Testing for carbohydrates - 3.4 - (1)

Question 1

In chemistry, what is reduction?

Answer 1

it is a reaction involving the gain of electrons

Question 2

What are reducing sugars?

Answer 2

all monosaccharides (glucose, galactose, fructose) and some disaccharides (maltose and lactose)

Question 3

What is meant by the term reducing sugars?

Answer 3

this means that they can give electrons, or reduce another chemical or molecule (e.g. copper ions)

Question 4

In the chemical test for a reducing sugar, what is the chemical called?

Answer 4

benedict’s reagent - an alkaline solution of copper(II)sulfate

Question 5

How is the chemical test for a reducing sugar carried out?

Answer 5

1. Place the sample to be tested into the test tube
2. You add the benedict’s reagent to the sample in the test tube
3. You place the test tube into the boiling water bath and allow it to heat for 3-5 minutes

Question 6

What will reducing sugars react with?

Answer 6

the copper ions in benedict’s reagent

Question 7

What does the reaction with the copper ions result in?

Answer 7

the addition of electrons to the blue Cu2 ions, which reduces them to brick red Cu+ ions

Question 8

When a reducing sugar is mixed with benedict’s reagent, and warmed, what happens?

Answer 8

a brick-red percipitate is formed which indicates a positive result

Question 9

The more reducing sugars that are present, the more of what is formed?

Answer 9

percipitate and the less blue Cu2+ ions are left in solution

Question 10

What will the actual colour seen be a mixture of?

Answer 10

brick-red (percipitate) and blue (unchanged copper ions) - this will depend on the concentration of the reducing sugars present

Question 11

What does this make the test?

Answer 11

qualitative

Question 12

What does not react with benedict’s solution?

Answer 12

non-reducing sugars

Question 13

How will the solution be after warming for non-reducing sugars?

Answer 13

it will remain blue and this indicates a negative result

Question 14

What is sucrose?

Answer 14

the most common non-reducing sugar

Question 15

If sucrose is first boiled with dilute hydrochloric acid, what will it then give?

Answer 15

a positive result when warmed with benedict’s solution

Question 16

Why will it give a positive result when warmed with benedict’s solution?

Answer 16

as the sucrose has been hydrolised by the acid to glucose and fructose - which are both reducing sugars

Question 17

Why is a reducing sugar a reducing sugar?

Answer 17

because it causes the copper ions which are present in benedict’s reagent to be reduced

Question 18

What could you also see?

Answer 18

different colours depending on the concentration - green ,yellow, orange, or red