RATE OF REACTION

How Does The Concentration of a solution affect the rate of osmosis

Osmosis can be defined as the net movement of water molecules from high to low concentration through a semi-permeable membrane. When a substance such as sugar and salt (which we will be using in the experiment being analysed) dissolves in water it attracts water molecules to itself and through that it stops them from moving freely. During osmosis in a plant cell, the water particles move through the cell membrane and into the large vacuole, where they are stored in a solution of sugars and salts. For this experiment, I will be using sucrose( and distilled water), which has large particles that cannot fit through the holes in the selectively permeable cell membrane, but water particles can fit through, as they are much smaller.

Before I start thinking about what I am going to test in my experiment and how I am going to carry it out, I have to know the range of factors that might influence these choices. I therefore have to identify and concentrate on the most important factor:

Temperature affect osmosis, because the greater the temperature the greater the energy (heat), the faster the molecules under the influence of the heat will move and the faster they will diffuse across the membrane. This isn’t the only problem because heat can affect this experiment physically as well. There is a possibility that the cell membranes will be denatured by heat and behave differently towards the experiment. The water molecules in the solution and in the potatoes might also evaporate which is uncontrollable.

Furthermore, the type of cell is also very vital in osmosis because of the fact that the permeability of the membrane and the surface area of the membrane can be different in potatoes of exactly the same type, even from the same region. If the membranes of potato I use in the experiment have different levels of permeability, water will flow more easily through some than others, changing the rate of osmosis. Again, if the surface areas of the membranes are very different between potato samples, diffusion will occur faster in cells where there is a greater surface area, as it means there are a more places to cross the membrane.

The shape of the samples; the shape of the samples should be the same as the volume of two shapes can be the same, but one may have a greater surface area than the other. If some samples are different shapes and so have different surface areas to other, there will be a greater chance for osmosis to occur, as the surface area affects the rate of osmosis. The greater the surface area, the more easily

water molecules can pass through the cell membrane either into or out of the cell.

The length/width of the potato samples; It would be impossible for me to get all of the samples to all be 100% the same in size, mass and shape. However, I can make them as similar to each other in size as possible. This is done by using the cork borer (so all samples should have the same diameter), the knife and ruler aid in keeping them the same length. If the width/length of the samples differed, then those that were longer and wider would have a greater surface area and so osmosis could occur at a greater rate leading to inaccurate results.

Concentration gradient between two solutions is a factor that affects the rate of osmosis a lot, and can be controlled. The greater the difference in free water molecules between two areas, the faster water will pass across the membrane from an area where there is more water to an area where there is less.

The time that the potato samples are left in the distilled water for; If some samples are left in longer than others then osmosis will have had a longer time in which to occur, depending on the concentration of the solutions that the samples were left in. This may mean that the amount of water entering or leaving the cells will be greater than of the other samples. However, if some have been left in for a shorter period of time than others, then osmosis will have had less time to occur in. Meaning that the volume of water that has left or entered the cell will be lower than that of the other samples. Therefore the mass reading obtained at the end will not be the value that it should be.

Finally pressure is the final factor affecting osmosis that I know can affect my experiment. If there is a greater pressure on the outside of a potato than the inside water will be forced into the potato from the outside.

It is my conclusion that the concentration gradient is the key factor, because it is easy to monitor in an experiment and has a definite effect on potatoes, which can easily be backed up by the osmosis theory.

I will vary the concentration of sucrose in the solution, and carry out various measurements on the potato cylinders before and after the experiment, to see what affect changing the concentration has had.

I predict that with the lowest concentration of sucrose, the potato will increase in mass, because of water moving into the cells. The percentage increase will then get smaller each time I increase the concentration of sucrose in the solution. At some point, the mass of the potato will not increase, and will be the same as it was before (when there is no net flow of water particles in or out of the potato cells).After this, the mass of the potato will start to decrease as I keep on increasing the concentration of the sucrose solution. At some point the potato cylinders will lose all the water that they are able to, and the percentage mass loss will stop decreasing.

Preliminary Test

This is a preview of the whole essay

Preliminary Test

I am investigating how temperature affects the rate of osmosis so I will keep all the variables the same except for temperature, which include the concentration of the dilute and the surface area. I will make sure that the potato cylinders are kept in different containers at different times as shown in the table below. I will also ensure that there is the same amount of distilled water in each test tube and the same amount of water in each water bath. I will not touch the potato cylinders with my hands as I might accidentally alter the mass and could affect the experiment. If I keep to my Fair Test guidelines I should receive accurate and reliable results. I will repeat the investigation and find averages to get the most accurate results as sometimes small mistakes can be made that cause outliers. Taking repeat results reduces the chance of having incorrect results

I also discovered that 10 minutes is not a lengthy enough time to show a great deal of change. However 20 minutes is too long as I would not have enough time in the 50minute period to complete more than two experiments. 15 minutes showed a substantial change and would allow me to complete at least three experiments in one lesson. Another important step in making this a preliminary test is to make sure that the potato is fully covered by the solution. This is because the potato should fully submerge, by having total contact with the solution.

When using the scales, we will make sure that the scale is reading zero before we put the potato pieces on it. This is so that we don’t get a false reading, with the weight of our potato with the reading it had before. And after the experiment, we will weigh the potato pieces that should be dried as much as possible, and weigh it the same way, taking the reading to the nearest 2 decimal places. Carrying out the experiment in a constant temperature for the duration of the experiment is important. And to avoid temperature change, which might affect the different sets of results, we will take the temperature of the room/solution into account.

In the experiment I am conducting, I have decided that the dependant variable is the mass of potato because it is more accurate than the length as the potato could have expanded width ways as well as increased in length.

Method

Apparatus:

Knife
Potato borer
Test tubes
A Test tube rack
Measuring cylinder x 1
Stopwatch
Weighing scale (Digital or electronic)
White tile
Paper towels
Potatoes
Ruler (30cm)
Distilled water
different sucrose solutions
Beakers
6 film canisters

First of all, I prepared a range of sucrose sugar solutions and distilled water and all the other apparatus I needed.

Using the cork borer, I will press it into the potato to get a long piece of potato. I will use a ruler to measure the pieces of potato to the same length (2.5 cm), and cut them into pieces of all the same size, so that I maintain the surface area of the potatoes constant. Using this process, I will cut out 18 potato cubes, all of which will be the same size. Each of these potatoes is then weighed using a scale, and the weights will be notated. The potatoes will be split up into groups of 3, so there will be 6 groups of 3. Each of the 3 potatoes in the group will have a different coloured pin( or anything else so that you can determine which one you are looking for) put into in (weighing 2.5g) so that we can identify which potato is which from the group. The first potato in the group will have a blue pin in it, the second will have a white pin in it and the third potato will have a purple pin in it.

We then put the first bunch of potatoes in the distilled water following this, we put the second bunch of potatoes in the sucrose solution, and a few minutes after this, we put in the third bunch of potatoes in the sucrose solution. The time for when each of the bunch of potatoes were put in the solutions were noted. After 5-10 minutes of putting the first bunch of potatoes in the distilled water solution, they were taken out of their beakers with tweezers(or drain the water from the beaker ), and placed onto tissue paper to dry off the excess solution on them, so that water that was not inside the potato due to osmosis was taken off. The potatoes were then weighed after being dried, and placed back into their solution. The masses of the potatoes were noted.

After 10 minutes of the second bunch of potatoes being in their solution , they were dried and weighed the same as the potatoes in the solution. The same process will occur for the potato cubes in the solution. After it has been in its solution 10 minutes they are weighed and dried. After another 10 minutes of the potato cubes being in their solutions, they are dried and re-weighed as before. This same process will occur after another 10 minutes in their solution, and again after 10 more minutes. After this has been done, I will have a full set of collected results for the potato cubes in the 0.0- 1.0(M) solutions.

I am going to measure the masses of the potato cylinders before and after having been in the sucrose solutions for 1 hour by using weighing scales. The weighing scales are accurate to 2 decimal places, so this should ensure that my results are very accurate.

I will make my results valid, precise and reliable in the following ways:

I will measure the lengths of the potato cylinders very carefully so that the surface area is not changed, as a change in the surface area may change the rate of osmosis taking place. I will use the same cork borer for each cylinder, as this would also change the surface area, which means that more water is taken into the potato by osmosis.
I will keep the volume of the solution the solution exactly the same as this would also affect the rate of osmosis. If there was more liquid then there would be a higher number of water particles which would move into the potato and therefore change the rate of osmosis.
I must keep all of the potato cylinders in the boiling tubes with the solution for the same amount of time, to make sure that the results are valid, as this too could alter the reliability of the results I collect. If they are in the water for different amounts of time then it would be unfair as some potatoes would have more osmosis happening to them.
I am doing three repeats for each concentration so that I can work out the mean for all of the readings I take for the same concentrations. If there are any odd results they will not change the shape of the graph too much.
I am using the same equipment throughout, including film canisters, boiling tubes, weighing scales and cork borers, so as to keep the results as accurate as they can be.
Although I am not controlling the temperatures at which osmosis takes place, I am going to measure the temperature of all of the solutions so that if there are any anomalies, they may be due to a change in temperature. Higher temperatures would increase the rate of reaction as particles have more energy to move faster, and so loses or gains more water than other potatoes.
I am going to place the potato cylinders on paper towels after they have been in the boiling tubes to soak up any excess water which may be on the outside of them and add to the mass.

Safety Precautions

To ensure my classmates and my own safety I will always wear safety goggles while partaking in the experiment especially near dangerous and harmful materials. I will always obey the laboratory rules and be careful when using sharp tools like scalpels, knives etc. and will use the tongs when handling hot materials. I will keep all my apparatus away from the edge of the working area as they may be accidentally spilt. After I finish using the apparatus I will put it all away and clean my working area.

Main Experiment

Rhys' Results

Conclusion

The evidence obtained from this investigation supports the prediction I made. It also shows that the potato cells increase mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration.

In this experiment, I believe that I have collected enough data to support my hypothesis. This investigation was, I think, successful meaning my results were similar to my predictions. The potato cells, working to stay alive, took in, or gave out the water depending on the concentration of the tissue and the concentration of the solution it is surrounded in.

The results were fine and by looking at the weights measured before the experiment, you can see that there is no reading, which seems to be out of the line. As the weights before the experiment range between 2.3g and 2.7g, this tells us that the potato pieces were cut well, and I believe accurate enough. There were, I believe, no anomalous results after the experiment, as the weights were very similar in their own category. This tells us that my experiment was successful.

The graphs and the results show that:

· Osmosis actually took place in the experiment.

· As the molars increased, the percentage of the weight difference decreased.

There doesn't seem to be any results, which undermine my predictions, and I didn't find any outliers, which means I didn't have to repeat any of our experiments more than twice times.

Having the results, which backs up my hypothesis, proves that this investigation was fully accomplished, and was an achievement.

In the table seen on the last page, the percentage shows a steady decrease, telling us that the percentage of the difference decreases as the water concentration decreases.

This experiment helped me find out that osmosis occurs between 2 liquid substances with a partially impermeable membrane, and that higher the water concentration is the larger increase in grams (mass). This means that the percentage will also be higher, increasing with the grams.

Evaluation

In my opinion the experiment that I carried out was successful. I obtained accurate results from which I was able to produce informative graphs and turn make a good analysis. I think the right amount of results to obtain an accurate percentage change for the different molar concentrations I used. I think that the amount of time I used for the experiment was enough to allow sufficient osmosis to occur. However, if I were to repeat the experiment I would carry out the experiment for a longer period of time to allow more osmosis to happen. The range of concentrations that I used was adequate for the experiment and I repeated the experiment so I used more concentrations and a bigger range. Therefore I obtained more varied results and the results gave me a more in depth study.

The main sources of error in order of importance are:

Cutting the potatoes
Drying the potato cores
The concentrations of the solutions
The accuracy of the balance
The room temperature and pressure

The limitation in the experimental procedure when cutting the potatoes was that the cores were not cut accurately. Ideally the samples should have come from the same part of the potato and this would have decreased the chances of variation in the texture. Although I was recording my results by mass, it could have affected the surface area and therefore the overall rate of osmosis may have been affected. Because I repeated this experiment I found a more accurate way of cutting the potato cores as the cored were cut by hand using a ruler for measurement.

When the potato cores were removed from the test tubes and dried I may have dried some of the potato cores more thoroughly than others and so some of them may have had more excess water than others, this may halved added to the mass. There may have been a slight degree of error but this would have been an insignificant factor to affect the results. Improvements to increase the accuracy of the concentrations are to be more accurate when measuring out the solutions.

The experiment was also limited by the accuracy of the balance used, although I used the sane balance each time I was recording a mass and therefore there was the same amount of error each time. I could have weighed the potato cores more accurately on a more accurate scale e.g. to 0.0000g.

It was unlikely that the room temperature and pressure remained consistent during the experiment and changes may have slightly altered the rate of diffusion. The room temperature could have been monitored using a thermometer throughout the experiment. Using a heating system or by open windows this may have been kept more constant.

However, I believe we did do enough tests of our investigation (three) as it is much easier to clarify the best results, and if outliers do appear (as in our case) you have more results to use to check it. The results and evidence I have produced were reliable to complete my investigation. I didn’t have any outliers, which were out of the ordinary, but some of the results were not close to the line of best fit as others.

However my results confirmed my initial hypothesis and I am pleased that I have produced accurate results from which I have analysed and drawn conclusion from. I have summarised my evaluation in the table below:

Bibliography