User:Anthony.Sebastian/Boiling point
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The ‘boiling point' of a liquid refers to the temperature at which bubbles of its vapor (gas) form within the body of the liquid. The bubbles form within the liquid at that temperature because the energy of motion of some collections of molecules has reached a level that the pressure they exert on the molecules surrounding them balances the total external pressure exerted upon the liquid and transmitted within it. At lower temperatures, the correspondingly lower 'kinetic' energies of any collections of the liquid's molecules cannot develop a pressure sufficiently great to counter the total external pressure transmitted through the liquid, and thus bubbles cannot form. At the boiling point temperature, the internal vaporizations produce bubbles with a pressure that balances the liquid pressure tending to crush them, and a pressure that reflects the equilibrium of rates of vaporization and condensations of molecules across the spherical liquid-vapor interface making up the boundary of the bubble as it floats upward, giving it a limited lifetime, in the end with continued heat input all the liquid becomes vapor.
It follows that no fixed value of boiling-point temperature exists for any given liquid, since it would differ depending on the total external pressure, increasing as a continuous positive function of the total external pressure exerted on the liquid.
For example, water in an open container exposed to sea-level atmospheric pressure of 1 atm (760 torr; 101.3 kPa) boils at 100 oC (373.15 K; 212 oF). At altitude (e.g., a mountaintop), where atmospheric pressure is lower than 1 atm, water boils at a temperature less than 100 oC, because with a lower pressure exerted on the water, and transmitted through its bulk, water molecules can achieve the necessary heat-induced kinetic energy to escape into aggregates as bubbles of vapor at a lower temperature. Conversely, at locations below sea level, where atmospheric pressure is greater, the boiling point of water is higher.
For a given total external pressure, the boiling-point temperature also varies depending on the nature of the liquid, inasmuch as different types of molecules have different forces, so-called intermolecular forces, attracting them to each other, thereby influencing the temperature effect on their kinetic energy. A liquid with loosely bound molecules boils at a temperature lower than one with more tightly bound molecules.
Formally, the boiling point of a liquid is expressed as the temperature at which the vapor pressure of the liquid equals the external environmental pressure surrounding the liquid.