, the percentage loss in mass increases. Although as the percentage loss in mass increases, the difference between them decreases. This is why the graph is in an “S” shape.
Evaluation
My method was suitable and I have no alterations to make towards it.
However, I have some ideas on how to improve the method.
- A small guillotine mechanism would be more accurate at making equally sized potato samples
- A dryer would do a better job at removing surface moisture than the paper towels. Although it would have to be a blow-dryer and at a cool temperature to avoid drying out the potato completely
- I could extend the time limit beyond 15 minutes to make sure that osmosis has taken place fully, although this could allow enough time for the temperature to change
