- Small molecules
- Low molecular mass
- Sweet tasting
- Crystalline
- Readily soluble in water
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Disaccharides: When two monosaccharide molecules undergo a condensation reaction (A reaction between two molecules which involves the formation of a bond between two subunits and a water molecule is released), a disaccharide molecule is formed and a water molecule is removed. The bond between the two monosaccharide subunits is known as a glycosidic bond.
The general formula for a disaccharide molecule is 2[(CH2O) n] – H2O. The properties of disaccharides are:
- Small molecules
- Low molecular mass
- Sweet tasting
- Crystalline
- Soluble in water, but less readily soluble in water.
An example of a disaccharide molecule is sucrose. Sucrose is formed by a condensation reaction between glucose and fructose (see below).
PREDICTION:
The glucose and fructose (monosaccharides) are reducing sugars and will react with Benedict’s solution and form insoluble copper (II) oxide (Colour Change: Blue to a Red-brown precipitate). The sucrose (disaccharides) is a non-reducing sugar and will not react with the Benedict’s solution and therefore no colour change will take place.
METHOD:
- 3 boiling tubes
- 250 ml beaker (for water bath)
- Bunsen Burner
- Tile
- Stand
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2cm3 of Glucose
-
2cm3 of Fructose
-
2cm3 of Sucrose
- Benedict’s Solution
- Fair Test: Use equal amounts of each sugar and Benedict’s solution. Also place all three test tubes in the same water bath to keep conditions constant.
- Safety: Benedict’s solution is highly alkaline. Avoid contact with skin. Also wear eye protection while using the Bunsen burner.
- Set up the apparatus as shown above in the diagram.
- Fill the beaker with water.
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Add 2cm3 of a sugar to the boiling tube i.e. Glucose, Fructose or Sucrose (Label the boiling tubes with the sugar it contains).
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Add 2cm3 of Benedict’s solution to each of the test tubes.
- Place the three boiling tubes in the water bath and heat it.
- Continue heating until the glucose and fructose solutions change colour.
CONCLUSION:
The above results prove that my prediction was correct. Glucose and fructose change colour to prove that they are reducing sugars and sucrose does not change colour proving that it is a non-reducing sugar.
Glucose is able to reduce the copper (II) [Cu++] in the copper sulphate to copper (I) [Cu+] in copper oxide because the aldehyde groups are oxidized to carboxyl (COOH) groups. Fructose on the other hand does not have a aldehyde group, so the carbonyl group on the carbon-2 changes place with the hydroxyl group on carbon-1, so they can also reduce the copper (II) to copper (I) when heated with Benedict’s solution.
Some disaccharides have an aldehyde group so a colour change occurs, but sucrose does not have an aldehyde group, thus no colour change takes place and it is described as a non-reducing sugar.
EVALUATION:
Although the results were positive the experiment can be bettered:
-
A fixed amount of water should have been used , preferably 50cm3 because water has a high specific heat capacity i.e. it take a large amount of energy to raise the temperature of 1 kg of water by 10 C. Thus less water in the water bath would save time.
- Also a fixed temperature should have been agreed upon for the water bath so that the result would be more accurate or a thermometer should have been placed to measure at what temperature the colour change occurs.
- A control should have been used so that it is certain that the colour change was due to the Benedict’s solution and not any other factor.
Even though sucrose is a non-reducing sugar it is possible for it to reduce copper (II) to copper (I) when heated with Benedict’s solution. The method for this experiment is shown below:
- Set up the apparatus (diagram)
- Fill the beaker with water.
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Add 2cm3 of sucrose to the boiling tube.
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Add 1 cm3 of dilute hydrochloric acid to the test tube.
- Place the boiling tube in the water bath and heat it.
- Heat the solution for 3 minutes.
- Let the solution cool.
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Add 1 cm3 of Benedict’s reagent to the above solution.
- Place the test tube in the water bath.
- Heat the solution until it changes colour.
Result/Analysis:
A brown-red precipitate occurs. This happens because sucrose has been hydrolysed (this reaction involves the breaking of a bond, between the two subunits of a large molecule, by the addition of H (hydrogen) and OH (hydroxide) from a water molecule) into its constituent monosaccharides: glucose and fructose.
(Although sucrose has reduced copper (II) to copper (I) that does not make it a reducing sugar because it was actually the glucose and fructose that make up sucrose that reduced copper (II) to copper (I).)
All the results obtained in the experiment are positive thus the experiment has been successful and the aim has been achieved.