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Chemical Differences Among Carbohydrates

1) How can you distinguish chemically between starch and three other carbohydrates (sucrose, glucose, & lactose)?

2) What would you expect to observe when adding one drop of iodine-potassium iodine reagent to each of the four carbohydrates?

3) How does Benedict's Reagent distinguish between carbohydrates?

4) When adding 5 drops of Benedict's reagent to 2 mL each of the above four carbohydrates and setting the test tubes in a hot water bath for 1-2 minutes, would you expect to see any color change and if so, what color and in which ones?

Solution Preview

Chemical Differences Among Carbohydrates
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1) How can you distinguish chemically between starch and three other carbohydrates (sucrose, glucose, & lactose)?

Response: Starch is stainable with the iodine-KI reagent while the mono & disaccharides are not. See below for how they might stain with Benedict's reagent.

2) What would you expect to observe when adding one drop of iodine-potassium iodine reagent to each of the four carbohydrates?

Response: The starch would stain purple (or black/blue) colour, while the other three solutions would remain uncoloured. Well, actually, since you added a little bit of iodine solution to the sugar solutions, the sugar solutions would have a slight yellow colour due to the iodine, but that's essentially irrelevant.

3) How does Benedict's Reagent distinguish between carbohydrates?

Response: Benedict's reagent reacts with reducing sugars. These are sugars that have the anomeric carbon (the carbon that exist in both alpha and beta positions when the ring closes) exposed and free so that it ...

Solution Summary

This solution contains a process that distinguishes between starch and other carbohydrates, effects and describes the effects of the iodine-potassium iodine reagent, Benedict's reagent and any color changes.

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