Question

When 2-3 drops of Benedict's agents are added to a urine sample and heated gently, it turns yellow. This color change indicates that
A) Urine contains 2% glucose
B) Urine contains 0.5% glucose
C) Urine contains 1.5% glucose
D) Urine contains 1% glucose

Answer 1

Hint: If any carbohydrate has a free functional group, then that sugar will test positive but if any such group is absent , the test will be negative. Positive meaning colour changes from blue and negative meaning it remains blue (as Benedict's reagent is blue in color).

Complete Answer:
- Sugars (Carbohydrates) are of two types : Reducing and Non-reducing. Reducing sugars are those that show presence of free aldehydic or ketonic groups whereas non-reducing do not have free groups.
- All Monosaccharides are thus Reducing sugars.
- Disaccharides can be Reducing or Non reducing.
- Disaccharides contain two sugar groups (monosaccharides) that are connected by glycosidic bonds. This bond formation is a condensation or hydrolysis reaction which means a water molecule is released during bond formation.
Examples : Sucrose, Lactose, Maltose, etc.

- The glycosidic bond in Sucrose is 1,2-glycosidic bond and the monosaccharides that form the bond are Glucose and Fructose. It means the first carbon of Glucose combines with the second carbon of Fructose to form the bond.
- Now the respective carbons of these monosaccharides show presence of functional groups too. (Aldehydic group in glucose and Ketonic group in fructose).
- Thus when sucrose is formed, the molecule does not contain any free functional groups and thus Sucrose is non reducing sugar.
- Lactose and Maltose show free groups after bond formation also and thus are Reducing sugars.
- Many tests can detect this property of carbohydrates. Benedict's test is the famous test for this.

Benedict’s Test:
This test detects the presence of reducing sugars.
Procedure: Add the reagent to the sample of sugar to be tested and wait for 4-10 minutes. Colour change will detect whether sugar is reducing or not.

Mechanism: The Cupric ions of the reagent produce cuprous ions if the sugar is of reducing type. Thus reduction takes place.

The table to show whether the given sample has glucose or not and if present what is the percentage of glucose in it.

Sl.NO	Color Change	Inference
1.	Blue	Reducing sugar is absent
2.	Green	0.1 to 0.5% glucose
3.	Yellow	0.5 to 1% glucose
4.	Orange	1 to 1.5% glucose
5.	Red	1.5 to 2% glucose
6.	Brick Red	More than 2%

Hence answer is Option D.

Note: Some other urine tests:
- A bilirubin in a urine test is only one measure of liver function.
- Urine Test called ACR. ACR stands for “albumin-to-creatinine ratio.” Three positive results over three months or more is a sign of kidney disease.