Biology Benedict’s Test

Question 1

What does Benedict’s solution contain?

Answer 1

Copper sulphate Cu2+SO42

This is blue in colour and soluble in water

Question 2

How can the copper part of the copper sulphate be reduced?

Answer 2

It accepts an electron to become Cu+

This is red in colour and insoluble in water

Question 3

What happens when the blue, soluble Cu2+ accepts and electron?

Answer 3

It becomes Cu+ (red and insoluble)

Question 4

What gives the colour change for copper?

Answer 4

The formation reduced copper that gives the colour change and because it is insoluble and it precipitates out of the solution

Question 5

What is only a small amount of copper is reduced?

Answer 5

The precipitate is formed when the red colour is not obvious as it is masked by the blue of the copper sulphate

The mixture of blue solution with a small amount of red precipitate is perceived as yellow or green

Question 6

What happens if more of the reduced precipitate is formed?

Answer 6

Then this will be perceived as being orange or even brick red, if all the copper sulphate is produced.

Question 7

What do reducing sugars mean?

Answer 7

Means they can donate electrons or reduce another molecule or chemical
All monosaccharides and disaccharides eg maltose and lactoe

Question 8

What do all monosaccharide do?

Answer 8

Have a reducing centre which will donate an electron and reduce the coppper in Benedict’s solution

Question 9

How can monosaccharides cause a positive result in Benedict’s test?

Answer 9

Because all monosaccharides are able to reduce the copper in the CuSO42- and will therefore cause a positive result for Benedict’s test

Question 10

What can reducing sugars do?

Answer 10

Donate an electron to another molecule

Question 11

All monosaccharides and some disaccharides are what?

Answer 11

Reducing sugars eg maltose and lactose (but some disaccharides are non reducing sugars)

Question 12

Give the method for testing for reducing sugars

Answer 12

• we start by grinding up the food with distilled water and filter away any food particles
• place about 4cm3 of test solution in a test tube
• add a few drops of Benedict’s solution (enough to make the blue colour obvious)
• stand the tube in the water bath at 85-100C for approximately 5 mins
• observe any change in colour

Question 13

Give the results for the test for reducing sugars

Answer 13

Benedict’s solution contains the copper ion Cu2+ which makes the solution blue

• if the solution remains blue there’s no reducing sugar present
• however if a reducing sugar is present then this adds an electron to the copper 2+ ion. This now forms the Cu+ ion and this forms a red precipitate
• if there’s only a very small amount of reducing sugar then only a very small amount of red precipitate forms (this causes Benedict’s solution to appear green)
• if more reducing sugar is present then the colour turns yellow
• a higher level produces an orange colour
• if a lot is present then we see a brick red colour

This test only gives an approximate idea of the amount of reducing sugar. That’s because it only shows a narrow range of colour changes

Question 14

Give the method for testing for non reducing sugars

Answer 14

• place about 1.5cm3 of test solution in a test tube
• add 3 drops of dilute hydrochloride acid
• shake the tube and place in the water bath at 100C for 5 minutes
• remove the tube, allowing it to cool
• add 1 spatula of sodium hydrogen carbonate and mix, to neutralise the acid
• add a few drops of Benedict’s solution (enough to make the blue colour obvious)
• stand the tube in the water bath at 85-100C for approximately 5 mins
• observe any change in colour

Question 15

How does maltose produce a positive result for a Benedict’s test

Answer 15

There is one reducing centre that is available (the other isn’t as it is involved with the glycosidic bond) so maltose is able to reduce the copper in Cu2+SO42- and will therefore produce a positive result for the Benedict’s test.

Maltose is a reducing sugar

Question 16

Why does sucrose produce a negative result for Benedict’s test

Answer 16

Because neither reducing centre is available; both are involved with the glycosidic bond

So sucrose is not able to reduce the copper in Cu2+SO42- and will therefore produce a negative effect for a Benedict’s test

Question 17

Why can’t we test for non-reducing sugars directly?

Answer 17

Because we must break the glycosidic bonds, releasing the monosaccharides

Question 18

How can we show that sucrose is not a reducing sugar but a non reducing sugar? (As any non sugar would also give negative results for the Benedict’s test)

Answer 18

If the glycosidic bond of the sucrose molecule is broken then the reducing centres will become available and would give a positive result for a subsequent Benedict’s test. When testing for a non - reducing sugar, the glycosidic bond has to be hydrolysed (broken) before the Benedict’s test is carried out

This could be achieved by using the enzyme sucrose but in the laboratory hydrolysis is achieved by boiling in hydrochloric acid

Question 19

What’s a colorimeter used for

Answer 19

The changes of the blueness in Benedict’s solution may be too subtle to detect by eye

So we quantify the blueness of a solution by using a machine called a colorimeter

Question 20

Explain about the blueness of Benedict’s solution and filtering off the red precipitate, leaving just the blue Benedict’s solution

Answer 20

If we shine white light through a sample of Benedict’s solution (white light has all the colours of the spectrum) then all the colours in white light will be absorbed apart from blue

That’s because Benedict’s solution allows blue light to pass through (that’s why Benedict’s solution has a blue colour)

The absorption of light can be used to quantify the levels of blueness

But rather than using white light the best light to use is actually red . This is because Benedict’s solution absorbs red most of all the different colours (as red is on the end of the spectrum to blue

Place a red filter in front of a lamp (this filter allows red light to pass through)

Less blue (reacted with glucose) can have some light absorbed and some light transmitted and the red light can be detected by the photoelectric cell (light detector)

Question 21

What’s a colorimeter used to measure?

Answer 21

When the Benedict’s solution is reacted it becomes less blue (colorimeter can be used to measure this)

Question 22

As the colorimeter cannot tell us the actual concentration of glucose, what do we need to do?

Answer 22

Prepare a whole range of known glucose concentrations

Question 23

What’s a calibration curve?

Answer 23

Then we react each of those with Benedict’s and filter off the red precipitate

Then use a colorimeter to see how much red light is absorbed by each solution (carry out the same procedure with the unknown solution and then compare it with the known solutions

Question 24

Give the colorimeter method

Answer 24

1. Label the cuvette tray with glucose concentration and unknown.
2. Fill a cuvette almost to the top with solution from the0.32 MB test tube, then paste into the slot on the cuvettes tray. Do the same with the solutions from 0.16 MB, 0.08MB, 0.04 MB, 0.02 MB, and 0.01 MB
3. Fill another cuvette almost to the top of distilled water. This will be a blank.
4. Make sure the colorimeter is set to 0.0 and the filter is set to 680nm
5. Place the blank cuvette into the colorimeter and press the reading button. This should give an absorbance of 0.0
6. Place the 0.32MB convert into the colorimeter and press the reading button. Records the absorbance. Do the same with the other glucose solutions
7. For the cuvette labelled U, use the blank cuvette and repeat steps 4,5,6. Record your absorbance

Question 25

Give the serial dilution method

Answer 25

• label 7 test tubes with permanent marker as 0.32, 0.16, 0.08, 0.04, 0.02, 0.01 M and then [place into the test tube rack
• using the 10cm3 measuring cylinder, measure out 10cm3 of the 0.32M glucose solution. It is important to be as accurate as possible, so use one of the dropping pipettes and to make sure the meniscus of the solution is exactly on the 10cm3 line of the measuring cylinder
• add the 10cm3 of 0.32M glucose solution to the test tube labelled 0.32M
• using the other 10cm3 measuring cylinder and a dropping pipette, add 5cm3 of distilled water to each of the other five test tubes
• using a 5cm3 measuring cylinder, take out 5cm3 of the solution in the 0.32M test tube and add it to the test tube labelled 0.16M
• invert the test tube 3 times, you do this by putting a test tube bung into the test tube and turning the test tube upside down
• using a new 5cm3 measuring cylinder, take out 5cm3 of the solution in the 0.16M test tube and add it to the test tube labelled 0.08M. Invert the test tube 3x
• repeat step 7 for 0.04, 0.02, 0.01 M test tubes