In test tube 2 when Benedict’s reagent is added the colour is blue also. It does not reduce the copper sulphate as the bonds between the fructose and glucose have not been broken and therefore sucrose remains to be sucrose and therefore not a reducing sugar.
Test tube 3 (Starch) is also initially blue. It does not change colour in neither tests, this proves that starch is neither a reducing or non-reducing sugar, as it cannot reduce copper sulphate to copper oxide.
Test tube 4 (Glucose) starts with blue and finishes with brown. This happens because it undergoes the same process with the Benedict’s solution.
Test tube 5 (Sucrose) starts with blue and ends with brown. This shows that the Hydrochloric Acid is required to break the bonds between fructose and glucose, and we know that glucose is a reducing sugar therefore when Benedict’s reagent is added it changes to brown, due to copper sulphate being broken down to copper oxide by the glucose. This shows that sucrose is non-reducing because it is needed to be broken down before it becomes a reducing sugar.
B) Test for Starch
The Iodine colour is red/brown. When it is added to starch is changes to blue black because this shows that amylase is present. Starch amylopectin does not give the colour. Amylose in starch is responsible for the formation of the deep blue colour in the presence of iodine. The iodine molecules slip inside the amylose coil. So when starch is heated the hydrogen bonds break between the glucose molecule and the iodine escapes resulting to a colourless solution. When the solution cools down hydrogen bonds are reformed and the deep blue black colour is present due to the presence of iodine in the amylose coil.
2) What is the purpose of adding NAHCO3 or NaOH after boiling non-reducing sugar acid?
The purpose of NaHCO3 is to neutralise the acid. We add it in excess so that the solution is slightly alkaline before the Benedict’s solution is added. It’s added after boiling the non-reducing sugar acid because if added when solution it will overflow with effervescence resulting to solution shooting out of the test tube vigorously (Health and safety reasons)
3) How would you detect sucrose in a mixture of glucose and sucrose?
First carry out the test for reducing sugar. If this is negative then test for non reducing sugar. If the test is positive then ensure that you have added excess Benedict’s reagent, that is ensure that all the reducing sugar has reacted. Then filter out the contents of the tube, and save the filtrate, if there is any non-reducing sugar present this is where it will be. Test the filtrate for the non reducing sugar, which in this case is sucrose.
4) How would you modify the reducing sugar test to make it -
a) Quantitative
b) Semi-Quantitative
- You would modify the reducing sugar test to make it quantitative by carry out the reducing sugar test. Then this would form a precipitate. Weigh the filter paper, then place solutions through the filter paper, weigh contents of filter paper with precipitate and then subtract the filter paper from the contents with filter paper, to get mass of precipitate.
- You would modify the reducing sugar test to make it semi-quantitative by adding Benedict’s solution and observing which one is the most brownest will tell you that there is most reducing sugar etc. Or by adding Benedict’s solution and then timing the reaction and seeing which one turns brownest the quickest. However the first one is not as accurate as colour is subjective, therefore more accurate method would be to time it.
Riaz Rampuri