Osmosis Eggs : IA Exercise ‘mini-lab report’
February 2012
AIM : To use vinegar-softened eggs, or ‘rubbery eggs,’ as a cell model to observe
osmosis effects.
Focused Research Question :
How will increasing sugar concentrations affect the dimensions of the rubbery eggs ?
Hypothesis : If the rubbery eggs are a good cellular model, then increasing external
sugar concentrations should cause more osmosis of water out of these eggs.
Predictions : Egg size should decrease the most in the solutions with the highest
sugar concentration. Eggs might also take in water (and grow in size) in the absence
of external solutes (plain water).
Background Information
Osmosis is the passive diffusion of water through a semi-permeable membrane.
Water moves down its concentration gradient, toward equilibrium, depending on the
relative amount of solute inside and outside. The greater the solute concentration, the
lower the water potential. Water moves by osmosis down this water potential
gradient (toward the area of high solute concentration).
Experimental Variables
Independent Variable : sugar concentration
Dependent Variable : mass and circumference of the eggs
Fixed Variables : volume of solutions, temperature of solutions, time of incubation
(all are kept constant)
Control : rubbery egg, allowed to re-harden and then put into water
Materials and Methods :
Eggs
Large beakers
Vinegar
Water
Sugar syrup (100% syrup : 1Kg sugar per liter water)
Scale, string, ruler,
Refrigerator
1) Measure egg circumference (with string and ruler, long dimension) and
weight. Record results.
2) Soften the eggshells by putting the eggs in vinegar, a weak acid which
dissolves the calcium carbonate of the shell, releasing carbon dioxide. This
and all subsequent incubations were performed at 4oC (in the refrigerator).
3) Measure and weigh the eggs again (24h later), incubating them subsequently
in the different solutions (100% syrup, 50% syrup, and plain water), 3 eggs in
each condition.
4) Measure sizes and weights at various times (3 and 6 days later) during the
course of the experiment.
5) Process data and analyze results.
Results
Measured values obtained in the course of the experiment are tabulated below.
Qualitative observations during the course of the experiment include the findings
that 1) the air pocket of the eggs was easy to find; 2) the ‘control’ egg did get
somewhat solid, but also quite wrinkly – and then water obviously could re-enter
between the 16-19th Jan; 3) the eggs sank in vinegar and in water, but floated
initially in the sugar solutions ; 4) a hard fold developed in many eggs that floated in
the sugar solutions ; 5) finally, the eggs sank part-way in the 50% sugar solution,
while the eggs still floated in the 100% solution.
6) cut eggs from the 100% sugar solution had a dense dark yellow yolk, while the
water solution yolk was soft and a paler yellow.
Average values, Standard Deviation and error bar values (+/- 2xStdDev)
Average Circumference Data :
date
H2O
50% sugar
100% sugar
circumferences
initial
after vinegar
12-Jan
13-Jan
16.9
17.67
16
17.17
16.33
17.17
expt
16-Jan
end
19-Jan
17.53
17.27
16.17
17.5
16.83
15.33
Standard deviation :
standard
deviations
H2O
50% sugar
100% sugar
Would be more
valid with a
0.36
1
0.29
2xStdDev for errorbars!
circumferences
1.15
1.04
0.29
larger data
0.06
0.32
0.58
set!!
1.32
0.29
0.58
0.72
2
0.58
H2O
50% sugar
100% sugar
2.3
2.08
0.58
0.12
0.64
1.16
2.64
0.58
1.16
Effect of increasing sugar concentration on circumference measurements
y-axis : average circumference(cm +/- 1mm)
x-axis : dates
21
20
19
18
H2O
50% sugar
17
100% sugar
16
15
14
10-Jan
12-Jan
14-Jan
16-Jan
18-Jan
20-Jan
As can clearly be seen, the egg circumference sizes change over the course of the
experiment. However, variability of the data set does not allow any conclusions on
significance, since all the error bars overlap to some extent. This means any apparent
differences between these average values could have arisen by chance.
Average Weight Data :
date
H2O
50% sugar
100% sugar
initial
after vinegar
expt
End
12-Jan
13-Jan
16-Jan
19-Jan
64.87
63.2
62
60.67
61.33
61.1
62.5
57.9
51.73
65.53
58.5
46.67
3.4
1.73
1.66
2.89
1.26
0.96
1.11
1.54
4.67
3.56
3.91
1.15
5.78
2.52
1.92
2.22
3.08
9.34
7.12
7.82
2.3
standard
deviations
2xStdDev for Error Bars :
weights
H2O
50% sugar
100% sugar
6.8
3.46
3.32
Effect of increasing sugar concentration on weight measurements
y-axis : average egg weights (grams +/- 0.1g)
x-axis : dates
75
70
65
60
H2O
50% sugar
55
100% sugar
50
45
40
10-Jan
12-Jan
14-Jan
16-Jan
18-Jan
20-Jan
Again, a change in egg dimensions is clearly visible. In this case, however, for
measurements of weight, overall trends are more evident, and the 100% sugar
condition results in what is very likely to be a significant difference between average
initial values (12/13 Jan) and the final value (19 Jan). Calculation of the t-test (even if
not really valid with such a small data set !) gives P=0.01, when comparing the final
to the intial values, and P=0.0001 for the comparison to the 13 Jan values. Since both
probabilities are less than 0.05, the decrease in mass in the 100% sugar solution is
indeed significant through these analyses. Nonetheless, all the other error bars show
some overlap, so little else can be understood about osmosis, especially from what is
seen in the 50% sugar solution. The increase of weight in plain water, very small in
comparison to the initial weight, is also uncertain from analysis of these results.
Results from the control egg are not depicted graphically, but the decrease in size
(weight only) over the first few days after the vinegar treatment was apparently
reversed by resubmersion in water. Thus, the re-generation of the eggshell in the
refrigerator from re-incorporation of carbon dioxide back into the calcium carbonate
must not have been fully effective. Water was able to enter into this ‘control’ egg by
the end of the experiment.
Conclusion and Evaluation :
The only clear result from this experiment is the striking decrease in mass for
the eggs in the 100% sugar solution. Supporting the initial hypothesis and prediction
of this experiment, water came out of the eggs by osmosis, moving out into the high
solute conditions of the sugar syrup, and resulting in a loss of over 15ml of water per
egg on average. The rubbery eggs are apparently a good model for a cell, and can
undergo osmosis.
Unfortunately, the majority of the results from this experiment are not useful,
because they are either going up and down too much, or simply non-significant
(differences in mean values could be due to chance because of the large spread of the
data around the means). It should be noted that the standard deviations calculated on
a sample set of ony 3 values is already suspect, however, so at least 5 measurements
for each condition would be necessary for real discussion of significance. Obviously,
measurements of egg circumference on the long axis with a string have several
sources of error, even if one day’s measurements, for the water alone condition had an
extremely low standard deviation. The mass measurement is certainly a better
measure of increase in the egg’s size.
More worrying, several potentially interesting aspects of osmosis (why did the
vinegar treatment cause some sets of eggs to seem smaller, and others larger ? why
did the 50% sugar solution cause the intermediate density of the eggs partially sunk
into the solution ? for instance…) were lost in the ‘noise’ of the generally large
uncertainties. Several modifications of the experimental design would certainly make
this a better experiment.
In the first place, actual negative and positive controls are necessary for valid
comparison to the experimental conditions. Several untreated eggs as negative
controls, and several rubbery eggs only in water (perhaps even deionized distilled
water would be best) as positive controls would be useful. Testing whether the
eggshell can re-harden properly from atmospheric carbon dioxide is certainly an
interesting topic for another experiment, but was not in any way a proper control for
these osmosis tests !
Additionally, a better range of sugar solutions could have been tested, perhaps
including a more concentrated ‘100%’ solution and certainly more intermediate
dilutions (25% and 75%, at least). For each of these conditions, more eggs (5-10) in
larger volumes would have increased the likelihood that the full effects of each
condition would be reliably observed. Larger sample sizes might also help make
averaged data less spread out (reducing the standard deviation and thus the size of
error bars).
If the floating eggs really did allow partial reformation of the egg-shell, so the
diffusion was partially limited, perhaps forcing eggs down into the solutions in which
they floated might also have been helpful. This would prevent access to the
atmospheric carbon dioxide that could regenerate hard calcium carbonate. Finally,
since these eggs were hard-boiled, with cross-linked proteins perhaps limiting
diffusion, an equivalent test with uncooked eggs might be also desirable to maximize
potential effects. In such a case, even exploding eggs from osmosis of water into the
eggs could be possible, according to some reports, however, which might make for a
messy experiment!