Introduction:

Boiling is a process of converting a liquid into vapor by heating it to its boiling point. It is considered a bulk phenomenon because it involves the entire liquid undergoing a phase change simultaneously. Unlike evaporation, which occurs at the surface of a liquid, boiling occurs throughout the entire volume of the liquid.

Explanation:

1. Definition of boiling:
Boiling is the rapid vaporization of a liquid when it is heated to a temperature where its vapor pressure exceeds the atmospheric pressure. At this point, bubbles of vapor are formed within the liquid, causing it to vigorously boil.

2. Heat transfer:
During boiling, heat is transferred from the heat source to the liquid through conduction. The heat causes the molecules in the liquid to gain energy and move more rapidly. As the liquid reaches its boiling point, the increased molecular motion overcomes the intermolecular forces holding the liquid together, and bubbles of vapor form throughout the liquid.

3. Nucleation and bubble formation:
The process of bubble formation during boiling involves two stages: nucleation and bubble growth. Nucleation is the initial formation of tiny vapor bubbles within the liquid when it reaches its boiling point. These tiny bubbles, called nucleation sites, can form on impurities or irregularities on the container's surface or from dissolved gases in the liquid.

4. Bubble growth and release:
Once nucleation occurs, the bubbles grow in size as more heat is transferred to the liquid. The growth of bubbles is facilitated by the continuous supply of heat from the heat source. Eventually, the bubbles become large enough to overcome the surrounding liquid's cohesive forces and rise to the surface, releasing the vapor into the atmosphere.

5. Bulk phenomenon:
Boiling is considered a bulk phenomenon because it involves the entire volume of the liquid undergoing a phase change simultaneously. The formation and growth of bubbles occur throughout the liquid, allowing for efficient heat transfer from the heat source. This is in contrast to evaporation, which occurs only at the liquid's surface, where molecules with sufficient energy escape into the gas phase.

Conclusion:

In summary, boiling is called a bulk phenomenon because it involves the entire liquid undergoing a phase change simultaneously. The heat transfer, nucleation, bubble growth, and release occur throughout the liquid, allowing for efficient vaporization. Understanding the concept of boiling helps in various practical applications, such as cooking, distillation, and power generation.