Question

Why boiling is called a bulk phenomenon?

Answer 1

because

evaporation does not starts in boiling till all the heat is not spread in the substance boiling

when the temperature is even and is at the boiling point

then it starts evaporating or boiling

Answer 2

Boiling is a process of simultaneous mass transfer and heat transfer taking place and the meaning of bulk is ENTIRE VOLUME OR ENTIRE MASS , the boiling is applicable to only liquid content , so when entire volume /mass of the given liquid / solution is allowed to boil at corresponding pressure and temperature it starts boiling and changes its phase from liquid to its vapor , thus boiling as such is a bulk phenomena , like distillation process by boiling is a bulk phenomena .
Some process of boiling is not clubbed under bulk phenomena like evaporative cooling of cooling tower water where part of water surface in contact with air transfers its heat content to air and the water -air film is getting evaporated is escapes from the water liquid and generates cooling water , the drying of wet solid takes place, i t initiates with surface drying which is in direct contact with heating media like hot air and surface drying is very fast , the bulk solvent/ liquid retained in the solid mass now starts vaporization , since it is in bulk and therefore time cycle for bulk phenomena of drying is slow