Science Osmosis

Osmosis

Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration. Similar to diffusion, osmosis is used in parts of the human body and in plant cells.

In plants osmosis causes the cells to swell up if they’re surrounded by weak solution and they become turgid. This is useful for giving the plants support so that it can stand, unsupported.

This is how the partially permeable membrane works;

Background

I already know that there will be a change in weight when there are two different concentrations in the same liquid because of a theory I found by D.G. Mackean: “If a dilute solution (water) is separated from a concentrated solution (sucrose, sugar) by a partially permeable membrane (potato chip cell membrane or visking tube), then water diffuses across the membrane from the dilute to concentrated solution. This process is called osmosis”. And because no molecules are reacted or evaporate in this process all the excess water molecules gained or lost will affect the weight accordingly.

Planning

My experiment is going to test the changes made to potato chips after they have been soaked in six different sucrose solutions: 0.0M (distilled water), 0.2M, 0.4M, 0.6M, 0.8M, and 1.0M. In this investigation I am trying to find out the weight change caused by the movement of water molecules in Osmosis and draw a conclusion from that.

For this experiment to be fair all the chips will be kept the same size in each set of the three repeats I have decided to do. The potato chips will be: 1.5cm, 2.0cm and 2.5cm. The changes made to each potato chip with the solution in a galley pot will be observed and made into a graph for clear visual help to see the weight change. The recorded changes will include softness, weight and colour of the chips.

In order to keep this test as fair as possible as many factors have to be controlled as possible, like keeping potato chips the same size by using the same cork borer; using the same volume of solution by measuring it accurately in a suitable cylinder; using the same size galley pot; using an accurate balance for all the chips; drying each chip after the set time in the galley pot with the solution; keeping all the galley pots in the same place and time; putting the chips into the galley pots in the same time, this is not possible to the exact second but since the pots will be left for a few hours a few seconds won’t matter.

The only factors that will change will be the 6 solutions and the three different chip sizes. Other factors like light and room temperature are much harder to control. For some experience and to help me make key decisions I will do a preliminary experiment.

I decided on all the measurements and apparatus I will use from my preliminary experiment.

Apparatus

Balance-to measure the weight of the chips accurately before and after they are left in the solutions.
Galley pot-to put the chips and the solution in, all the same size.
Potato chips- 1.0cm, 2.0cm and 3.0cm, to use in the experiment, record changes that are made.
Solutions-0.0M (distilled water), 0.2M, 0.4M, 0.6M, 0.8M, 1.0M, to react with the chips.
Measuring cylinder-to accurately measure the solutions.
Cork borer-0.6cm, for all the chips to be the same in diameter.
Knife-to cut the potato chips into measured lengths.
Ruler-to measure the chips.
Pen and pencil-to write up activities and draw a graph.
Paper towel-to dry the potato chips so the scale has a reliable reading.

Method

Fill 6 Galley pots with 40 ml solution measured with a cylinder.
Fill one pot with distilled water, one with 0.2M sucrose solution, one with 0.4M sucrose solution, one with 0.6M sucrose solution, one with 0.8M sucrose solution and one with 1M sucrose solution. Take care not to cross-contaminate the solutions. Do not overfill them or you will not have enough room for the potato chips to fit in. 40 mil is enough as I found in my preliminary experiment.
Label each pot so you know what is in each one.
Cut out the potato chips ...

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Method

Fill 6 Galley pots with 40 ml solution measured with a cylinder.
Fill one pot with distilled water, one with 0.2M sucrose solution, one with 0.4M sucrose solution, one with 0.6M sucrose solution, one with 0.8M sucrose solution and one with 1M sucrose solution. Take care not to cross-contaminate the solutions. Do not overfill them or you will not have enough room for the potato chips to fit in. 40 mil is enough as I found in my preliminary experiment.
Label each pot so you know what is in each one.
Cut out the potato chips with the same cork borer.
Measure and cut the chips into three different sizes 6 times.
Weigh the chips and record the results observe what each chip looks and feels like.
Put the potato chips in the Galley pots.
Leave the pots in a controlled environment for a few hours.
Take out the out the potato chips, dry, and Record any observations that you make about the look and feel and weigh.
Explain the results and observations as scientifically as possible.

To make the experiment as safe as possible I will use all the equipment properly and ask before using any unlabeled solutions.

Preliminary experiment

I did a preliminary experiment so that I have a good idea on how the experiment is going to work, what things may go wrong and so that I can decide on how much of everything I am going to use and what factors I am going to keep the same.

The experiment is going to test the change in weight of three different solutions I carried out the experiment by using three visking tubes that had been pre-soaked and tied at one end were used in this investigation which is going to investigate osmosis using model cells.

Visking tubing is useful for observing osmosis as it acts like a semi-permeable membrane.

Method

Collect 3 pieces of visking tubing, which have been tied at one end and pre-soaked before.
Fill one bag with distilled water, one with 0.5M sucrose solution and one with 1M sucrose solution. Take care not to cross-contaminate the bags. Do not overfill them or you will not have enough room to tie the tops off.
Tie the top of each bag securely with strong thread and gently pat the outside of it dry.
Weigh each bag and record the results in a table. Observe what each bag looks and feels like.
Collect 3 beakers and fill with 0.5M sucrose solution.
Place a bag in each beaker and label beakers so you know what is in each one.
Leave the experiment set up for at least 30 minutes.
Remove the bags from the beakers and gently dry. Record any observations that you make about the look and feel of the bags. Reweigh the bags and record the results in a table.
Explain the results and observations as scientifically as possible.

I predict that the 1M solution will lose weight because the water will enter the beaker by osmosis, the 0.0M solution will gain weight by water entering the visking tube to make the solution more balanced and the 0.5M solution will stay the same because the concentration gradient will be the same.

Results table

As I predicted the Distilled water gained weight in the 0.5M solution because when there is a unbalanced level of concentration particles move around by osmosis, the 0.0M solution had less molar than the 0.5M solution and therefore the water would have entered the visking tube resulting in a gain in weight. The 0.5M solution pretty much stayed the same because although the visking tube would allow particles to move around - semi-permeable – the concentration of the sucrose was the same in the beaker. The 1.0M solution lost weight because there was a difference in the molar concentration level and therefore the water from the 1M tubes would defuse in to the 0.5M solution resulting a loss in weight for the 1M solution.

From this experiment I decided to use six different concentration solutions for a wide range of results. These are: 0.0M, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M solutions instead of three like I used in the preliminary experiment so I will have a more accurate and detailed set of results. I learnt the procedure on how to accurately weigh the visking tube and I decided to repeat my experiments until I get a set of results that all look coherent and to a certain degree of accuracy. I am also wiser on how to carry out the experiment because of my experience with the preliminary experiment. I will also know what trend of results I am expecting.

Prediction

From what I found in my preliminary experiment I predict that potato chips in the lower concentrated molar solutions will get bigger and therefore gain weight. Like the 0.0M visking tube in 0.5M solution, the potato chips in the 0.0M sucrose solution will gain weight because there is an unbalance in the concentration, water through osmosis will enter the partially permeable membrane call of the potato chip causing the cells to become Turgid and gain weight (from the extra water molecules). The potato chips will probably feel firmer because of the turgor pressure.

In the mid range 0.4M to 0.6M solution I predict that there will be not much change in the weight of the potato chip because the level of concentration would be around the same so there would be no need for water particles to move around as much.

In the higher concentrations I predict the potato chips will lose weight, as did the 1M Visking tube. because there will be an unbalanced molar gradient causing the water molecules from the potato to move to the beaker leaving the potato chips and therefore loosing weight, they should also feel soggy as they become more flaccid. The changes in weight are caused by the natural process called osmosis where water molecules move from a partially permeable membrane from a region of high water concentration to an area of low water concentration resulting in less water molecules (lower weight).

Obtaining Evidence

I used all the equipment safely to carry out the experiment, everything went well after I discovered one mistake I made which resulted in an anomalies result but I changed that and got all the results I was expecting.

Analysis and considering evidence

It is clear that osmosis has caused the weight change in the potato chips. As I predicted, in the lower Molar sucrose concentration 0.0M-0.4M the potato chips clearly gained weight because there was obviously an unbalance in the concentration gradient as a result, the natural process of osmosis to happened and that caused the potato chips in the lower molar concentration sucrose to fill with water molecules causing them to gain wait. The opposite happened when the higher molar sucrose solutions caused water molecules to leave the potato chips to balance the solution, less water meant a decrease in weight.

Osmosis happens when there are unbalanced concentration gradients in the solutions therefore through Osmosis water molecules enter or leave the potato chips making the potato cells become turgid or flaccid. There is a clear pattern because the loss in weight was relative to the Molar concentration: as the concentration increased the weight decreased as shown clearly on the graph. The Potato chips lost weight because through osmosis the potato cells released water into the solution to try and balance the solution therefore the potato cells became flaccid and lost weight.

From all the results I can conclude that as the molar sucrose solution concentration increases the potato chips lose less weight and in the opposite case as the molar concentration decreases the potato chips gain weight.

The pattern is shown on the graph, the line starts positive meaning there is a gain in weight but as the sucrose solution increases the line goes steeply down until the isotonic point (where there the solution is balanced) the curve becomes less steep after 0.4M because the weight loss is getting less.

I was correct in my prediction where I predicted that Lower molar concentrations would gain weight and that the higher molar concentrations would lose weight, this is exactly what happened however when I predicted the isotonic point I thought it would be between 0.4M-0.6M but I was wrong, the actual isotonic point is between 0.2m and 0.4M (0.28) but I know why I made this mistake, the reason is that I thought the isotonic point would be similar to the visking tube but I was wrong, that is why my prediction was wrong. I made the prediction thinking that the potato chips would be similar to the visking tubes but obviously now I realise that maybe the membranes are different. I now know from my experiment that the isotonic point for the potato chips and sucrose solution is exactly 0.28M.

Evaluating Evidence

I was happy with the outcome of the results. The method I used was one of a few but I definitely think it was the best because it gave me accurate results that were consistent and therefore the graph was a curve with no point out of place except for the anomalies which I did not include in order to make it a fair test. I collected enough data for an accurate set of results; I experimented with six solutions and three different sets of potato chips. My prediction was right because the graph and table of results show a pattern and I got a curve as I expected.

My results were accurate but could have been more accurate if I had done more repeats. I didn’t have any difficulties with my results except for one anomalous which I left out of the averages for a fair test and I repeated it again to find a more accurate average. The result I left out was an after weight of the 1cm chip in the 0.6Molar solution. I got the weight 0.74 grams and a change of -0.11 and after I calculated the averages I realised it was wrong because the weight was more than o.4M instead of less. I made a mistake when I transferred the results from my rough table to the new table; I looked over my other results to make sure there weren’t any other mistakes, there weren’t.

My results are very reliable because I did everything myself, I followed a plan and the final result looks coherent and accurate, there is nothing out of place which I haven’t altered already but there are things that can be improved. There are other investigations I could carry out if I were to extend the osmosis study. I could extend the period of time I left the potato chips and see if the time factor would make a difference. I could experiment with onion and other plant cell; I would do this by cutting a piece off the root of a plant (where water is absorbed by the root) and doing exactly what I did with this experiment but for a longer period of time.

I had to make sure certain variables were kept the same such as the size of potato chips, mass of chips, same pot, same volume of solution, accurate measurement, same temperature and when possible in the same light.

There are obvious things that I can improve next time, I would pay more attention when writing the results as the smallest mistake can make a big difference in the outcome when the results are transferred onto graphs. I will carry out the method step by step because this time I weighed the potato chips before drying them with my first set and so after I realised this I dried them and weighed them with slightly altered results but the small difference is enough to change the average because I only got an average of 3 results, this makes me realise that if I want a more accurate and reliable set of results I will need to repeat it many more times. Also the graph showed me that the biggest difference in weight (steepest point on graph) was between 0.2M-0.4M hence the isotonic point was here therefore next time I will concentrate more on the section maybe experiment with 0.2M, 0.22M, 0.24M, 0.26m, 0.28M, and 0.30M so I can get the isotonic point even more exact but if I do this I will need to make the surface area of the potato chips precise to the millimetre which would be hard with class room facilities.

Problems other than the anomalies and not drying the first set of results I had were to try in five minutes from when I dried and weighed the first one until I weighed the last one which proved to be difficult however I can overcome this problem if next time I work in a group, this was not possible this time.

Another improvement I can make is there were three different sized potato chips in each pot; ideally there would be a Galley pot for each potato chip, this may have caused problems like two potato chips on top of each other may mean there is less surface are for osmosis to happen and the molecules in the solution would have more volume to try and make balanced maybe this would slow the process. Next time I could try to have a larger range of potato chip sizes and solutions for a more detailed experiment.

Next time I could change other factors like volume of solution, temperature, light and other materials like plant cells to see if they would make a difference in weight.