3.4 - Testing for Reducing and Non Reducing Sugars

Question 1

How are sugars classified as reducing or non reducing?

Answer 1

Their classification depends on their ability to donate electrons

Question 2

What colour is the Benedict’s test for reducing sugars?

Answer 2

•Benedict’s reagent is a blue solution that contains copper(ll) sulfate ions

Question 3

In the presence of a reducing sugar what is formed ?

Answer 3

Copper ( I) oxide
- not soluble in water so it forms a precipitate

Question 4

What’s the method for the Benedict’s test?

Answer 4

1. Add Benedict’s reagent to a sample solution in a test tube
2. Heat the test tube in a water bath or beaker of water that has been brought to boil for a few minutes
3. If a reducing sugar is present - a colored precipitate will form as copper (II) sulfate is reduced to copper ( I ) oxide which is insoluble in water
4. Important that a excess of Benedict’s solution is used so there is more than enough copper (II) sulfate present to react with any sugar present
5. A positive test result is blue —> green, yellow and orange
   ( low to medium concentration of reducing sugar ) to brown/brick red ( high concentration)
6. This is a semi quantitative test as the degree of colour change can give an indication of how much the concentration of reducing sugar is present

Question 5

Give some examples of reducing sugars

Answer 5

• galactose
• glucose
• fructose
• maltose

Question 6

Give an example of a non reducing sugar

Answer 6

Sucrose

Question 7

Describe the benedicts test for a non reducing sugar

Answer 7

1. Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to boil
2. Neutralise the solution with sodium hydrogencarbonate
3. Use a suitable indicator such as red litmus paper to identify when the solution has been neutralised
4. Add a little more sodium hydrogen carbonate as the conditions require to be slightly alkaline for the Benedict’s test to work
5. Carry out the Benedict’s test as normal
6. Add Benedict’s reagent and heat in a water bath
7. If remains blue it’s a negative test , any other colour a reducing sugar is present

Question 8

Why do we add a acid to the solution?

Answer 8

•The addition of acid will hydrolyse any glycosidic bonds present in carbohydrate molecules
• the resulting monosaccharides left will have an aldehyde or ketone function group that can donate electrons to copper ( ii ) sulfate allowing a precipitate to form

Question 9

Describe the iodine test for starch

Answer 9

1. Add a few drops of orange/brown iodine in potassium iodide solution to the sample
2. The iodine is in potassium iodide solution as iodine is insoluble in water
3. If starch is present - iodide ions in the solution will interact with the center of starch molecules producing a blue- black colour
4. Color change from orange ——> blue/black

Question 10

State the role and chemical symbol of hydroxide and phosphate ions

Answer 10

•OH ions affect pH and can interact with bonds in 3° protein structure to cause denaturation
• PO4^ (3-) is a component of ATP/ADP for energy and release and NADP

Question 11

Are lipids polar or non polar molecules?

Answer 11

• Non polar molecules
• do not dissolve in water but will dissolve in organic solvents such as ethanol

Question 12

Describe the emulsion test for lipids

Answer 12

1. Add ethanol to the sample to be tested
2. Shake to mix
3. Add the mixture to a test tube of water
4. If lipids are present a white cloudy emulsion will form

Question 13

What’s a limitation of the lipids emulsion test?

Answer 13

Test is qualitative - does not give a quantitive value as to how much lipid may be present in the sample

Question 14

Describe the test for proteins

Answer 14

1. Place 3cm cubed of the food solution into a test tube
2. Add 3cm cubed of dilute sodium hydroxide solution and mix
3. Add 10 drops of dilute copper (ii) sulfate solution and mix again
4. Colour change of positive should go from blue —> purple/ lilac

Question 15

How can the concentration of a solution be measured quantitatively?

Answer 15

• use colorimetry to measure absorbance / % transmission
• use biosensors - detects the presence of a chemical - a transducer converts the response into a detectable electrical signal

Question 16

What is a colorimeter ?

Answer 16

A device that is able to mess the light absorbance or light transmission
The more concentrated a solution the more light it will absorb and the less light it will transmit

Question 17

How do you place the curvettes in ?

Answer 17

• curvettes have 2 clear transparent sides and 2 translucent sides
• place the corvettes into the colorimeter so light passes through the transparent sides