Question

The leaf that has photosynthesised starch, shows a colour change, when boiled in ethanol and then treated with iodine solution. What is the colour change?

• A. Green leaf⇢Red colouration
• B. Green leaf⇢Blue colouration
• C. Green leaf⇢Black colouration
• D. Green leaf⇢Blue-black colouration

Answer 1

The correct option is D

Green leaf⇢Blue-black colouration

The correct option is (D)

Explanation for the correct option

1. Starch is a complex polysaccharide formed by the polymerization of glucose units.
2. To do the starch test in the plant leaf, the performer should first take a leaf, then boil the leaf in alcohol using a water bath, followed by washing and adding iodine solution.
3. In the presence of iodine solution, the color of the leaf with starch changes from green to blue-black depending upon the starch concentration.

Explanation for incorrect options

Option (A)

1. The iodine solution used is used to detect the presence of starch.
2. The color of the iodine solution is red.
3. When a leaf does not contain starch, then the color of the iodine solution will not change, and the solution remains red.

Option (B)

1. The starch test is a method to detect the presence of starch using an iodine solution.
2. If the starch amount is less in a green leaf, adding iodine will change the color from green to blue.

Option (C)

1. The starch test is a method to detect the presence of starch using an iodine solution.
2. If the starch amount is high in a green leaf, adding iodine will change the color from green to black.

The color will change to blue-black when a leaf having photosynthesised starch is boiled in ethanol and then treated with iodine solution.