Determining the Effect of Different Sugar Concentrations on Benedict's Solution

Determining the Effect of Different Sugar Concentrations on Benedict's Solution

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Determining the Effect of Different Sugar Concentrations on Benedict's Solution


Hypothesis and prediction:

My hypothesis is that the sugar solution with the highest
concentration will turn the precipitate the darkest colour. This will
be the 10% sugar solution. This is because the more amount of sugar it
contains the more it will reduce the Copper. It will be a much darker
precipitate compared to the other two.

Background theory:

Benedict's solution is an aqueous solution of Copper (II) Sulphate,
Sodium carbonate and Sodium citrate. It is an alkaline solution used
to test for the presence of aldehyde groups (RCHO). The reducing sugar
(Glucose) reduces the copper (II) Sulphate to Copper (I) oxide. The
colour of the precipitate varies dependent on the strength of the
reducing sugar present. The colour can vary from blue to red-brick:
indicating a high concentration of sugar. Glucose contains an aldehyde
group, so it is able to reduce the Benedict's solution and form a
precipitate. An aldehyde contains the general formula; RCHO, where the
R represents Hydrogen. They are formed from partial oxidation of
primary alcohols. An aldeyde is formed due to cabonyl groups; these
contain an Oxygen atom joined by a double bond to carbon. If the
carbonyl is joined to a hydrogen atom, then the compound is an
aldehyde. Glucose is a monosaccharide. Monosaccharides can take the
form of linear or ring structures. The carbonyl on the carbon 1
supplies the electron which joins the carbon 1 to carbon 5. When this
bond breaks, there are extra electrons which are then used to reduce
other molecules. The heating thus breaks these bonds, so then the free
electrons reduce the Copper (II) Sulphate to Copper (I) oxide in the
Benedict's solution. The reason why the Benedict's solution is readily
reduced is because it has a high PH, hence alkaline, whereas the
Carboxyl group in the Glucose gives it acidic properties. It splits
open the ring structure, consequently releasing the electrons which
are accepted by the Copper. The Copper (II) ions act as a mild

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oxidising agent. When the reagent is mixed with an aldehyde and
heated, the aldehyde is oxidised to give a carboxylic acid, while the
Copper (II) ions are reduced to a red-brick precipitate of Copper (I)


4 test tubes

Test tube rack

Bunsen burner

200 millilitres distilled water

15ml Benedict's solution

2 syringes

10% Glucose solution



500 millilitre measuring cylinder

300 millilitres of tap water

500 millilitre Glass Beaker



1 test tube holder


Masking tape

Reason for choosing apparatus:

I have chosen to use four test tubes because I will be not be
restricted in the data I acquire, which would be the case if I had
used two. The Bunsen burner is required to heat the solution and
subsequently carry out the Benedict's test. The temperature needs to
be high so that it can break the bonds between the Carbon 1 and Carbon
5, thus open up the ring structure. The thermometer will be required
to measure the temperature and also make sure that the same
temperature is used for all solutions. The distilled water will be
used along with the Glucose to make a 10% sugar solution, which will
be discussed in the method. The measuring cylinder will be used for
measuring the solutions and the syringe for extracting the Glucose
solution. The beaker will be used as a water bath to carry out the
Benedict's test. The gauze and tripod will be used to hold the beaker
above the Bunsen burner. The test tubes will be extremely hot, hence
the need for test tube holders. They will be used to transport the
test tubes from the water bath to the test tube rack. The stopwatch
will enable me to carry out the test under timed conditions, which
will be four minutes. I have chosen to use a control so that it can be
used to compare it with the test tubes that contain the Glucose


I will be carrying out Benedict's test, which will require me to use a
Bunsen burner; therefore, I will have to be careful whilst handling
it. I will also use goggles to protect my eyes. Furthermore, I will
keep the flame away from myself so that I do not come into contact
with it. Moreover, when using a boiling water bath I will need to
point the test tubes away from myself because the water could be
ejected from the test tubes and can burn the skin. Finally, I will be
handling glassware; hence I will need to be very careful not to break


The independent variable will be the sugar concentration, this is the
variable I will be controlling and testing its effect on the dependent
variable: which is the colour of the precipitate. I will be able to
control this variable by using a certain amount of concentration for
the solution, this will therefore enable me to acquire the relevant
data required to carry out my investigation.

The first constant will be temperature; I will keep this constant for
all three solutions, this is so that it is a fair test and will enable
me to accumulate my data accurately. I will control the temperature by
using a thermometer to determine the temperature; 90oC. I will control
this because it will enable me to understand the relationship between
the independent and dependent variable. The second constant in this
experiment is the Benedict's solution, this will also need to be
controlled to make the experiment fair and will show what effect the
sugar concentration has on the solution. Therefore, I will be using
equal amounts in all three solutions, which is 5millilitres.

The variable which I will not control will be the colour of the
precipitate; this will be the dependent variable and is a result of
the manipulation of the independent variable.


1- Label all test tubes by covering with masking tape at the top and
writing the test tube number accordingly.

2- Place 10 millilitres of the 10% solution in test tube number 1.

3- Extract 1cm3 of this solution using the same syringe and place in
another test tube. Then add 9cm3 of distilled water with the second
syringe, thus the second test tube will now contain 1% sugar solution.

4- From the second test tube take 1cm3 of the solution and add 9cm3 of
distilled water. The third test tube now contains 0.1% sugar solution.

5- The fourth test tube should contain 10 millilitres of distilled
water because it is a control.

5- Place all test tubes (except the control) on the test tube rack and
add 5millilitres of Benedict's solution to each test tube.

6- Add 300ml of tap water to the beaker and place on top of the gauze,
which will be placed on the tripod.

8- Goggles should be worn at this point.

9- Bunsen burner should be underneath the tripod and turned on. Put it
on a high flame.

10- Water should start to boil, measure with thermometer, so that it
is 90oC and leave.

11- Using the test tube holder, place all three test tubes one by one
in the boiling water bath and time it for four minutes using the

12- After four minutes, using the same test tube holder, take out the
test tubes and place them on the test tube racks.

13- Carefully look at test tubes and record results.


Test tube

Solution (%)

Colour of precipitate













Results 2:

Test tube

Solution (%)

Colour of precipitate



Dark red






Dark yellow





It is clear to see from the results that as the concentration of the
sugar increases, the colour of the precipitate becomes more intense.
The Glucose acts as a reducing agent and reduces the Copper (II) to
Copper (I), hence the change of colour. The colour was more intense
for the 10% Glucose solution because there were more sugar molecules
to reduce the Copper sulphate. The Copper oxide is insoluble,
therefore forms a precipitate and manifests the colour. The 0.1% sugar
concentration was not as intense because there were not as many sugar
molecules which could reduce the Copper sulphate. The high temperature
ensured that the bonds between the Carbon 1 and the Carbon 5 were
broken; this consequently opened up the ring structure and enabled the
free electrons to reduce the Copper molecules to form a precipitate.

Colour intensity


The qualitative data did not enable me to create a valid graph;
therefore a superficial graph was created to reflect the overall
trend. Additionally, the graph suggested that the relationship between
sugar concentration and colour intensity is directly proportional; as
the concentration of the sugar increases, the colour of the
precipitate becomes more intense. Moreover, the control was compared
with the remaining test tubes to show that there had been a dramatic
colour change in the sugar solution. The copper-red was put at the top
because if this experiment is cross-referenced with the use of a
colorimeter, then it suggests that the darker, more intense colour has
a higher wavelength, which in this case is copper-red.

A colorimeter was required to be able to correctly plot a graph and
analyse the trends. This is a probe that determines the solution
concentration from its intensity. The graph for the increased
concentration would show that there was more colour absorption, hence
intense colour. If a colorimeter was used it would display the
following graph:


In conclusion, the overall trend from the data suggests that the
greater the sugar the concentration, the more intense the colour of
the precipitate. This is because the higher sugar concentration has
more molecules which can reduce the Copper Sulphate to Copper oxide,
which forms a precipitate.


The overall reliability of the experiment was increased by repeating
the experiment again. A fair test was secured through using equal
amounts of Benedict's solution; this therefore enabled me to infer a
cause and effect relationship. The control was affective because it
enabled me to compare it to the remaining test tubes to deduce that
there was an overall change in colour intensity. Furthermore, the
experiment was carried out methodically and thoroughly, which was
important to investigate the effect of sugar solution on Benedict's
solution and also make it reliable. Reliability was also increased
because the method was replicable, this was important because
replicability is an important trait of a science. If this is
undermined, then the experiment is not valid. However, an important
limitation to the study and which could have affected it was the lack
of a colorimeter. The observation was carried out using the naked eye.
This would not be an accurate technique because it is difficult to
measure the colour intensity of the precipitate without any technical
equipment. The colorimeter would be used to reflect the transmittance
of the solution. Transmittance is the amount of light that penetrates
the solution; therefore a solution with a high concentration will have
a higher transmittance. Furthermore, the fact that the experiment
yielded qualitative data meant that I was unable to represent the data
graphically. This was disadvantageous because it affected the analysis
and did not allow me to discuss overall trends in graph. Overall, the
results should be treated carefully. The absence of any technical
equipment to measure colour intensity proves disadvantageous and does
not entirely support the hypothesis. A secure way to support the
hypothesis would be to repeat the experiment several times and also
use a colorimeter, this would then enable me to make a concrete
judgment and will allow me to corroborate my hypothesis. A further
source of error whilst conducing my experiment was using the syringes.
As I was using only two, I was obliged to use the same pipette for all
solutions and the other syringe for adding distilled water. This was
not entirely accurate because I was not able to empty all the contents
of the solution into the test tube. So if I was making a 1% sugar
solution, I would not be adding the full 9 milliliters of the Glucose
solution, since approximately 0.5% of this solution remained as
droplets in the syringe. This would affect the subsequent solution
being made because if a little amount of the Glucose solution
remained, then the following solution; 0.1% would not exactly be that
but would be approximately 0.2%. The repeats also yield similar
results, but are not entirely compatible with the first results. The
reason for this can be down to experimental error. As there was no
technical equipment such as a colorimeter, it was difficult to measure
the colour intensity. Therefore, if compared with a colorimeter, the
dark yellow solution may not be that but darker. This would be best
measured using transmittance, which would only be possible whilst
using a colorimeter.

Changes were also made to the preliminary method, this some apparatus
chosen were not suitable. The fist change made was the size of the
beaker, this is because the beaker was to large to fit the tripod,
therefore a smaller beaker had to be used. Also, I decided to add a
control (as discussed earlier).

Further research

If I was conducting the experiment again the fundamental change
regarding the experiment would be to add a colorimeter. This would
enable me to measure the transmittance of the sugar molecules, as
opposed to the colour intensity. Transmittance is the amount of light
absorbed, therefore the sugar molecule that absorbs the highest amount
of light will have higher transmittance. The following method would be
used to carry out this experiment:

Set up the colorimeter by following the instructions below:

1- Connect the colorimeter to a power socket.

2- Turn it on.

3- Set the device at 0% firstly and then at red.

4- Open the device up and insert the sugar solution.

5- Click on start and it should begin to record he data. The
transmittance will be recorded as numbers, for example 450nm.

Beer's Law can be used to define the concentration of the unknown
solution. This is the absorption coefficient for light passing through
a solution. It is expressed as I=Io10-cx, where C is the
concentration, whilst x is the thickness.
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