Question

Benedict solution provides:
 A.\[{\text{A}}{{\text{g}}^ + }\]
 B.\[{\text{L}}{{\text{i}}^ + }\]
 C.\[{\text{C}}{{\text{u}}^{ + 2}}\]
 D.\[{\text{B}}{{\text{a}}^{ + 2}}\]

Answer 1

Hint: The benedict solution or reagent is a mixture. It contains sodium carbonate, sodium citrate and copper (II) sulfate pentahydrate. The reagent reacts with the reducing sugar to give a color range from blue to brick red along with a precipitate.

Complete answer:
The benedict's test is used to detect the presence of reducing sugars. The reducing sugars include all the carbohydrates containing a free aldehyde, ketone or hemiacetals, which will be able to reduce the various reagents. These reagents include Fehling’s, tollens and the benedict's reagent.
 In this mixture of the benedict’s solution, the role of the sodium carbonate \[({\text{N}}{{\text{a}}_2}{\text{C}}{{\text{O}}_3})\] is to provide an alkaline medium or in other words to provide a medium for the reaction to take place. The role of copper sulfate \[({\text{CuS}}{{\text{O}}_4}.{\text{5}}{{\text{H}}_2}{\text{O)}}\] is to give the cupric ions and the role of sodium citrate \[{\text{(N}}{{\text{a}}_3}{{\text{C}}_6}{{\text{H}}_5}{{\text{O}}_7})\] is to provide the precipitation of the cupric ions. The cupric ions \[({\text{C}}{{\text{u}}^{ + 2}})\] will get reduced to \[({\text{C}}{{\text{u}}^ + })\] . So, by the addition of sodium citrate the complex formation takes place with cuprous ions and thus acting as a chelating agent while the solution is stored.
Thus, it is clear from the mixture of the solutions that \[({\text{C}}{{\text{u}}^{ + 2}})\] is the one which is provided to undergo the reduction.
And hence the correct answer is option C.

Additional information: During the test the reagent is added to the sugar solution. The sugar due to the alkaline condition forms enediols which are strong reducing agents. Thus \[({\text{C}}{{\text{u}}^{ + 2}})\] gets reduced to \[({\text{C}}{{\text{u}}^ + })\] and gives a red precipitate on the reaction with the free aldehyde, ketose or hemiacetal. This red precipitate \[{\text{C}}{{\text{u}}_2}{\text{O}}\] is red copper oxide which is insoluble in water, resulting in the confirmation of the reducing agent.

Note:
 First write the contents of the solution. Understand the role of various chemicals. Re-check this with the options you are provided with. If you have two possible answers, then try to analyze the role of the solution.