An Investigation of Factors Affecting the Rate of Osmosis.

An Investigation of Factors Affecting the Rate of Osmosis

Introduction

Osmosis is the movement of water molecules across a semi permeable membrane from a region of high water concentration to a region of low water concentration.

A semi permeable membrane is a membrane with very small holes in it; they are so small that only water molecules can pass through them. Bigger molecules such as glucose cannot pass through it. In actual fact water molecules pass both ways through the membrane, but because there are more water molecules in the high concentration region than the other there is a steady net flow into the lower concentration region. The lower concentration is the stronger solution, such as a glucose solution. This movement causes the glucose-rich region to fill up with water. The water movement is diluting the solution so that the concentration on both sides is equal.

This diagram illustrates the net flow of water movement from a hypotonic solution, low solute concentration, to an area of high solute concentration. In other words this shows water movement across the semi permeable membrane from a high concentration of water to a low concentration of water.

Water moves from a high water potential to a low water potential. A low water potential is a high solute concentration. Water potential has the symbol Ψ, sigma.

When water potential on both sides is equal then it is known as equilibrium. In this case there is not an absence of movement between the equal water potentials but a movement of water in both directions, maintaining the equilibrium. Osmosis is a continuous process which does not require any energy to take place.

Factors Which Affect the Rate of Osmosis

Temperature: The higher the temperature is the faster the molecules will move. This means that the movement of water molecules across the semi permeable membrane will be faster.

Surface Area: When there is a larger surface area there will be more space for the molecules in and so they will be able to move across more easily. In a small area the movement across the semi permeable membrane will be restricted with less molecules being able to get through at a time. So the rate will be faster with a bigger surface area.

Difference in Water Potential: If the difference in water potential is higher then the rate of osmosis will be faster. This is because if there are not many water molecules in the low concentration region then water molecules from the high concentration area can come in quickly and easily. When there is less difference it will take longer because there are already a lot of water molecules occupying the space in the low concentration.

Pressure: When there is more pressure the molecules move faster. This is because when there is not much pressure they are not being forced to move quickly, but in high pressure they are being pushed across to the low concentration.

Hypothesis

I believe that the bigger the difference in water potential the quicker the molecules will move and the quicker the rate of osmosis will be.

Why Use Potatoes

They are cheap

They have homogenous tissue. This means that they have the same type of tissue throughout the potato, and so results won’t be indifferent or unfair.

The cell surface membrane acts as a semi permeable membrane, allowing osmosis to occur.

Molarity of Sucrose

The molarity of sucrose is 0.27; we can tell this from the graph above. The dotted line across the graph shows the equilibrium and if we see where the curve joins this line and then trace it down from that point to the x axis we find the solute concentration to be 0.27. The Roberts textbook was our source for this information. Using this we now know that the equilibrium is at 0.27M. This means when the solution outside the potato is 0.27M it is matched by the concentration of 0.27M by the potato. Therefore at this point no mass change will occur. When the solution outside the potato is lower than the equilibrium, then it will cause the potato to gain mass. This is because the water moves into the potato from the concentration, because the potato has less water than the solution. When the solution is higher in concentration than 0.27M then the potato should lose mass, because it has more water than the solution. To balance this out it goes into the solution.

If the liquid that the potato is in has a solute concentration of 0.1; the potato has a solute concentration of 0.27. This means that the potato has higher sugar concentration than the outside solute, so there are not many water molecules present there. Therefore water molecules move into the potato, increasing its mass.

If for example the liquid outside the potato had a solute concentration of 0.4 and the potato has its solute concentration of 0.27, then the mass will decrease. This is because there is not a high concentration of water molecules in the liquid, so water ...

This is a preview of the whole essay

At equilibrium the mass stays the same.

The rate at which the water moves into the potato is not always constant. As it gets closer to equilibrium the difference of water between the potato and the solute is less. Therefore the rate of osmosis decreases. Also the rate of osmosis changes with time because as osmosis happens the potato cells start to become turgor. This is where the cells swell up and become rigid because so many water molecules enter the cell. The cell has a rigid cell wall and so it does not burst like animal cells, but the cell stops taking in water molecules.

We know that when the solution is above equilibrium, 0.27M, then the potato will not stop losing water until it cannot lose anymore. This means that the rate of osmosis will continue to increase, until it gets to a point where it cannot increase anymore and has to stop. For example, a plant cell when the cell becomes turgid. A graph to illustrate this is a graph of rate of osmosis against concentration. This is ideally what our results should show.

The lower the concentration of the solution the more weight it should gain and the higher the concentration of the solution the more weight it should lose.

A graph of what the mass change should look like ideally will look this:

This graph shows that as the solution reaches equilibrium then the percentage mass change begins to slow down, and does not change quite so much.

Apparatus List

Potato: we need the potato because this is what will be conducting the experiment on
Solute 0M – 1M: we need the solution to put the potato in so we can test the rate of osmosis
Weighing scales: we need this because to calculate the rate of osmosis we need to weigh the potato at intervals to see how quickly it is gaining or losing water molecules
Stopwatch: this is necessary to know when to measure the potato at the right time interval
Cork borer: we need this to obtain the potato sample
Tile: we need this to cut the potato on, so we don’t damage the work benches
Scalpel: we need this to cut the potato samples skin off
Beakers: we need 6 different labeled beakers to put the 6 different solutions into
Permanent pen: we need this to label the beakers
Tweezers: we need this to pick up the potato sample out of the beaker to weigh it
Paper towel: we need this to dry the potato sample of any excess liquid after it has been taken out of the solution. If it was not dried then it would effect the results
Recording table: this is to record the results quickly and clearly

Planning the Preliminary Experiment

Firstly I will do a preliminary experiment, to test my method and then see how I can improve it for the final experiment. Also a preliminary experiment will show me what results to expect if it goes correctly. For this preliminary experiment I will use a cork borer to produce the sample potato cells. They should have approximately the same surface area and mass. If possible the samples should be taken from the same potato. The amount of solute but into each beaker will be 40cm3.

The range of sucrose values I will use will be from 0-1 molar at o.2 intervals. I will leave the cylinders in the beakers for at least half an hour allowing enough time for osmosis to happen and to see a change in mass.

The potato should be weighed every 5 minutes to give an idea of how fast osmosis is occurring. We need repeats at each concentration because one set of data is not 100% reliable, we need to take several to make sure our readings are accurate.

Method of my Preliminary Experiment

First get six beakers and label them all accordingly, from OM (distilled water) to 1M in 0.2 intervals.
Now measure 40cm3 of each solute into each appropriate beaker
Get a potato and a cork borer of appropriate size, if the potato small you cannot have a large size cork borer as you will not be able to get enough potato cylinders from the potato
Cut the skin off the potato cylinders as the skin is not homogenous
Weigh the potato and make sure all the samples are off similar weights and shapes
Now put the cylinders into each beaker and begin to time
Every five minutes take the potato samples out and weigh them, then put them back into the beaker, and record the weight
Continue to weigh the samples at 5 minute intervals for 30 minutes

Problems Encountered and How to Solve Them

The preliminary experiment helped me very much in conducting my final experiment. My preliminary experiment had many errors which I identified and corrected in my final experiment. Firstly I didn’t use the 0M solution; this was because this solution had run out as many people were not economic in pouring it into the beaker. This can be solved by using only the amount needed in your experiment and pouring the solution cautiously as to avoid spilling any.

I did not in this experiment use more than one potato sample, to check if my results were consistent or if they were a one off. Therefore in the final experiment I will take more than one sample at each solution.

Practically doing the experiment was a huge rush when it was time to weigh the potato sample. I did not put each potato sample in at different times, meaning I had to weigh them all at the same time, which was not possible. To overcome this in the final experiment I will put first sample in, and then wait 1 minute before putting in the next potato sample. By doing this I will have enough time to weigh all the potato samples correctly. Also I have decided to take the weight reading every 10 minutes. This is due to the fact that measuring the weights every 5 minutes was too chaotic and could easily lead to errors. Also by staggering the time of weighing the samples taking the last sample of the first five minutes would overlap with taking the first sample at the next five minutes. Therefore it would not be possible to take the sample every 5 minutes.

Furthermore in this experiment my potato was too small for the size of my cork borer, and I ran out of potato. I then had to use a different potato to obtain more sample cylinders. This is not at all ideal as this would have affected my results as the potato would be different to the original. Consequently in my final experiment I will select a smaller cork borer size and a large potato to ensure I don’t run out of potato to take my sample from.

Lastly I decided that I didn’t get enough results from conducting the experiment over half an hour only. So I have decided to do the experiment over a longer period of an hour.

Method

First get six beakers and label them all accordingly, from OM (distilled water) to 1M in 0.2 intervals.
Now measure 40cm3 of each solute into each appropriate beaker
Get a potato and a cork borer of appropriate size, if the potato small you cannot have a large size cork borer as you will not be able to get enough potato cylinders from the potato
Cut the skin off the potato cylinders as the skin is not homogenous
Weigh the potato and make sure all the samples are off similar weights and shapes
Now put the first cylinder into the 0M beaker and begin to time, after one minute put in the next potato sample into the 0.2M beaker. Continue to put in the samples into the beakers in ascending order, until there are none left
At ten minutes take the 0M potato sample out, dry it of excess solution, weigh it, return it to the beaker and record the weight. At 11 minutes take out the 0.2M and repeat the procedure. Do this for the rest of the potato samples at the right time
Continue to weigh the samples at 10 minute intervals for 60 minutes

Results

Because I did the experiment using two samples in each beaker to ensure my results were correct, I can now take the average of both sets of data for the most accurate results.

Using these results I can now draw three graphs to analyze the rate of osmosis. These will be: 1) Percentage mass change Vs. solute concentration

2) Rate of osmosis Vs. Solute concentration

3) Percentage mass change Vs. time

The equation to find the percentage mass change is:

Change in mass (new mass – original mass) divided by the original mass multiplied by 100.

To calculate the rate of osmosis you will need to use the equation:

Final mass – starting mass divided by starting mass, multiplied by 100.

Using these equations I produced the following tables from which I produced my graphs:

Percentage mass change Vs. solute concentration

2) Rate of osmosis Vs. Solute concentration

3) Percentage mass change Vs. time

Analysis

1) This graph shows us that the weight of the potato decreases as the concentration of the solution increases. We know this is correct as water moves across a semi-permeable membrane from a high concentration to a low concentration, trying to achieve equilibrium. In a high concentration inside the potato there is more water molecules than in the solution, so they move into the solution, and consequently the potato loses mass. From this graph we can see that the equilibrium is around 0.3M. This is very accurate as the correct value is 0.27m.

2) This graph shows how the rate of osmosis is affected by the different solute concentrations. The graph shows that as the concentration of solution increases the rate of osmosis increases. This is as there is a bigger difference between the solution and the potato. This means that the water can easily leave the potato to go into the sucrose solution, because there is a lot of free space in the concentration.

The graph also shows that the equilibrium was around 0.23M, again close to the actual figure. This is the point where the line meets the axis and where we no that no water movement is occurring because both sides outside and inside the potato are balanced.

This graph shows how the percentage mass changes over time, comparing them with all the other solute concentrations. From this we can analyze the rate of osmosis. We can see that both the first and second lines, of 0M and 0.2 M are relatively flat. This suggests that the rate of osmosis slows down towards the end of the experiment. This is expected as it becomes harder for the molecules to pass through the membrane as it is getting closer to equilibrium. There is less space inside the potato for the water molecules to move into, so it is taking them longer to push through.

The other lines which go into the negative half of the graph do not show that their rate of osmosis is slowing down. Therefore this is probably because they have not yet reached equilibrium, and longer time would be needed to measure this in more detail.

Conclusion

In conclusion we have found that the rate of osmosis increases as the difference in water potential increases. This is evident from the graphs produced. We can that as the solutes get more and more concentrated the rate increases more and more. The reason being for this is that as the solution gets more concentrated, there are fewer water molecules in the solution. This means that the water entering the solution can do so readily, without being slowed down by many existing water molecules already there.

Also, although we did not set about intending to prove the molarity of sucrose as being 0.27M, we found that it accurate as we produced readings of 0.23 and o.28 for the molarity of sucrose.

We can also conclude that in solute concentrations lower than 0.27 the potato will gain mass and in solute concentrations above 0.27 it will lose mass.

Evaluation

Overall I would say that the experiment was a successful one, proving my hypothesis to be correct. However the results may not be entirely accurate due to some problems which came up in the experiment. The main problem was weighing the potato samples, as there were not many scales. Therefore you could have easily missed your time to put the sample back into the beaker. Also another problem was drying the potato samples. This is because there was no way whether we could be sure if we had dried all the samples equally. Some samples may not have been dried enough and other may have been dried too much. Doing this would give rise to anomalous results.

To carry out the experiment safely it was imperative we were extremely cautious when using the scalpel knives. This could have easily cut someone.

To further improve the experiment we could conduct it with a longer time limit, so that we can specifically see how long it takes for the samples in different solutions to reach equilibrium.

Hypothesis

I believe that the higher the concentration of the solute the longer it will take for it to reach equilibrium.

Method

First get six beakers and label them all accordingly, from OM (distilled water) to 1M in 0.2 intervals.
Now measure 40cm3 of each solute into each appropriate beaker
Get a potato and a cork borer of appropriate size, if the potato small you cannot have a large size cork borer as you will not be able to get enough potato cylinders from the potato
Cut the skin off the potato cylinders as the skin is not homogenous
Weigh the potato and make sure all the samples are off similar weights and shapes
Now put the first cylinder into the 0M beaker and begin to time, after ten minutes put in the next potato sample into the 0.2M beaker. Continue to put in the samples into the beakers in ascending order, until there are none left
Every hour measure the sample, by taking it out and then drying it, weighing it and putting it back into the beaker. This should be done for until all the solutions have reached equilibrium

I made the time interval an hour because it is clear this experiment would take much longer than the previous one.

An Investigation of Factors Affecting the Rate of Osmosis.

This is a preview of the whole essay

Document Details

Related Essays

An Investigation of Factors Affecting the Rate of Osmosis.

An investigation into the factors affecting osmosis

Investigate the factors affecting the rate of Osmosis.

An Investigation into the factors affecting osmosis.