Prediction with reason
I predict that the mass will increase for the lower concentrations and decrease for the higher concentrations. The reason I predict this is because of my knowledge of osmosis. Osmosis is a special case of diffusion. Osmosis is the movement of water particles through a semi- permeable membrane. The water diffuses across the permeable membrane from where there is a high water potential to where there is a low water potential. (They go from where there are lots of water particles to where there are less water particles.) The diagram below shows this information:
Water particles
Permeable membrane
The mass of the potato will increase when the concentration is low because there are more water particles in the solution than in the potato. When the concentration is of the salt solution is high (I.e. 1.0%) the mass of the potato will decrease. This will happen because there are more water particles in the potato than in the solution, so they pass from the potato to the solution.
Size does affect the rate at which osmosis occurs and generally it is that water moves faster through the membranes and other molecules like salt move slower because of their size.
Results
As you are aware for each concentration there were three test tubes. The results were calculated for all of them but when the graph was plotted an average point was used.
The values in the percentage mass change column are either positive or negative. If the values are positive then the weight of the potatoes had increased by that amount. If the values are negative then the weight had decreased by that amount.
Analysis
From the graph and the table of results it has become evident that as the concentration of salt solution increases the average change in the percentage of mass decreases.
The higher the concentration of salt the less the change in the mass of the potato chip. This can be seen on the graph and in the table, for example when the potato chip was in distilled water the mass increased by 18%, then at 0.25% the potato chip only increased in mass by 13%.
There are no specific patterns in the way osmosis occurred. For example as the concentration doubled the change in mass did not halve or anything like that.
The reason for this is that in the pure water solution the potato chips swell up because water enters them by osmosis, in the rich salt solution water leaves the potato chip by osmosis, and so they shrink in size. Osmosis is trying to balance the amount of water particles on each side of the partially permeable potato membrane. When there is hardly any salt in the salt solution (i.e. it is weak) there is more water in the solution than in the potato so water goes form the solution into the potato. When the salt solution is more concentrated (i.e. 1%) there are more water particles in the potato than in the solution so they leave the potato and enter the solution by osmosis.
If you think very technically then there should be a pattern that as the concentration doubles the change in percentage mass should halve. Because at 0.25% there was a + 13% mass change, at 0.5% there was an + 8.8% change, which is nearly half. In the 0.5% salt solution there are twice as many salt particles than in the 0.25% solution, this would mean that there would there is less water in the 0.5% salt solution and therefore less osmosis is needed to reach a water Equilibrium. The lower the concentration of the solution the more osmosis would occur to reach equilibrium of water particles (there would be lots and lots more particles in the solution than the potato so water would go into the potato and make it swell or increase in mass).
Later on when the concentration is higher the this pattern will carry on but after a certain point (that can be seen in the graph and results table) instead of the potatoes increasing in mass they will start to decrease in mass and in this way as the concentration doubles the potatoes will get lighter.
These results support my prediction because in my prediction is said that the mass will increase for the lower concentrations and decrease for the higher concentrations and this is what happened. After 0.75% the potato chips started to decrease in weight. The diagram below show why this happens.
Diagram showing
osmosis for low
concentrations of
salt solution!
=Water particle
Salt solution potato = Salt particle
Diagram showing
osmosis for high
concentrations of
salt solution!
Evaluation
When writing up the plan for this investigation I tried to make the method as fair as possible, I did this by keeping things constant like the potato etc. but it cannot be a 100% fair because there will always be a slight human error no matter how perfect you try and make things.
I tried to be as accurate as possible when carrying out the experiments for this investigation because accuracy in things like measuring can make all the difference. When measuring the potatoes to a length of 3cm I just simply used a ruler and a knife. On my part I tried to keep all human error to a minimum however the equipment that was used to carry out this investigation was very basic and therefore it is likely that the chips may still have been slightly different lengths. Because I was aware of this inaccuracy taking place I decided to repeat each concentration three times and use an average of the results. The results of the repeats were varied for some concentrations the results were similar which proved that the inaccuracy was small, however for some concentrations the results were not as close and this showed that there was a lot of error.
The experiment that was carried out was suitable because overall the results made sense and proved what a lot of different textbooks quote. A lot of things were kept constant to make this experiment as fair as possible. They included things like; the length and width of the potato, the volume of salt solution, the potato used, and the time the experiment was left for.
The range of concentrations that I chose to use was enough to support my prediction because within the five concentrations the different mass changes are visible. However it is only for one concentration that the mass decreases and to make sure that this pattern carries on you could use six or even seven different consecutive concentrations. The experiment is fairly simple therefore there were not many difficulties when carrying it out.
The results are reliable and they provide enough information to support a firm conclusion because they all make sense. Also if I was to make a conclusion from these and then check it up with theory from textbooks it would have matched.
The odd results may have occurred because the equipment was not as reliable as it could have been.
The investigation could have been improved in a number of ways. For example the equipment could have been better (more scientific?), this in turn would have also made the results more accurate and reliable.
If a bigger range was used then there would have been more evidence and therefore, it would be easier to draw a firm conclusion. Even with the five concentrations that were used there was enough evidence; if more concentrations were used then there would have been more proof.
The main improvements that could be made were to the equipment because the way the experiment was carried out was fine; the main errors were due to the equipment. Also to get very definite end results the experiment could have been left for longer so that all the osmosis that was going to happen would have happened. A water equilibrium would have been reached, and then the mass of the potatoes would be in some sort of proportion to the concentration of the solution and it may have also been possible to come up with a formula or rule that linked the two.
This investigation could have been extended for a different variable. Many things affect the rate at which osmosis occurs like, the concentration gradient, the size of the ions or molecules, and the distance over which it occurs. It is possible to extend this particular investigation by using a different substance instead of the potato. Some thing with a much thicker membrane would increase the distance over which osmosis has to occur and consequently the results would be different if the experiment was left set up for the same period of time. This investigation would then be to see what factors affect the rate at which osmosis occurs.
Alternatively an investigation could be carried out to see how temperature affects the rate. This could be done by setting up the experiment in the same way as before but instead of simply leaving the test tubes to stand in a test tube rack; they could be put into water baths that are kept at certain constant temperatures.
