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Determine which of the three sugars tested (Glucose, Fructose and Sucrose) is a reducing sugar by heating them with Benedict's solution.

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Introduction

QUALITATIVE EXPERIMENT [REDUCING SUGARS] AIM: To determine which of the three sugars tested (Glucose, Fructose and Sucrose) is a reducing sugar by heating them with Benedict's solution. BACKGROUND INFORMATION: A carbohydrate is an organic compound that is composed of atoms of Carbon, Hydrogen and Oxygen in the ratio 1:2:1. Carbohydrates used in the experiment can be divided into two groups: > Monosaccharides: All monosaccharides have the general formula (CH2O) n; where 'n' can be any number between 3 and 9. All monosaccharides also contain a carbonyl (C=O) group and at least two hydroxyl (OH) groups. The monosaccharides used in this experiment are called Hexose, which have six carbon sugars. All the hexoses have the general formula C6H12O6. They can exist as straight chains or rings. Two common hexoses are glucose and fructose (see figure below). Hexoses are sources of energy in respiration and are the monomers, which link together to form disaccharides and polysaccharides. ...read more.

Middle

1. Set up the apparatus as shown above in the diagram. 2. Fill the beaker with water. 3. Add 2cm3 of a sugar to the boiling tube i.e. Glucose, Fructose or Sucrose (Label the boiling tubes with the sugar it contains). 4. Add 2cm3 of Benedict's solution to each of the test tubes. 5. Place the three boiling tubes in the water bath and heat it. 6. Continue heating until the glucose and fructose solutions change colour. > Results: Name of Sugar Original Colour Colour Change Final Colour Glucose Blue Green Brick-red Fructose Blue Turquoise Orange-red Precipitate Sucrose Blue No change Blue 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) ...read more.

Conclusion

4. Add 1 cm3 of dilute hydrochloric acid to the test tube. 5. Place the boiling tube in the water bath and heat it. 6. Heat the solution for 3 minutes. 7. Let the solution cool. 8. Add 1 cm3 of Benedict's reagent to the above solution. 9. Place the test tube in the water bath. 10. 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. ...read more.

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