Deep Water research

Sea-Level Water Boils at 100°C Under One Atmosphere Pressure

What is the boiling point of water at sea level in Celsius?

Sep 11, 202622 sources reviewed

Key Takeaways

Adopt 100°C as sea-level boiling point at 101.325 kPa per NIST and Wikipedia [4][13], rounding observed 99.97°C [15] and displacing IUPAC 1-bar and altitude values [4][15].

  • Water boils at 100°C (212°F) at 1 atm when vapor pressure matches surrounding air pressure, per NIST Chemistry WebBook data and Wikipedia definition [13][4].
  • Retain 1-atm 100°C for classrooms and sea-level tables, accepting ~0.39°C excess over IUPAC 99.61°C at 100 kPa [4][15].
  • Altitude lowers boiling markedly, about 1°F per 500 ft toward ~71°C on Everest per Wikipedia High-altitude cooking and Mountain House, forcing adjustments above ~2,000 ft [4][6][11].
  • Enter measured local pressure for precision work because ThermoWorks documents weather shifts of ±0.2–0.5°C and Wikipedia notes salt elevation [8][4]. | Choose 100°C at 1 atm when… | Choose adjusted value when… | |---|---| | teaching sea-level basics [4] | reporting at IUPAC 100 kPa, 99.61°C [15] | | using tables calibrated to 101.325 kPa [13] | calculating from measured pressure [8] |

[!WARNING] Reusing sea-level 100°C high up undercooks food, with Wikipedia High-altitude cooking noting potatoes staying hard for hours at 11,000 ft [4][6].

Abstract

At sea level under one standard atmosphere, ordinary water boils at 100°C [1][4][2]. Lower surrounding pressure lowers the boil [1][4][7]. NIST Chemistry WebBook pins the normal point at 99.97°C (373.17 K) from seven laboratory determinations, a figure textbooks round to 100°C, separate from the IUPAC one-bar standard at 99.61°C [13][15][4]. Climbing thins overhead air and drags boiling down to about 95°C at 1,905 m, about 71°C atop Everest and about 82°C in La Rinconada, losing roughly 1°F per 500 ft and forcing recipe adjustments above roughly 2,000 ft [3][11][6], while sealed pressure cookers push the point higher by raising internal pressure and dissolved salt nudges it upward by suppressing vapor pressure [4][7][9], and ThermoWorks notes day-to-day weather wobbles add about ±0.2–0.5°C unless cooks enter measured local pressure [8]. ThermoWorks and Omni calculators invert vapor-pressure relations including the Antoine equation to predict location-specific values from measured pressure [3][8], but controlled comparisons of glass versus metal vessels lack replication [9].

Table of Contents

Key Takeaways Abstract

  1. Introduction
  2. Background
  3. Findings 3.1 Boiling Point of Water at Sea Level 3.2 How Altitude Pressure and Impurities Shift Boiling 3.3 Authoritative Sources Confirming 100C Boiling Point
  4. Discussion
  5. Conclusion References

1. Introduction

Determining the precise boiling point of water at sea level in Celsius anchors cooking guidance, laboratory calibration, and safety planning [4][6][9]. Pressure governs boiling [4][6][9]. Evidence suggests high-altitude cooks extend heating times because thin air lowers boiling temperature, per Wikipedia and MountainHouse guidance [6][11]. Pew Research polling probes understanding; one report suggests many Americans remain unsure about altitude effects [12]. Water passes into full boiling when vapor pressure matches external pressure, a mechanism described in University of Maryland notes, Wikipedia’s boiling-point entry, and EBSCO’s research starter [1][4][7]. A shared sea-level reference therefore lets kitchens and laboratories directly compare results, a function reflected in calculators from OmniCalculator, ThermoWorks, and Pearson [3][8][10].

This report examines only pure liquid water under standard sea-level atmospheric pressure and reports temperature in Celsius. Analysts use calculators from ThermoWorks, Pearson, and OmniCalculator that treat sea-level pressure as the baseline for modeling open-air boiling shifts with elevation [3][8][10]. Scope stays tight by design. Laboratory analysts consult standard thermodynamic compilations; one report suggests NIST Chemistry WebBook data support boiling-point work for pure water [13].

Dissolved salts elevate boiling temperature, and evidence suggests the shift holds across common solutes [4][9], so this investigation excludes those mixtures. Pressurized vessels shift boiling behavior away from the open-pan reference, and evidence suggests the deviation matters for calibration [4][6], so they fall outside this review. Boundary holds firm. Elevation depresses boiling temperature, and evidence suggests Compound Interest and MountainHouse guidance document the drop for general readers [5][11], so detailed altitude modeling stays outside scope. Superheating, dissolved gases, and isotopic variants remain outside scope for clarity.

Background explains how vapor pressure interacts with atmospheric pressure to permit boiling and defines sea level for this analysis. Findings compiles the sea-level Celsius reference. After establishing the reference, Discussion compares agreement across sources and identifies limits without settling interpretation. Conclusion synthesizes the answer without introducing new data. By previewing logic before results, the chapter keeps the question open.

2. Background

Physical chemistry defines boiling as the condition where a liquid's vapor pressure matches external pressure, permitting bubbles to form throughout the volume according to Wikipedia, Wikidoc, and EBSCO Research Starters [4][9][7]. Evidence suggests evaporation strips molecules from the surface at any temperature, while boiling agitates the bulk liquid once vapor pressure overcomes confinement according to Wikipedia and Wikidoc [4][9]. One report suggests bulk bubbles grow freely per Wikipedia [4].

Atmospheric pressure sets the reference for any boiling-point measurement according to Wikipedia, Wikidoc, and University of Maryland notes [4][9][1]. Standard atmosphere equals 101.325 kPa or 760 mmHg at sea level according to Wikipedia, Wikidoc, and University of Maryland [4][9][1]. One report suggests altitude lowers pressure per Omni Calculator [3]. Evidence suggests weather systems and elevation shift local pressure away from standard according to Omni Calculator and Pearson [3][10].

Pure water boils at 100 °C under one standard atmosphere according to NIST WebBook records and Compound Interest [13][15][5]. Early Celsius calibrations fixed 0 °C at ice melting and 100 °C at water boiling under standard pressure according to Wikipedia, Compound Interest, and EBSCO [4][5][7]. One report suggests definitions preserve that interval per Wikipedia [4]. Evidence suggests laboratories calibrate thermometers against NIST WebBook phase-change data and ThermoWorks references [13][8].

Lower ambient pressure reduces the temperature needed for vapor pressure to match surroundings according to Omni Calculator, Pearson, and Mountain House [3][10][11]. Calculators from Pearson, Omni Calculator, and ThermoWorks implement Clausius-Clapeyron relations to estimate boiling-point decline with elevation [10][3][8]. One report suggests cooking slows up high per Wikipedia [6]. Evidence suggests extended cooking times and pressure cookers compensate for lower boiling temperatures according to Wikipedia and Mountain House [6][11].

Dissolved salts and impurities elevate the observed boiling temperature according to Wikipedia, Wikidoc, and EBSCO [4][9][7]. Evidence suggests precise measurement requires degassed pure water, stable pressure, and calibrated thermometry according to NIST WebBook and ThermoWorks [13][8]. One report suggests purity affects results per EBSCO [7]. One report suggests many American adults remain uncertain that altitude lowers water's boiling point per Pew Research [12].

3. Findings

3.1 Boiling Point of Water at Sea Level

Water boils at 100°C (212°F) at sea level under standard atmospheric pressure of 1 atm or 101.325 kPa [2]. The sea-level value is always 100°C or 212°F, according to Omni Calculator [3]. Water's boiling point is 100 degrees Celsius at 1 atm, as University of Maryland chemistry notes record [1], and Wikipedia's high-altitude cooking article gives 100°C (212°F) at sea level [6]. That fixity forces thermometers and cooking assumptions to treat 100°C as the sea-level baseline [3]. Vaia's Chemistry: Matter and Change textbook identifies sea-level atmospheric pressure as 1 atm or 101.325 kPa [2], typically about 101.325 kPa where water boils at 100°C [2]. EBSCO gives 100 degrees Celsius at sea level [7], and WikiDoc gives 100°C (212°F) at standard pressure [9]. Sea-level pressure is 1 atmosphere or 101,325 pascals, according to Compound Chemistry [5]. One atmosphere equals about 14.7 pounds per square inch, according to EBSCO [7], which locks gauge conversions to the same boiling threshold. Los Angeles is close to sea level and serves as the sea-level reference, according to Pew Research [12], with 14.7 pounds per square inch there, according to Pew Research [12]. The anchor holds.

Boiling begins only when vapor pressure equals surrounding pressure, as University of Maryland chemistry notes and Wikipedia's boiling-point article record [1][4]. That match lets vapor bubbles survive against the atmosphere instead of collapsing [5]. Evidence suggests the normal boiling point is the special case where vapor pressure equals the defined sea-level pressure of one atmosphere, as Wikipedia and EBSCO record [4][7]. The boiling point is the temperature where liquid vapor pressure equals external pressure, according to EBSCO [7], which prevents sustained boiling below that temperature. A liquid boils when internal vapor pressure equals atmospheric pressure, according to Pew Research [12], so sea-level air sets a fixed threshold. One atmosphere is equivalent to air pressure at sea level, according to EBSCO [7]. Compound Chemistry explains the sea-level result: vapour pressure equals atmospheric pressure at 100˚C, so this is the temperature where water boils [5]. No match, no boil [5].

100°C is a rounded convenience with limited thermodynamic exactness [4]. Wikipedia's boiling-point article gives water as 100°C rounded from scientific precision of 99.97°C (211.95°F) under standard pressure at sea level, according to Wikipedia [4]. That rounding lets textbooks print 100°C without changing kitchen outcomes [4]. The scientifically precise normal boiling point is 99.97°C at 1 atm (101.325 kPa), according to Wikipedia [4], given as 211.9°F in that thermodynamic definition. That 0.03°C gap costs nothing in cooking but matters for metrology [4]. Boiling points may be published with respect to NIST standard pressure of 101.325 kPa (1 atm) or IUPAC standard pressure of 100.000 kPa (1 bar), according to Wikipedia [4], so the chosen reference shifts the reported value. IUPAC has defined the standard boiling point since 1982 as boiling under one bar, according to Wikipedia [4], which creates a separate reporting basis from the sea-level atmosphere. The Celsius scale was defined until 1954 by 0°C freezing and 100°C boiling at standard atmospheric pressure, according to Wikipedia [4], which locked the sea-level boiling point into the unit itself. The freezing point at sea level is constant at 0°C or 32°F, according to Omni Calculator [3], bracketing the liquid range from below. Labs need the extra digits [4].

Sea-level values anchor every altitude calculation, as the boiling-point formula uses the sea-level boiling value as its basis, according to Omni Calculator [3]. The Fahrenheit boiling point equals 49.161 times ln of pressure in inHg plus 44.932, according to Omni Calculator [3], so a pressure reading converts directly to boiling temperature. Pressure in inHg equals 29.921 times (1 minus 0.0000068753 times altitude in feet) to the power 5.2559, according to Omni Calculator [3], which forces boiling temperature down as altitude climbs. Pearson converts altitude h to pressure with the standard-atmosphere formula P = P0(1 − Lh/T0)^gM/RL for 0–11 km, according to Pearson [10], so elevation alone predicts ambient pressure. For custom liquids Pearson solves ln(P2/P1) = −ΔHvap/R(1/T2 − 1/T1) with P1 = 1 atm at normal boiling point T1 to find T2 at ambient P2, according to Pearson [10], which extends sea-level data to any pressure. Water at ambient pressure of 90 kPa boils at about 96.7°C by Antoine inversion, according to Pearson [10], costing about 3.3°C versus sea level. ThermoWorks calculators use adjusted barometric pressure as typically reported by airports and news outlets, according to ThermoWorks [8], so public weather reports feed directly into the estimate. Omni Calculator finds pressure at altitude by assuming constant sea-level pressure of 1013 hPa (1.013 bar), according to Omni Calculator [3], which simplifies math but ignores weather shifts. Math starts at sea level [3].

Leave sea level and the fixed point collapses quickly. From Mount Everest to the Dead Sea water's boiling point varies from just below 70°C to over 101°C, according to Compound Chemistry [5], which ruins single-temperature recipes. On Everest at 8,848 m (29,029 ft) pressure is about 34 kPa (255 Torr) and boiling point is 71°C (160°F), according to Wikipedia [4], so tea brews far cooler. Tabulated boiling points are based on standard sea-level atmospheric pressure courtesy of NOAA, according to Wikipedia's high-altitude cooking article [6], which makes sea level the required baseline. Denver air is only about 12 pounds per square inch, according to Pew Research [12], compared with 14.7 pounds in Los Angeles, according to Pew Research [12], which explains lower boiling in Denver. In Death Valley at 282 feet below sea level water boils slightly above 212 degrees, according to Pew Research [12], adding a small premium to the sea-level figure. At 5,000 feet water boils at about 203 degrees F, according to Mountain House [11], lengthening cooking times. Water boils at 212 degrees Fahrenheit under sea-level conditions, according to Pew Research [12], and at 100 degrees Celsius under sea-level conditions, according to Pew Research [12].

Sea-level reference compared with altitude boiling points.

Condition Pressure Boiling point
Sea level 1 atm or 101.325 kPa [2] 100°C (212°F) [2]
1,000 ft above sea level lower than sea level 210.1°F (98.9°C) [7]
5,000 ft (1,524 m) lower than sea level 203°F (95°C) [6]
8,000 ft (2,438.4 m) lower than sea level 92°C (198°F) [6]
10,000 ft above sea level lower than sea level 193.2°F (89.6°C) [7]
1,000 ft below sea level higher than sea level 213.9°F (101.1°C) [7]
Machu Picchu, 2,430 m (7,970 ft) 22.25 inHg [3] 197.44°F [3]

3.2 How Altitude Pressure and Impurities Shift Boiling

Water boils at 212°F at sea level only because the atmosphere presses that hard, according to ThermoWorks [14]. Pressure sets the temperature, according to OmniCalculator [3]. Boiling is the temperature at which water starts changing from liquid to gas, evidence indicates from OmniCalculator's definition [3]. Pew Research, University of Maryland chemistry notes, EBSCO and CompoundChem agree the boiling point of water or any liquid varies with surrounding atmospheric pressure, temperature and vapour pressure [12][1], with liquids boiling hotter at low altitude and cooler at high altitude [7][5].

Vapour pressure wins when it equals the air pushing back, evidence suggests from CompoundChem descriptions of escape tendency into the gas phase above the liquid and temperature response [5]. Liquid-to-gas change requires energy, according to University of Maryland [1]. During boiling the more energetic molecules change to gas, spread out and form bubbles that rise to the surface and enter the atmosphere, evidence indicates from University of Maryland chemistry notes [1]. Higher temperature raises vapour pressure because molecules move faster and more have energy to escape the liquid, according to CompoundChem [5]. Liquid temperature stays constant during boiling because departing gas molecules carry away heat energy, and adding more heat will not raise it, as multiple sources report in University of Maryland and EBSCO notes [1][7].

Climbing lowers boiling because the weight of the atmosphere above decreases since you are above some of it and so pressure also decreases, evidence suggests from CompoundChem and EBSCO elevation accounts [5][7]. Less pressure needs less vapour, according to CompoundChem [5]. CompoundChem and Wikipedia's high-altitude cooking entry agree differences in atmospheric pressure at different elevations cause variation in water's boiling point, with lower pressure yielding lower temperatures at higher altitudes [5][6], while OmniCalculator and ThermoWorks tie that pressure itself to altitude and elevation shifts [3][8]. Water's boiling point drops by approximately 1°F for every 500-ft (152.4-m) increase in elevation, steadily eroding cooking heat, according to Wikipedia's high-altitude cooking entry [6]. That effect starts to become relevant at altitudes above approximately 2,000 feet (610 m), forcing recipe adjustments above that line, according to Wikipedia's high-altitude cooking entry [6].

Denver already boils well below sea level, with water at ~1,609 m and ~83.3 kPa reaching only ~94.6°C and cutting cooking heat, according to Pearson's altitude calculator [10]. That is about 202 degrees in Denver due to lower air pressure at such high elevations, cutting cooking heat, according to Pew Research [12]. At 1,905 metres (6,250 ft) water boils at 93.4°C (200.1°F), shaving more than 6°C off sea-level heat, evidence indicates from Wikipedia's boiling-point entry [4]. At 11,000 feet potatoes stayed hard for hours, per Wikipedia [6]. They stayed nearly as hard as ever in boiling water because diminished atmospheric pressure lowered the boiling temperature, according to Wikipedia's high-altitude cooking entry [6]. Real weather-related pressure shifts change water boiling temperature by ~±0.2–0.5°C relative to the standard-atmosphere calculation, evidence indicates from Pearson's calculator notes [10]. If local pressure is known, entering it directly in mmHg, kPa or atm overrides the altitude-derived pressure and prevents weather error from biasing the estimate, according to Pearson's calculator guidance [10].

Putting a liquid in a partial vacuum lowers its boiling point by lowering atmospheric pressure, according to Pew Research [12]. Less air means less heat, according to Pew Research [12]. A pressure cooker compensates for low atmospheric pressure by providing higher pressure inside the cooking vessel and thus higher temperatures, evidence indicates from Wikipedia's high-altitude cooking entry [6]. The boiling point increases with increased pressure up to the critical point, where gas and liquid properties become identical, and decreases with decreasing pressure until the triple point is reached, according to Wikipedia's boiling-point entry [4], while NIST lists the triple-point pressure of water as 0.0061 bar from Sato, Watanabe, et al., 1991 with TRC-assigned uncertainty of 1e-7 bar, according to NIST [13]. Thermodynamics Research Center compiled data at NIST Boulder Laboratories, evidence indicates from NIST compilation notes [13]. NIST models water vapor pressure with the Antoine equation log10(P) = A - (B / (T + C)) where P is in bar and T is in K, enabling boiling-point calculation from pressure, according to NIST [13].

Adding solutes causes water boiling-point elevation, as multiple sources report in OmniCalculator and Wikipedia accounts [3][4]. Salted water boils hotter than plain, according to EBSCO [7]. Adding any solute increases boiling temperature by reducing vapour pressure, meaning slightly higher temperature is required for vapour pressure to equal atmospheric pressure and boil the water, evidence indicates from CompoundChem solute notes [5]. Each sample of clear water starts boiling at the same temperature at a given terrain elevation, according to OmniCalculator [3].

Comparison of factors that lower versus raise the boiling point.

Driver Pressure shift Boiling outcome
Higher altitude, Denver at ~1,609 m [10] Ambient pressure ~83.3 kPa [10] Boils at ~94.6°C, lower [10]
Higher altitude at 1,905 m (6,250 ft) [4] Lower atmospheric pressure [6] Boils at 93.4°C (200.1°F) [4]
Partial vacuum [12] Lower atmospheric pressure [12] Lower boiling point [12]
Dissolved salt or solute [4] Reduced vapour pressure [5] Higher boiling point [4]
Pressure cooker [6] Higher pressure inside vessel [6] Higher temperature [6]

Most Americans get altitude boiling wrong, with only 34% knowing water boils lower in Denver (Mile High City) than Los Angeles (close to sea level), while 26% said higher and 39% said same, according to Pew Research [12]. Confusion persists despite simple physics, according to Pew Research [12].

3.3 Authoritative Sources Confirming 100C Boiling Point

Water boils at 100°C under sea-level standard pressure, and NIST data agree within tenths of a kelvin [4][13]. Wikipedia's boiling-point entry gives 100°C rounded from 99.97°C (211.95°F) at sea level, which locks 100°C as a rounding convention rather than an infinite-precision constant [4]. Water boils at 212°F (100°C) in Pew Research and EBSCO accounts [12][7]. The convergence is tight [13].

The benchmark applies to ordinary water identified by NIST Chemistry WebBook as H2O with CAS Registry Number 7732-18-5 [13][15]. NIST Chemistry WebBook lists formula H2O with molecular weight 18.0153, which fixes the substance for thermodynamic tables [15]. NIST Chemistry WebBook also groups Water vapor, Distilled water, Ice, H2O, Dihydrogen oxide and steam under the same entry, so the data cover vapor-steam equilibrium rather than a specialty fluid [15]. NIST Standard Reference Database 69, the NIST Chemistry WebBook, supplies the data [15][13]. The Thermodynamics Research Center at NIST Boulder Laboratories compiled the data under director Chris Muzny, which centralizes responsibility for evaluation [15]. Tboil means boiling point according to NIST's symbol key [15].

NIST measurements cluster at 373.15–373.2 K, forcing the 100°C rounding used in textbooks [15]. NIST Chemistry WebBook lists average Tboil as 373.17 ± 0.04 K as an average of 7 individual experimental values according to its main table, which shrinks random error below 0.05 K [13]. Multiple sources report 373.15 K for water, including Rajendran, Renganarayanan and co-workers in 1989 with TRC-assigned uncertainty 0.2 K and Zieborak, Brzostowski and co-workers in 1958 with 0.05 K, so two independent labs land on the same tenth [15]. Multiple sources report 373.16 K, including Arce, Martinez-Ageitos and co-workers in 1998 with 0.05 K uncertainty and Sada, Morisue and co-workers in 1975 with 0.1 K uncertainty, which narrows the band to 0.01 K [15]. Multiple sources report 373.2 K, including Fandary, Aljima and co-workers in 1999 with 0.1 K uncertainty and Sokolov, Sevryugova and co-workers in 1969 with 0.1 K uncertainty, which still rounds to 100°C [15]. The spread is tiny [13].

Comparison of authoritative boiling-point values for water.

Benchmark Boiling-point value
Average Tboil 373.17 ± 0.04 K, average of 7 values [13]
Single-study normal point 373.15 K, TRC uncertainty 0.2 K, Rajendran et al. 1989 [15]
IUPAC standard at 100 kPa (1 bar) 99.61 °C (211.3 °F) [4]
Textbook reference 212°F (100°C) [7]

The Celsius scale itself anchors 100° to boiling water, which explains why the benchmark persists in EBSCO accounts [7]. By 1747 Celsius's scale was inverted on some thermometers so that 100° was the boiling point and 0° was the freezing point of water according to EBSCO Research Starters, fixing classroom thermometry to steam [7]. The TerpConnect chemistry page defines the boiling point as the temperature at which boiling occurs for a specific liquid, which ties the number to phase change rather than arbitrary marking [1]. The best-known boiling point is that of water at 212°F (100°C) according to EBSCO Research Starters, which makes 100°C the standard pressure benchmark for general reference [7]. IUPAC recommends 99.61°C (211.3°F) at 100 kPa (1 bar) as the standard boiling point according to Wikipedia's boiling-point entry, which sits 0.39 degrees below the 100°C sea-level rounding [4]. That gap is pressure [4].

Vapor-pressure equations reproduce the 1–100°C boiling region accurately enough to calculate altitude shifts according to Pearson's boiling-point-at-altitude calculator [10]. Pearson's calculator uses Antoine constants A = 8.07131, B = 1730.63 and C = 233.426 for water over 1–100°C according to its methods page, and those three constants let pressure be converted directly to temperature [10]. Pearson's calculator treats water's Antoine constants as standard and accurate in 1–100°C while using Clausius-Clapeyron with ΔHvap for other liquids because reliable constants vary according to its documentation, which makes water the reference fluid [10]. Pearson's calculator obtains boiling temperature by inverting log10 P(mmHg) = A − B/(C + T(°C)) for T according to its methods page, so a pressure measurement forces a boiling temperature without new experiments [10]. NIST provides Antoine coefficients 5.08354, 1663.125 and -45.622 for 344. to 373. K from Bridgeman and Aldrich, 1964 according to its WebBook, covering the boiling region up to 373 K and enabling the same inversion near steam [13]. ThermoWorks built its calculator to find the exact boiling point for any specific location according to its learning center, which turns the 100°C benchmark into a location-corrected value [8]. The math is settled [10].

The 100°C figure collapses outside sea-level pressure or in sticky vessels, as shown in Pew Research and Compound Chem data [12][5]. Water boils at about 162 degrees at 29,029 feet on Mount Everest according to Pew Research, which would leave tea under-extracted if cooks trusted 100°C [12]. Water boils at about 181 degrees in La Rinconada at more than 16,700 feet, the highest permanently inhabited town in the Peruvian Andes, according to Pew Research, which cuts sterilization margins [12]. Experiments show water boils at different temperatures in metal versus glass vessels at the same pressure according to Compound Chem, which prevents pressure alone from fixing the point [5]. Water is theorized to boil higher in vessels to which its molecules adhere more strongly according to Compound Chem, so surface chemistry adds degrees [5]. NIST Chemistry WebBook lists critical temperature Tc as 647. ± 2. K as an average of 7 values according to its tables, which marks the end of liquid-vapor boiling altogether [13]. NIST Chemistry WebBook lists critical pressure Pc as 220.64 bar with TRC uncertainty 0.05 bar from Sato, Watanabe and co-workers in 1991, and critical density as 17.9 mol/l with 0.2 mol/l uncertainty from the same team, which bounds how far pressure can push boiling before supercriticality [13]. Limits matter [13].

4. Discussion

Pressure definition controls the answer. Under 101.325 kPa, water turns to vapor at 100°C (212°F) [1][3][5], with data compiled by NIST Chemistry WebBook clustering around 99.97°C [4][13][15]. That cluster rounds to 100°C because the Celsius scale fixes boiling there at standard pressure [2][4][7]. Pressure decides. One report suggests the competing 99.61°C figure reflects 100 kPa under IUPAC convention [4] (see 3.3). Keep 1 atm for sea-level work.

Elevation moves the thermometer without rewriting the standard. Because vapor pressure must match ambient pressure before bubbles break free [1][4][9], thinner air aloft lowers the boiling temperature [3][5][6]. Water boils cooler at Denver and at 1,905 metres in data from ThermoWorks calculators and mountain-house guides [1][8][11]. Evidence suggests cooks lose about 1°F per 500 feet and adjust recipes above about 2,000 feet [6][10]. Daily wobble stays small, evidence suggests [8][14]. Evidence suggests daily pressure swings shift the point only ±0.2–0.5°C unless cooks enter measured local pressure in data from ThermoWorks [8][14] (see 3.2). Salt suppresses vapor and nudges the point up, pressure cookers push it up by pressurizing the vessel, and partial vacuums drag it toward the triple point in Pearson and vapor-pressure accounts [4][9][10].

The strongest challenge invokes modern standards and messy reality. One report suggests standard pressure under IUPAC convention reads 1 bar [4]. Evidence suggests field pressure shifts with weather [8][14]. Thermometers drop far lower on mountains [1][3][11]. Why retain 100°C? That objection deserves weight because modern practice and field variability challenge textbook simplicity. Data compiled by NIST Boulder laboratories outweigh Wikipedia accounts for the number itself, and those measurements average 373.17 K with tight clustering [4][13][15]. Treat 100°C as the normal boiling point at 1 atm [1][13][15] (see 3.1). Accept 99.61°C for work specified at 1 bar.

Gaps temper confidence away from the core value. One report suggests vessel adhesion in metal versus glass nudges the reading [14]. Thermometers drop to about 162°F atop Everest and about 181°F in La Rinconada [1][3][11]. One report suggests many Americans misjudge Denver versus Los Angeles in Pew Research testing [12]. Prediction tools from Pearson extrapolate through Antoine and Clausius-Clapeyron equations [4][10][13] (see 3.2). Pressure definition dominates; rounding explains the rest.

5. Conclusion

At sea level under one standard atmosphere, ordinary water boils at 100°C [1][4][13].

| Reader scenario | Recommended choice | Deciding factor | Confidence | Reversal assumption | | General use at sea level | Quote 100°C | Vapor pressure matches 101.325 kPa [1][4][7] | High | Barometer reads far from 1 atm | | High-precision thermodynamics | Use 99.97°C (373.17 K) | Seven experiments average 373.17 ±0.04 K in NIST Chemistry WebBook [13][15][2] | High | Specification switches to IUPAC 100 kPa | | IUPAC 1-bar or mountain work | Use 99.61°C or altitude-corrected temperature | Lower reference pressure or thinner air lowers boiling [4][9][10] | High for definition; Medium for altitude estimate | Return to sea-level 1-atm conditions |

The 1-atm sea-level figure settles decisively at rounded 100°C, because seven measurements compiled in NIST Chemistry WebBook cluster between 373.15 and 373.2 K and the Celsius scale anchors boiling there [13][15][2]. Use that number. The IUPAC 99.61°C value steelmans the rival case: it governs when the stated standard means one bar rather than one atmosphere [4][7][9]. That definition flips the default [4][7][9]. Climbing flips it downward too, with inversion tools from ThermoWorks and OmniCalculator mapping pressure to temperature [3][8][6]. One report suggests vessel adhesion in metal versus glass may nudge observations, a wrinkle that leaves classroom practice untouched [9]. Salt raises the point slightly by lowering vapor pressure, while pressure cookers lift it and vacuums drop it toward the triple point [4][7][9]. One report suggests many Americans misjudge Denver versus Los Angeles boiling, per Pew Research [12]. Evidence suggests weather-driven pressure wobbles shift boiling only ±0.2–0.5°C, so field accuracy comes from entering local pressure [8][14].

A calibrated thermometer on Everest next year will still read near 71°C where the sea-level pot reaches 100°C [3][5][13].

References

[1] Boiling Point — https://terpconnect.umd.edu/~wbreslyn/chemistry/pressure/boiling-point.html · academic [2] Step by Step Solution — https://www.vaia.com/en-us/textbooks/chemistry/chemistry-matter-and-change-2007-edition/chapter-3/problem-40-boiling-water-at-what-temperature-would-250-ml-of/ · general [3] Boiling Point at Altitude Calculator — https://www.omnicalculator.com/chemistry/boiling-point-altitude · general [4] Boiling point — https://en.wikipedia.org/wiki/Boiling_point · general [5] Compound Interest: What Temperature Does Water Boil At? Boiling Point & Elevation — https://www.compoundchem.com/2016/03/22/boiling-point/ · general [6] High-altitude cooking — https://en.wikipedia.org/wiki/High-altitude_cooking · general [7] Boiling points | Science | Research Starters | EBSCOhost — https://www.ebsco.com/research-starters/science/boiling-points/ · general [8] The Boiling Point of Water Calculator — https://www.thermoworks.com/blogs/learning-center/boiling-point-calculator?srsltid=AfmBOoo3rZnIPmcbvUPuJWzlSlpfMqfNqYTjmpIlRB9hc7-1V-uAWsK9 · general [9] Boiling point - wikidoc — https://www.wikidoc.org/index.php/Boiling_point · general [10] Boiling Point at Altitude Calculator | Water & Clausius–Clapeyron Equation — https://www.pearson.com/channels/calculators/boiling-point-at-altitude-calculator · general [11] Impacts of High Altitude on Water’s Boiling Point — https://mountainhouse.com/blogs/backpacking-hiking/effects-of-altitude-on-water-boiling-time?srsltid=AfmBOopJ9oV8SHwB95xQQVWUS8MWAAhlRAIb7cjjXJF03B7a-UfiMiP- · general [12] Does water’s boiling point change with altitude? Americans aren’t sure — https://www.pewresearch.org/short-reads/2015/09/14/does-waters-boiling-point-change-with-altitude-americans-arent-sure/ · general [13] Water — https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=4 · government [14] The Boiling Point of Water Calculator — https://www.thermoworks.com/blogs/learning-center/boiling-point-calculator?srsltid=AfmBOopxGXWcwxFKhrGC1wHH8Pb5r5cpkctYY6m0V7pTdmXVbyIf2-5g · general [15] Water — https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Type=TBOIL · government

Source quality: 1 academic, 2 government, 12 general.