To investigate how osmosis in potatoes is affected by varying the concentration of a salt solution.

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Investigation into the effect of Concentration on the Rate of Osmosis

Aim:

Introduction:

Osmosis is the movement of water molecules between selectively permeable membranes. It takes place in all cells, at the surface of the organism. In order for osmosis to take place, a concentration gradient is needed on each side of the cell membrane. The water molecules move across the membrane during osmosis and always travel from a higher concentration to a lower concentration. If a potato chip is put into water, where the number of particles inside the potato is less than that compared to the number of particles outside the potato, the mass of the potato should increase, due to the fact that the water would be able to travel through it; the concentration outside the potato is less than that of the concentration inside the potato.

However, if the water concentration was to be changed, i.e. salt was added, the concentration of the water would increase and the solution would become stronger than that of the potato, then the water molecules would be able to move out, rather than in, and so the potato would decrease in mass.

In order to complete this experiment, the concentration of the salt solution will be changed and therefore the molarity of the solution will be changed. Potato chips will be placed in dishes of different concentrations of water, at regular intervals. Each dish will contain three identical potato chips, so that the experiment is made sure to be accurate and reliable. The first solution will be that of distilled water, and will go up in intervals of 0.5M; i.e. there will be five dishes, each containing three potato chips, one dish being ‘distilled water’, the second being ‘0.5M salt solution’, the third one being ‘1.0M salt solution’, the fourth one being ‘1.5M salt solution’ and the last dish will be of ‘2.0M salt solution’. The potato chip will be weighed before and after the experiment, and the weight change will then be measured (mass before – mass after). This variation in mass will indicate how much water has left or penetrated the potato cells during osmosis, hence showing us, the amount of osmosis has occurred. If the mass change can be calculated via varied concentrations of salt solution, a trend should appear and a conclusion drawn as to how varying concentrations of salt solution affects osmosis.

Key Variables:

In order to complete this task, the concentration of the water solution will be changed. The rest of the factors which can potentially make the test unfair will be kept the same.

Concentration: by increasing the concentration of a substance, the amount of certain particles in that substance is also changing. By doing this and maintaining the concentration of the potato chip, we are increasing the concentration difference between the two. The whole point of osmosis is to balance the concentration in both the potato chip and the solution, so more osmosis must take place when the ...

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Key Variables:

In order to complete this task, the concentration of the water solution will be changed. The rest of the factors which can potentially make the test unfair will be kept the same.

Temperature: when the temperature of a reaction is increased, the thermal energy is changed into kinetic energy. The higher the amount of energy the particles have, the higher their speed they move around, and so the quicker they are able to penetrate through the cell membrane. Therefore, the hotter the temperature of the salt solution, the higher the amount of osmosis occurring.

Type of Potato: every potato has a unique chemical structure, and this may affect the amount of water going in and out of the potato. The higher the amount of water in the potato, the less the amount of osmosis occurring, as the concentration difference of the water in the potato and the water solution is less. The lower the levels of water in the potato, the more the amount of osmosis, as the concentration gradient is more.

Time: the longer the potato chip is left in water, the more osmosis produced, as the more time the water molecules will have to enter the potato through the cell membrane.

Scientific knowledge and Hypothesis:

Osmosis is defined as the movement of water molecules from a high concentration to a low concentration. The molecules travel trough a semi-permeable membrane, and enter into the substance. This occurs when a concentration gradient is established. The water molecules which move across the membrane during osmosis always travel from the higher to the lower concentration. So if we leave a potato chip in distilled water, where the number of particles (water molecules) in the potato is less than the solution outside of the potato than that inside the potato, water should travel into it and its mass should increase. Now if the distilled water were replaced with 0.5 molar (0.5M) salt solution, the concentration of water in the potato would become higher than that of the solution to which it is being compared and therefore, water molecules would move out of the potato and its mass would decrease.

When an identical potato chip is then placed in a weak solution of 0.5M, the concentration of water would be higher in the potato chip and therefore the rate of water loss in the potato chip would be greater than the rate of water entering the potato chip and therefore the cells in the potato chip would become flaccid (when the contents of the cell pull away from the cell wall, the process is called plasmolysis and the cells have become plasmolysed) this is because of osmosis as the concentration gradient has tried to be lessened. This is an example of hypertonic osmosis.

From this, I predict that as the concentration gradient increases the amount of osmosis will also increase.

Apparatus:

1 Potato
1 Cork Borer
1 Scalpel
1 Ruler
5 Test Tubes
1 Electronic Weighing Scale
1 Test Tube Rack
1 Measuring Cylinder
2 Paper Towels
1 Ruler

Diagram:

Method:

I need to take the following steps when undertaking my experiment:

Get together all the apparatus and position them safely on the bench, as shown above.
Using the cork borer, cut out long cylinder shaped pieces of potato.
Do this numerous times and then cut them into evenly shaped potato chips. Weigh these potato chips using the electronic weighing scale.
Make the selected solutions.
Place these potato chips into a Petri dish and on top of them place 20ml of the selected concentration of salt solution required for that particular part of the experiment; in this case: distilled water, 0.5 molar, 1.0 molar, 1.5 molar and 2.0 molar.
Place lid on Petri dish and leave for 24 hours, or hover long the teacher has stated.
After the designated time, collect the Petri dishes and find out the results by measuring weights of potato chips on electronic measuring scales, letting the potato chip dry on a paper towel before each weight. Record the results in a Results table by taking away the weight before from after.

Fair Test:

In order for this experiment to be fair, the following precautions will be taken into account:

The potato chips will have to be the same weight, shape and size, so that the amount of osmosis is not affected.
The temperature will gave to be kept constant throughout the experiment (i.e. room temperature), as varying temperatures will effect the amount of osmosis.
The potato chips will be drained so that no excess water is present in the potatoes while they are being measured.
The volume of the solutions will be measured whilst they are on the bench, so it is as accurate as possible.
The average mass of the potato chip after the reaction will be measured for accuracy.

Safety:

The following safety precautions will be taken into account whilst conducting the experiment:

Do no walk around the laboratory holding a scalpel unnecessarily.

Be extreme cautious when using the scalpel to cut the potato chips to ensure you do not cut yourself.

In case of any emergencies, ask teacher for assistance

Make sure you are standing at all times in case of any spills and that all bags are under the desks and not in the isle to prevent tripping
Keep desk clear at all times in case of spills.
Always place loose boiling tubes in a rack to avoid breakages
Clear up any breakages and spills immediately with a dustpan brush or a paper towel, being very careful not to cut yourself.
Do not distract others while they conduct their experiment.

Preliminary Experiment: To ensure that my experiment would be carried out without any problems and inconveniences, I conducted a preliminary experiment prior to my actual experiment.

I will now plot the results I have on a piece of graph paper. This will enable me to read the results more clearly and to draw a definite conclusion as to whether my prediction was correct or not. I will also be able to see which changes I need to make, where I will have an anomalous result, which I will be able to change during the actual experiment.

Graph (on next page):

Analysis of Preliminary results:

As evident by the graph, my prediction has been proved; as the concentration gradient increases the amount of osmosis will also increase.