Question

If a dried cobalt chloride paper is clipped on the undersurface of a leaf its colour changes from blue to pink because 
(a) It reacts with the chlorophyll of the leaf.
(b) The clipper puts pressure on the paper.
(c) Paper is moistened by the transpiring water.
(d) It comes in contact with green leaves.

Answer 1

Hint: We know that the colour of the cobalt chloride changes under certain conditions like the presence of water, heat etc. We also know that the leaves bear very minute, small openings on the surface known as stomata, which has various functions.

Complete Step by Step answer:
When dried cobalt chloride paper is clipped on the under surface, the colour changes from blue to pink. The surface of the leaf contains small, tiny microscopic openings called the stomata. These openings have various functions like the exchange of gases, and regulation of water movement which is known as transpiration. As the water transpires through the stomata comes on the surface of the underside of the leaf, the dried cobalt chloride is moistened by the water droplets.

Additional information: The transpiration is more on the lower side of the leaf. Thus the change in colour is also rapid on the strip of cobalt nitrate on the under surface. The transpiration is high in the underside of the leaf due to the presence of a high number of stomata on the underside surface of the leaf. The colour change happens because the dried paper gets moistened by the water droplets released during transpiration. We know that the cobalt chloride turns from blue to pink under moist conditions. The change in colour indicates that water is released on the undersurface of the leaf, which turns the paper moist.
So, the correct answer is,' paper is moistened by transpiring water.'

Note: Cobalt chloride turns pink under moist conditions and is one of the evidence to prove transpiration. The colour change proves that the transpiration occurs through the stomata of the plants.