Carbohydrates (B)

Question 1

Which monosaccharide or disaccharide is a non reducing sugar?

Answer 1

Sucrose

Question 2

Explain Benedict’s test for reducing sugars

Answer 2

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 3

Is the Benedict’s test for reducing sugars a qualitative or quantitative test?

Answer 3

A qualitative test

Question 4

Negative result for Benedict’s test

Answer 4

The Benedict’s solution remains blue

Question 5

Positive result for Benedict’s test

Answer 5

A brick red or orange precipitate

Question 6

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

Answer 6

Comparing the concentration of reducing sugars in different solutions

Question 7

What colour changes occur in the semi-quantitative Benedict’s test for reducing sugars?

Answer 7

Blue: No reducing sugars present
Green: Very low concentration
Yellow: Low concentration
Orange: Medium concentration
Brick-red: High concentration

Question 8

When is the quantitative Benedict’s test used?

Answer 8

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

Question 9

What is important about the control variables?

Answer 9

The control variables must be standardised

Question 10

How can you standardise the control variables?

Answer 10

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

Question 11

What is a colorimeter?

Answer 11

A device that measures how much light is absorbed

Question 12

How does a colorimeter work?

Answer 12

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Question 13

What is a calibration curve?

Answer 13

The line of best fit on a graph that measures the absorbance (y-axis) against concentration (x-axis)

Question 14

Explain the test for non reducing sugar

Answer 14

Take a small sample and heat in a 95C water bath with Benedict’s solution to confirm a negative result
Then hydrolyse another sample of the sugar by heating with dilute acid (HCl) in a 95°C water bath.
When cooled neutralise with an alkali (NaOH)
Add the same volume of Benedict’s solution and heat in a 95C water bath
A brick-red precipitate indicates the presence of reducing sugars, confirming a non-reducing sugar was initially present.