The point at which heat added to a liquid is no longer used to increase temperature but instead to form gas from the liquid. Formally, the boiling point temperature is reached when a liquid's vapour pressure equals external pressure. Boiling points thus decrease with altitude. Water may be boiled at room temperature by decreasing the pressure around it.
This article is about the boiling point of liquids. For other uses, see Boiling point (disambiguation).The boiling point of a substance is the temperature at which it can change its state from a liquid to a gas throughout the bulk of the liquid at a given pressure. A liquid may change to a gas at temperatures below the boiling point through the process of evaporation. Any change of state from a liquid to a gas at boiling point is considered vaporization. Boiling on the other hand is a bulk process, so at the boiling point molecules anywhere in the liquid may be vaporized, resulting in the formation of vapor bubbles.
A somewhat clearer (and perhaps more useful) definition of boiling point is "the temperature at which the vapor pressure of the liquid equals the pressure of the surroundings." Hot liquid will boil as it rises through the bulk liquid if the pressure of the environment drops to the vapor pressure of the liquid at its temperature.
Saturation temperature and pressure
A saturated liquid or saturated vapor contains as much thermal energy as it can without boiling or condensing. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase.
If the pressure in a system remains constant (isobaric), a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy (heat) is removed. Similarly, a liquid at saturation temperature and pressure will boil into its vapor phase as additional thermal energy is applied.
The boiling point corresponds to the temperature at which the vapor pressure of the substance equals the ambient pressure. The boiling point increases with increased ambient pressure up to the critical point, where the gas and liquid properties become identical. The boiling point cannot be increased beyond the critical point. Likewise, the boiling point decreases with decreasing ambient pressure until the triple point is reached. The boiling point cannot be reduced below the triple point.
Saturation Pressure, or vapor point, is the pressure for a corresponding saturation temperature at which a liquid boils into its vapor phase.
If the temperature in a system remains constant (an isothermal system), vapor at saturation pressure and temperature will begin to condense into its liquid phase as the system pressure is increased. Similarly, a liquid at saturation pressure and temperature will tend to flash into its vapor phase as system pressure is decreased. As heat is added to a liquid at its boiling point, all of this heat goes toward the phase change from liquid to gas, thus the temperature of the substance remains constant even though heat has been added. Since heat is added with no corresponding change in temperature, the heat capacity of the liquid is essentially infinite at the boiling point.
Intermolecular interactions
In terms of intermolecular interactions, the boiling point represents the point at which the liquid molecules possess enough heat energy to overcome the various intermolecular attractions binding the molecules into the liquid (eg.
The boiling point of water is 100 °C (212 °F) at standard pressure.
For purists with a knowledge of thermodynamics, the normal boiling point of water is 99.97 degrees Celsius (at a pressure of 1 atm, i.e. Until 1982 this was also the standard boiling point of water, but the IUPAC now recommends a standard pressure of 1 bar (100 kPa). At this slightly reduced pressure, the standard boiling point of water is 99.61 degrees Celsius. Both the boiling points of rhenium and tungsten exceed 5000 K at standard pressure.
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