Investigating the Effect of Concentration of a Certain Salt Solution on the Amount Tissue of Potato Slices

Investigating the Effect of Concentration of a Certain Salt Solution on the Amount Tissue of Potato Slices

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Investigating the Effect of Concentration of a Certain Salt Solution on the Amount Tissue of Potato Slices

Aim:

The aim of the experiment is to investigate the effect of
concentration of a certain salt solution on the amount tissue of
potato slices.

Osmosis is the movement of a solvent (e.g. water) through a semi
permeable membrane, from a solution that has a lower concentration of
solutes (e.g. salt) to one that has a higher concentration of solutes.
In osmosis, the membrane does not allow the solute to pass through,
but does allow the solvent to pass through. The rate in which osmosis
occurs depends on the temperature of the solution, the concentration
of the solute, and the electrical charge of the solute. Another factor
is the difference between the pressures that both sides of the
solutions exert onto the membrane. Osmosis stops when the two
solutions are equal in concentration.

Prediction:

For this investigation I think that the lower the concentration of the
salt solution the larger the mass of the potato will be at the end.
This is because the water molecules pass from a high concentration
(solution) to a low concentration (potato). As a result the potato in
lower molar concentrations will gain a larger mass than in higher
molar solutions.

To create a fair test variables of the experiment will have to be kept
the same whilst only one variable is changed. I have chosen to vary
the morality of the salt solution. If the solution volume, duration of
experiment, temperature, solution or scales are not kept constant it
would not be a fair test. For example if one of the potato slices was
longer than another then the surface area of the potato would be
larger and there would be more space for osmosis to occur. Doing all
the tests at one temperature will control the temperature, I will try
to work at room temperature and will leave all of the tubes in the
same place so that they are all experiencing the same conditions.

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The
volume of the solution that the potatoes are kept in must be exactly
the same in each tube, to do this I will use a pipette filler and
pipette to ensure 25cm³ of solution is in each tube. All of the
potatoes will have to be covered also. I am also going to use the same
balance to weigh my potatoes. This is because the measurements may
slightly vary between scales. The mass of the potato will be measured
before and after the experiment. I will measure the mass in grams.
This will allow us to see whether osmosis has taken place and how much
the mass has differed.

Preliminary Results:

Molarity

Mass Before

Mass After

Change

% Change

0.00

1.66

1.74

0.08

4.8193

1.58

1.66

0.08

5.0633

0.25

1.70

1.62

-0.08

-4.7059

2.06

2.07

0.01

0.4854

0.50

1.69

1.62

-0.07

-4.1420

1.78

1.67

-0.11

-6.1798

0.75

1.76

1.60

-0.16

-9.0909

1.71

1.56

-0.15

-8.7719

1.00

1.74

1.59

-0.15

-8.6207

1.40

1.25

-0.15

-10.7143


These preliminary results show me the kind of results expected. If I
have drastically different results then I will know that there has
been a mistake made somewhere in the method.

Planned method:

A range of salt solutions will be prepared with concentrations 0.00
molar, 0.25 molar, 0.5 molar, 0.75 molar and 1.00 molar. Sections of
potato will be cut using a scalpel and will be measured using a ruler.
All of the potato slices will be as close to 15mm as I can make it.
This must be done accurately as a change in the surface area may
change the results. The mass of each potato will be taken so that more
results can be obtained. Three slices will be placed on a wooden pin
in each tube each time so that I can take an average for each tube. I
will use 25cm³ of each concentration of salt solution and once in the
test tubes they each will be labelled. The potato pieces will then be
left overnight, for 25 hours. Then the potato pieces will be removed,
the excess solution absorbed using paper towels and then they will be
re-weighed.

Skill Area O: Obtaining evidence

Method:

1. I took a potato and washed it.

2. Using a cork-borer and scalpel I cut potato slices that were 15mm
long and 10mm wide. I made 15 slices.

3. I measured and weighed each slice, weighing them in a particular
order to ensure I knew which one was which.

4. I took 5 specimen tubes and labelled each one with my initials and
the molarity of the solution.

5. Using a pipette and pipette I measured out different amounts of
salt solution and distilled water which I then put into the test tubes
in a percentage ratio giving me the various molar concentrations.

6. I put 3 potato slices into each beaker and then sealed the tubes. I
sealed the tubes to prevent anything from getting into them and also
to stop water from evaporating

7. Whilst waiting I drew up a basic table for my results.

8. After 25 hours I drained out the solutions in the sink and placed
all the slices on the paper towel in the order I had put them in the
test tubes as to not confuse myself as to which slice came from which
solution.

9. I dried each slice with the paper towel and then placed each one on
the scales so that I could weigh them. I recorded the weights and put
them in the table with the previous results.


Results:

Molarity

Mass Before (g)

Mass After (g)

Change in Mass

% Change

0.00

1.72

1.85

0.13

7.5581

1.64

1.89

0.25

15.2439

1.63

1.85

0.22

13.4969

Average

1.66

1.86

0.20

12.0996

0.25

1.56

1.46

-0.10

-6.4102

1.66

1.64

-0.02

-1.2048

1.51

1.15

-0.36

-23.8410

Average

1.58

1.42

-0.16

-10.4853

0.50

1.65

1.45

-0.20

-12.1210

1.66

1.52

-0.14

-8.4337

1.88

1.71

-0.17

-9.0425

Average

1.73

1.56

-0.17

-9.8657

0.75

1.64

1.36

-0.28

-17.0730

1.49

1.34

-0.15

-10.0670

1.87

1.74

-0.13

-6.9518

Average

1.67

1.48

-0.19

-11.3639

1.00

1.61

1.46

-0.15

-9.3167

1.66

1.49

-0.17

-10.2400

1.56

1.42

-0.14

-8.9743

Average

1.61

1.46

-0.15

-9.5103

The attached graph gives the line of best fit for the percentage
change in mass of the potato. The graph is a downward curving line.
Because the line is not straight it means that the percentage gain and
loss in mass and molarity are not directly proportional. Although
there is a pattern on my graph: as the concentration of the solution
increases, the percentage change in mass decreases. The graph shows
that the percentages gain and loss in inversely proportional to the
concentration. The gradient does change in my graph. It gets less
steep as X axis gets bigger. This is because the potato is becoming
flaccid and so the change in mass of each molar concentration is
becoming closer and closer together. The graph shows the results
accurately and from this you can see that the result for 0.75M does
not fit, it is an anomaly. This could be because of various reasons
such as contamination: I could have accidentally got something into
the tube or I could have made a mistake when making up the solutions.

It shows that the potato cells increase in mass in solutions with a
high water concentration and decrease in mass in solutions with a low
water concentration. This graph of the change in mass helps prove the
point of complete plasmolysis, whereby the potato cannot expand and
take in any more water. Plasmolysis occurs when a plant cells membrane
shrinks away from its cell wall. This happens when water is removed
from the cell into the outside cell fluid. The movement of water
occurs across the membrane moving from an area of high water
concentration to an area of lower water concentration outside the
cell. As you can see as the molar concentration increases the change
in mass decreases. From right to left the first two points on the
graph are very spread out indicating that there was a large change in
the mass. This decreases throughout the increasing molar concentration
until the change is very small.

This graph attached shows a clear indication that there was an overall
decrease in mass during the experiment. At the point 0.00 M the line
for after the experiment is above the line for before the experiment
unlike any of the others. This is because the water potential of the
salt solution is higher than that of the potato.

Skill Area E: Evaluation

The experiment was very successful in my opinion. I obtained accurate
results from which I was able to create graphs. I think I took enough
results for the amount of concentrations that I was using, and the
time that I left the experiment for was enough to allow sufficient
osmosis to occur. However if I was to repeat the experiment I might
well increase the time of the result to allow more osmosis to happen
and possibly find out the saturation point of the potatoes. The range
of concentrations was adequate but I would possibly create more
concentrations if I repeated the experiment so that I would have more
varied results, i.e.0.1, 0.2, 0.3, and so on. This way would have
allowed me to get more accurate results and would have allowed me to
produce a more detailed graph.

The cutting of the potatoes was the most difficult part of the
experiment as although I was recording my results by mass, it could
well have affected the surface area and so the overall rate of
osmosis. If I were to repeat the experiment I would have possibly
found a machine to cut the potato as it would ensure that all potatoes
would be the same weight and dimensions. As well as the potato, I
could have found a more accurate way to measure out the solutions and
to determine the molar concentrations. I could also weigh each slice
on a more accurate scale, e.g. not to 0.00g but to 0.0000g.

There was only one anomaly and this may have been caused by me. When
the potato slices were removed from the test tubes and dried I may
well have dried some potatoes more thoroughly than others and so some
would have more excess water, which would add to the mass. If the
experiment was repeated I could find another way to dry the potatoes
that would ensure that all were dried in the same way for the same
time. However with all this said I think that the experiment was truly
successful and I was very pleased with the complete comparison of my
results with my initial prediction.
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