Boiling occurs when the vapor pressure reaches or exceeds the surrounding pressure from the atmosphere or whatever else is in contact with the liquid. At standard atmospheric pressure (1 atmosphere = 0.101325 MPa), water boils at approximately 100 degrees Celsius.

Understanding Boiling Point
The boiling point of a liquid is a fundamental concept in the study of thermodynamics and physical chemistry. It is the temperature at which a liquid transitions to a gas when the vapor pressure of the liquid equals the atmospheric pressure surrounding it.
Key Points about Boiling Point:
- Definition: The boiling point is the temperature at which a liquid's vapor pressure is equal to the pressure of the surrounding atmosphere. At this point, bubbles of vapor form within the liquid and rise to the surface.
- Atmospheric Pressure: At standard atmospheric pressure (1 atm), the boiling point of water is 100°C (212°F). However, this temperature can change with variations in atmospheric pressure.
- Phase Transition: Boiling is a type of phase transition, where a liquid becomes a gas. This involves the absorption of energy, specifically in the form of latent heat of vaporization.
- Factors Affecting Boiling Point:
- Pressure: Higher atmospheric pressure raises the boiling point, while lower pressure decreases it. For example, water boils at lower temperatures at high altitudes because of reduced atmospheric pressure.
- Impurities: The presence of dissolved substances can also alter the boiling point, a phenomenon known as boiling point elevation.
Conclusion
Understanding the boiling point is crucial in various scientific and industrial processes, including cooking, distillation, and chemical reactions. Therefore, the correct answer to the question is option 'B' — the boiling point, as it accurately describes the temperature at which a liquid starts to boil under atmospheric pressure.