Concept:The boiling point is rigorously defined by thermodynamics as a state of mechanical equilibrium between the internal push of the liquid's vapor and the external push of the atmosphere.
Formula:$$ P_{\text{vapor}} = P_{\text{atm}} $$
Solution:- When a liquid is heated, its molecules gain kinetic energy, and its vapor pressure steadily rises.
- Evaporation occurs only at the surface. However, for a liquid to boil, massive vapor bubbles must form within the bulk interior of the liquid and rise to the top.
- These bubbles can only form and survive without collapsing when the internal outward pressure pushing them open (the vapor pressure) matches or exceeds the downward crushing force of the atmosphere (the atmospheric pressure).
- Thus, boiling strictly occurs at the exact temperature where Vapour pressure = Atmospheric pressure.
Why other options are incorrect:Surface tension and viscosity are fluid dynamic properties, not pressures. Equating them to atmospheric pressure is dimensionally and physically nonsensical.
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