The controlled variables:
Materials needed:
30 pieces of potato strips with the following dimensions: (0.5cm x 0.5cm x 4.0cm)
6 beakers of size 50ml
Knife
Ruler ( measuring up to 15cm)
6 different sugar (sucrose) solute concentrations :
0.0 mol dm-3 ( distilled water )
0.2mol dm-3
0.4 mol dm-3
0.6mol dm-3
0.8mol dm-3
1.0mol dm-3
The method:
The different Sugar solutions were poured a beaker and each beaker was marked according to the concentration of sucrose poured into it .
Potato pieces were cut into 30 strips using a ruler and a knife to have the same size with dimensions (0.5cm x 0.5cm x 4cm).
Diagram 1: sketch showing the approximate size of each potato strip
4 cm
0.5cm
0.5 cm
5 potato strips were placed into a beaker.
Stopwatch was used to set time (30 minutes) from time of dropping the strips into the beakers.
After 30 minutes. Each sample was drained off and measured using the same ruler again to see if the length of the potato strips has changed.
Method for recording the raw data was to make a table and to write up any other observations
Data collection and processing:
Raw data table:
The table below shows the Quantitative raw data :
Table 1: the table below shows the length of the potato strips after the incubation in various sucrose concentrations for a time of 30 minutes (±0.01 minute).
Original length of the potato strips before the incubation is 4.0 cm.
The table below shows the qualitative raw data :
Table 2: showing the observations of the situation (sinking – floating) of potato strips after the incubation in various sucrose concentrations for a specific time of 30 minutes:
Diagram 2 : showing the observations of the situation (sinking – floating) of potato strips after the incubation in various sucrose concentrations for a specific time of 30 minutes:
Processing raw data:
Table 3 : the table shows the mean initial length of potato strips, the mean final length after the incubation in different sucrose solutions and the change in length (change in length = mean final length – mean initial length )in addition it shows the standard deviation of the mean final lengths using excel :
Graph (1) the graph shows the change in the length of the potato slices after the incubation in different sucrose concentration for 30 min.:
Best fit line
The solutions are isotonic (0.15 mol dm-3)
Conclusion and evaluation:
The results obtained from the experiment support my hypothesis. By osmosis , water molecules diffused into or out of cells of the potato strips since they are a collection of thousands of cells all stacked on one another that are surrounded by partially permeable membranes. The concentration of sucrose solution affected direction of the movement of water. The potato strips decreased more in length, the higher the concentration of sucrose in the surrounding solution was in the beaker, and on the other hand the potato strips increased more in length, the lower the concentration of sucrose in the solution in the beaker surrounding the potato strips.
The different solutions in which the potato strips were incubated in resulted in different final lengths of the potato strips:
In 0.0 mol*dm-3 sucrose concentration, the length of the potato strips increased as seen in graph (1). This is due to osmosis which is the passage of water from a region of high water concentration (hypotonic solution) which in this case is the 0.0mol*dm-3 concentration of sucrose through partially permeable membranes (potato cell membrane) to a region of low water concentration (hypertonic solution) which in this case is the concentration inside the cells of potato strips. There was a difference between the solute concentrations between the two solutions. The one with highest was hypertonic in comparison to the one with lower solute concentration of sucrose. The solution that has the lowest solute concentration of sucrose is hypotonic (hypo means under or less than). As water diffuses into potato cell, from the 0.0 mol*dm-3 concentration, the cells will expand, and as a result ,the strips expand and become more rigid.
The (best-fit) line crosses the x-axis (the horizontal line) at a sucrose concentration of 0.15 mol*dm-3 (see graph 1 ), at this point there is no change in length of the potato strips since the two solutions (i.e. the solution inside of the cells of potato strips and the solution in the beaker have the same solute concentration. They are said to be isotonic in relation to each other. If the two solutions are separated by a selectively permeable membrane. Water will diffuse between the two solutions resulting in that the concentrations will remain the same. But there will be no net diffusion of water in either solution. In this case the two solutes are isotonic and as a result the length of the potato strip stays the same because of this.
In the (0.2 , 0.4 ,0.6 , 0.8 , 1.0 mol*dm-3 sucrose concentrations), the length of the potato strips decreased as seen in graph 1, This is due to osmosis which is the passage of water from a region of high water concentration (hypotonic solution) through a partially permeable membrane . In this case, the cells making up the potato strips have lower concentration of solutes (hypotonic solution) in comparison to the different concentrations of sucrose solutions that surrounds the cell . The water molecules will move out of the cells to beakers containing different solute concentrations, as water leaves the potato strips, the length of the potato cells shrinks and the strips become more flexible and elastic.
- it is clear from look as graph 1 ,that all the results lie near to the 'curve of best fit , showing that there were not many anomalies and that the results were reliable
The sinking and the floating observations are generally due to weight gain. Because in the( 0.0 )mol*dm-3 concentration of sucrose) the potato strips gains weight when the water diffuses into the potato cell and the density becomes higher. In the other concentrations (0.2 , 0.4 ,0.6 , 0.8 , 1.0 mol*dm-3 sucrose concentrations) the water diffuses out of the potato cells ,as a result the potato cells lose weight and float as the density decreases.
A general conclusion could be drawn from the results of this investigation, that the higher the level of solute sucrose concentration, the more rapidly osmosis takes place.
Evaluation:
I believe that the experiment was carried out well, where the results shown in tables and on the graph were as I had expected, based on my background knowledge. The results gave evidences to the process of diffusion and answered my research question. However the results were not completely accurate due to human error and not using very accurate apparatus to measure lengths and cut potatoes.
Limitations and improvements: