Question

what is the purpose of boiling the leaf in water in the experiment demonstrating chlorophyll is essential for photosynthesis?

Answer 1

It's purpose to prove that chlorophyll is essential for green leaves to prepare starch as food in plants.
Hope it helps u dear

Answer 2

The purpose of boiling leaf in the water is to break down the cell membrane, soften the cuticle layer and cells walls.  

Boiling makes it easy to extract chlorophyll from the leaves which proves its presence in leaves and also justify the process of photosynthesis requires chlorophyll to occur in the presence of sunlight.