Question

1.Water was used as a negative control. If you had observed the water change in color from clear to red after adding Benedict's solution and then heating to 100 ^oC, what could you conclude?

A.The water was contaminated with glucose.

B.The Benedict's solution was contaminated with glucose.

C.The Benedict's solution was contaminated with sucrose.

D.The water was contaminated with sucrose.

2. Glucose was used as a positive control . If you had NOT observed the glucose solution change in color to red after adding Benedict's solution and then heating to 100 ^oC, what could explain this result? Select ALL correct answers.

A.The concentration of glucose was too high.

B.The Benedict's solution was no longer working as intended.

C.The temperature probe was broken and the water bath was actually 50 ^oC

D.The glucose solution had been contaminated with ribose.

Answer 1

Ans 1: Benedict's test is used as a simple test for reducing sugars. A reducing sugar is a carbohydrate possessing either a free aldehyde or free ketone functional group as part of its molecular structure. These includes all the monosaccharides and some disaccharides including maltose and lactose. Originally Benedict's reagent is of blue color and on reaction with reducing sugar it's color changes to green,yellow and brick red according to the concentration of Glucose.

Now in the question we are using water as a negative control and we are observing the water change in color from clear to red after adding Benedict's solution and then heating to 100^oC, so from this we can say that water is contaminated with glucose and hence we are observing the shift in water color.

Hence option A is correct.

2: In Benedict's test  a greenish precipitate indicates about 0.5 g% concentration; yellow precipitate indicates 1 g% concentration; orange indicates 1.5 g% concentration; and red indicates 2 g% or higher concentration. So this means that whatever is the concentration the glucose is still going to reduce by the Cu⁺² ions present in the Benedict's reagent.

Generally, Benedict's test detects the presence of aldehydes, alpha-hydroxy-ketones, and hemiacetals, including those that occur in certain ketoses. So this means that Benedict's reagent shows positive result for Ribose, so if Glucose is contaminated with Ribose sugar the tets will still showing positive response.

Benedict's test operates by reducing sugars that are heated in the presence of an alkali into powerful reducing species known as enediols. The newly formed enediols proceed to reduce cupric compounds (Cu²⁺) present in the Benedict's reagent to cuprous compounds (Cu⁺) which are precipitated as insoluble red copper(I) oxide (Cu₂O). So this means that 100^oC is essential for the test to be working.

Hence from all these we can conclude that in only two cases Benedict's test will not give positive result even in the presence of reducing sugar and these are: when the temperature probe is broken and the water bath is actually 50 ^oC, and when the Benedict's solution is no longer working as intended.

Hence option B and C are the conditions responsible for the negative result even in the presence of Glucose.