Carbohydrates (B)

Question 1

Which monosaccharide or disaccharide is a non reducing sugar?

Answer 1

Sucrose

Question 2

Is this test a qualitative or quantitative test?

Answer 2

Qualitative test

Question 3

Benedict’s test for reducing sugars

Answer 3

A small amount of the sample is placed in a test tube with the same volume of Benedict’s solution
This is heated to 95C in a water bath

Question 4

Negative result for Benedict’s test

Answer 4

The Benedict’s solution remains blue

Question 4

Positive result for Benedict’s test

Answer 4

A brick red or orange precipitate

Question 5

When is the semi-quantitative Benedict’s test used?

Answer 5

Comparing the concentration of reducing sugars in different solutions

Question 5

How can you standardise the control variables?

Answer 5

Use the same volume of Benedict’s solution and samples
Heat samples for the same period of time and at the same temperature

Question 6

When is the quantitative Benedict’s test used?

Answer 6

To obtain numerical data to compare the concentration of reducing sugars in different samples or to find the concentration of an unknown sample

Question 6

What is important about the control variables?

Answer 6

The control variables must be standardised

Question 7

How to perform it?

Answer 7

Standardize control variables.
Test reducing sugar solutions of known concentrations
Measure absorbance value of each solution with a colorimeter
Plot a calibration curve of concentration against absorbance value and add a line of best fit

Question 8

If we are looking to find the concentration of an unknown solution what do we do?

Answer 8

Perform the Benedict’s test with the unknown sample (making sure to standardise the control variables)
Measure absorbance value of unknown using a colorimeter
Use the absorbance value of the unknown to read off the calibration curve in order to find its concentration

Question 9

Explain the test for non reducing sugar

Answer 9

Take a small sample and heat in a 95C water bath with Benedict’s solution to confirm a negative result
Hydrolyse another sample of the sucrose by heating in a 95C water bath with dilute acid (e.g. Hydrochloric Acid)
When cooled neutralise the acid with an alkali (e.g. Sodium Hydroxide or Sodium Carbonate)
Add the same volume of Benedict’s solution and heat in a 95C water bath
A positive brick red colour (precipitate) indicates the presence of reducing sugars, confirming a non reducing sugar was originally present