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Tests for carbohydrates practical

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Results Non - Quantitative Test for reducing sugars (1, 2, and 3) Test for Non-reducing sugars (4, 5, and 6) Test Tube Number Sugar Initial Colour Final Colour 1 Glucose Blue Brown/Red 2 Sucrose Blue Blue 3 Starch Blue Blue 4 Glucose Blue Brown 5 Sucrose Blue Brown 6 Starch Blue Blue Test for Starch Iodine colour = Red/Brown Iodine added to solution = Blue/Black The when boiling goes = Colourless Then when cooled = Blue/Black Questions 1) Explain fully of all the tests? a) Non - Quantitative Test for reducing sugars All three test tubes initial colour is blue. This blue comes from the Benedict's solution as it is made up of Copper Sulphate (which gives it the blue colour) and Sodium Hydroxide (used to make the solution alkali) which is colourless. When the Benedict's reagent is added to test tube 1 (glucose) ...read more.


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. ...read more.


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 A) 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. B) 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 ...read more.

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    A positive result produces a nice brick orange precipitate. (see fig.2) (www.chemed.chem.wisc.edu - Nov.2007) (fig.2 - Benedict's test positive result for reducing sugars - note Sucrose is a non-reducing sugar hence the blue hue) c) Seliwanoff's test for Fructose - this reagent consists of resorcinol with concentrated hydrochloric acid.

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