Question

Glucose on reacting with benedict’s solution may give the following precipitates except:
A) Violet
B) Orange-red
C) Brick red
D) Green-yellow

Answer 1

Benedict’s and Fehling’s solutions are two commonly used reagents to test for the presence of reducing sugars. Most commonly this is to test for the presence of glucose in a given solution. This reagent is made from sodium citrate, copper sulphate and sodium carbonate.

Complete answer:

Reducing sugars have either a free aldehyde or a free keto group and can therefore act as good reducing agents. This includes all of the monosaccharides as well as some disaccharides and oligosaccharides. Glucose is a monosaccharide with a free aldehyde group.
Benedict’s reagent is a bluish solution. On heating with glucose it turns a range of colours, particularly yellow, orange, and deep red. The stronger the shade or orange-red, the more positive the result.
With Benedict’s glucose does not give a violet colour. Option A is the correct answer.
Solutions with reasonably high concentrations of glucose will give an orange red colour. Option B is incorrect.
Solutions with very concentrated glucose, particularly a test solution of only glucose, will go brick red. Option C is incorrect.
Occasionally a yellowish tinge may show in a green solution which is very mildly positive. Option D is incorrect.

Hence the correct answer is option ‘C’.

Note: Benedict’s solution cannot be used to test for starch as starch is not a reducing sugar, but a long chain homopolysaccharide of glucose units. If starch is treated with a mild acid solution, it will undergo hydrolysis releasing glucose molecules and disaccharides amongst other break-down products. These will then react with Benedict’s reagent to give a positive result.