During a lab we mixed Benedict's reagent with glucose and after boiling it, it turned lime green. Why?

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sciftw | High School Teacher | (Level 1) Educator Emeritus

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Benedict's reagent is solution of copper sulfate, sodium carbonate, and sodium citrate.  It is used as an indicator to test for the presence of sugars.  It works the same way that phenolthalein works with acids and bases.  When it is mixed into a solution it will be pink or red without an acid present.  As an acid is introduced, it will turn clear.  Benedict's reagent works the same way.  At the start of a test, it is a bluish color.  Food is mashed up or ground up and placed in the solution.  If no sugars are present, the Benedict's reagent remains the same color.  If a color change occurs that means some form of sugar (glucose) is present.  The amount of color change indicates the amount of sugars present.  It will turn from blue, to green, to yellow, to orange, to pinkish red.  Since you know that you put glucose in your heated solution, a color change is expected.  Since the color change was to green, not much sugar was added.  

The only downside to Benedict's solution is that it won't change color in the presence of sucrose.  In that case, you would need to boil the sucrose with a bit of dilute hydrochloric acid to split the sucrose into glucose and fructose (which Benedict's reagent does indicate).  

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