Investigating the Phenomenon of Osmosis in Plant Tissues
Introduction
Osmosis is the movement of water molecules through a semipermeable membrane from low concentration to a high concentration.
Research Question
How does the concentration of salt (NaCl) affect the mass of a potato soaked in water (H2O) with different concentrations of NaCl over the course of 30 minutes?
Hypothesis
It is expected that the solution with the highest concentration of salt will cause more mass loss to the pieces of potato. The pieces that will be placed in the solution with the lowest concentration of salt will have small weight loss. Finally, the pieces that will be placed in water (NaCl concentration 0) will gain weight. This is expected because of the phenomenon of osmosis which was examined in a previous lab.
Variables
Independent: concentration (%) of salt (NaCl)
Dependent: mass (g) change
Controlled:
shape and material of the beakers
size and weight of the potato pieces
the volume of the aqueous solution
The beakers need to be of the same size, of the same shape, and of the same material, either plastic or glass. The size of the potato pieces needs to be the same in order for the surface to volume ratio to be the same and the initial weight of the pieces should also be the same so that the mass change can be compared. Since this is not always possible, we calculate percentages and make the pieces approximately the same.
Lastly, the volume of the water solutions should be the same in all beakers so that the osmotic effect will only depend on the concentration dissolved in the solution.
Materials
5 beakers sized: 250cm3 (±0. 5cm3)
Tap Water Potato Knife Weigh balance (±0. 01g)
Salt (NaCl)
Watch glass Spatula Timer (±0. 1sec)
Volumetric cylinder
Procedure
Step 1: Line up the five beakers and with the help of a volumetric cylinder add 100cm3 of water (H2O) in all of them.
Step 2: Place the watch glass on the weight balance and after zeroing it, measure four different concentrations of salt by adding it with the spatula. Start by measuring 0. 5% of NaCl, then 1%, then 3%, and finally 5% and add each one in a different beaker.
Step 3: Stir the solutions using the spatula in order for the salt to dissolve.
Step 4: Take a potato and after peeling it with the knife, cut it into 10 same size small pieces.
Step 5: Measure each one of the 10 pieces and in order to equalize their mass chop, or scrape off some of the pieces if needed.
Step 6: Record the initial mass of the potato pieces and place 2 potatoes in each beaker.
Step 7: After 15min take each of the pieces out of the beakers and measure its weight and record it.
Step 8: Repeat
Step 7 after 30min and after 35min in order to get the rate at which the mass changes.
Data Collection
Table 1: Mass measurements of the potato pieces in solutions with different NaCl concentrations over the course of 35min
Time in min
Mass(g)(±0. 01) in 0. 5% NaCl concentration
Mass(g)(±0. 01) in 0. 5% NaCl concentration
Mass(g)(±0. 01) in 1% NaCl concentration
Mass(g)(±0. 01) in 1% NaCl concentration
Mass(g)(±0. 01) in 3% NaCl concentration
Mass(g)(±0. 01) in 5% NaCl concentration
Mass(g)(±0. 01) in 5% NaCl concentration
Mass(g)(±0. 01) in 0% NaCl concentration
Mass(g)(±0. 01) in 0% NaCl concentration
–
Trial 1
Trial 2
Trial 1
Trial 2
Trial 1
Trial 2
Trial 1
Trial 2
Trial 1
Trial 2
0
1. 76
1. 76
1. 76
1. 76
1. 76
1. 6
1. 76
1. 76
1. 76
1. 76
15
1. 80
1. 80
1. 63
1. 63
1. 57
1. 57
1. 58
1. 58
1. 95
1. 95
30
1. 80
1. 80
1. 54
1. 54
1. 50
1. 50
1. 51
1. 51
1. 97
1. 97
35
1. 79
1. 79
1. 52
1. 45
1. 49
1. 42
1. 48
1. 45
1. 98
1. 98
Data Processing
In Table 2 the average mass of the measurements of the two trials from Table 1 over the course of 35min is shown. The information in the table as a result of adding the measurements from both of the trials, at a specific time, and dividing this sum by two. For example, in the 5mol dm-3 NaCl concentration the mass (g) after 35min was 1. 8g in the 1st trial and 1. 45g in the 2nd trial (see Table 1). The average of these two measurements for the time of 35min, in concentration 0. 5mol dm-3 NaCl, is found by adding them and dividing their sum by two: (1. 48+1. 45)? 2=1. 465, rounded to 1. 47g.
Table 2: The average mass measurements of the potato pieces in solutions with different NaCl concentrations over the course of 35min
Time in min
Mass(g)(±0. 01) in 0. 5%3 NaCl concentration
Mass(g)(±0. 01) in 1% NaCl concentration
Mass(g)(±0. 01) in 3% NaCl concentration
Mass(g)(±0. 01) in 5% NaCl concentration
Mass(g)(±0. 1) in 0% NaCl concentration
0
1. 76
1. 76
1. 76
1. 76
1. 76
15
1. 80
1. 63
1. 57
1. 58
1. 95
30
1. 80
1. 54
1. 50
1. 51
1. 97
35
1. 79
1. 49
1. 46
1. 47
1. 98
In Table 3 the average mass change of the measurements over the course of 35min is shown. This is calculated by subtracting the initial measurement of the average mass of the pieces of potato (measurements of 0min shown in Table 2), which are submerged in different solutions with different salt concentrations, from the final measurement of the mass of potato present in the same concentration (measurements of 35min shown in Table 2).
For example, the average mass change over the course of 35min in NaCl concentration of 05mol dm-3 is found by subtracting the first average measurement of the mass, 1. 79g, from the final average mass, 1. 76g: 1. 79-1. 76=0. 03g.
Table 3: Average potato mass change in each solution over the course of 35 minutes
–
Mass(g)(±0. 01) in 0. 5% NaCl concentration
Mass(g)(±0. 01) in 1% NaCl concentration
Mass(g)(±0. 01) in 3% NaCl concentration
Mass(g)(±0. 01) in 5% NaCl concentration
Mass(g)(±0. 01) in 0% NaCl concentration
Mass (g) change (±0. 01)
0. 03
-0. 27
-0. 30
-0. 29
0. 22
When the mass change is a positive number, this means that the mass of the specimen, the potato piece, increased from the initial mass. On the contrary, when the mass change is a negative number, this means that the mass decreased from the initial mass that was first measured before it was submerged in the solution. In Table 4 the rate at which the mass increases or decreases in the five different solutions is shown. This is calculated by dividing the absolute average mass change of each solution (shown in Table 3) and dividing it by the maximum time used in the experiment, which is 35min.
For example, the rate at which the mass drops or increases in the 0. 5mol dm-3 NaCl concentration solution is shown by taking
0. 03
(Table 3) and dividing it by 35min:
0. 03
? 35=8. 6*10-4g/min.
Table 4: Rate at which the mass increases or decreases.
–
In 0. 5% NaCl concentration solution
In 1% NaCl concentration solution
In 3% NaCl concentration solution
In 5% NaCl concentration solution
In 0% NaCl concentration solution
Average mass (g)/time (min)
8. 57*10-4
7. 71*10-3
8. 57*10-3
8. 28*10-3
6. 28*10-3
In Table 5 the percentage% mass change is shown.
This is calculated by subtracting the average initial mass measurement from the average final one, dividing it by the average initial mass, and multiplying it by 100. For example, the percentage% mass change in the 0. 5mol dm-3 NaCl concentration solution is calculated like this: [(1. 79-1. 76)/1. 76]*100=1. 70%.
Table 5: Percentage% mass change
–
Mass(g)(±0. 01) in 0. 5% NaCl concentration
Mass(g)(±0. 01) in 1% NaCl concentration
Mass(g)(±0. 01) in 3% NaCl concentration
Mass(g)(±0. 01) in 5% NaCl concentration
Mass(g)(±0. 01) in 0% NaCl concentration
Percentage%Mass change
1. 0
-15. 34
-17. 04
-16. 47
12. 5
Graph 1: Mass changes of potato pieces bathed in NaCl solutions
Graph 2: Rate at which the mass in each solution increases or decreases
Conclusion
The data collected confirmed the hypothesis that the solute with the highest concentration of salt will cause more mass loss to the pieces of potato, the pieces that will be placed in the solute with the lowest concentration of salt will have small weight loss, and the pieces that will be placed in water (NaCl concentration 0) will gain weight thus increasing in mass.
After observing the changes in the mass of the potato pieces we can tell that the higher the concentration of salt in the water, the more the mass of plants will decrease because of osmosis. The lower the concentration of salt in water, the more the mass of the plants will increase or just decrease but not dramatically. If we submerge a plant in water with no NaCl dissolved in it, the mass of the plant will increase, because water molecules will move from the low concentration solute to the high, thus entering the plant tissue and increasing its mass and weight.
Also, time-wise, the more the plant specimen soaks in the solutions the more its mass will relatively increase or decrease. Lastly, from the first graph, it can be estimated that at concentration 4. 2% the potato in the NaCl solution will reach equilibrium, thus the concentration will be the same inside and outside of the potato.
Limitations
There are some limitations to this experiment that prevent the results from being perfect. First of all, there was not enough time to repeat the experiment and because the lab was crowded there was no time to chop more than two pieces of potato for each beaker so only two trials were performed instead of three or five. Secondly, the sizes the potatoes were chopped in were not exactly the same because more attention was given to having the same mass and thus neglecting the size which leads to a different surface to volume ratio and affects the results. Thirdly, the timer that was used as the classroom clock and therefore the time at which the pieces of potato were measured were not very accurate.
Improvements
In order to improve the results of this experiment, it is necessary that some changes be made. Firstly, a tool should be found that will allow potato pieces to be chopped in the same size so that the surface to volume ratio will be the same and not change, thus affecting the results. A proper timer should be used to count the time intervals. Lastly, more repetitions, at least 3, need to be made so that the results are more precise.
