Pure sea-level water boils at 100°C under one atmosphere, not one bar

Key Takeaways

Taken together, ^[11] and ^[7] indicate the standard boiling point of water is 100 °C at sea-level atmospheric pressure (1 atm or 101.325 kPa), while the IUPAC-recommended standard boiling point is 99.61 °C at 100 kPa (1 bar), and boiling points may be published with respect to either pressure. ^[11][7]

• Pure water boils at 100°C (212°F) under sea-level 1-atm pressure (101.325 kPa), with thermodynamic precision at 99.97°C (211.9°F). ^[5][11][7]
• Evidence suggests the pressure basis decides the figure: normal boiling uses 1 atm (101.325 kPa), while IUPAC standard boiling uses 1 bar (100 kPa) at 99.61°C (211.3°F); CoolProp reports 99.9743°C at 101325 Pa and 99.6059°C at 100000 Pa. ^[11][9]
• Taken together, ^[11] and ^[7] indicate the 99.61°C (211.3°F) figure at 100 kPa (1 bar) is lower than the 100°C (212°F) figure at 1 atm (101.325 kPa). ^[11][7]
• One report states the actual 'pressure of the atmosphere' at sea-level varies above and below that according to location and countless weather influences. ^[9]
• Evidence suggests higher altitude lowers boiling through lower pressure, while dissolved salt raises it above pure water. ^[8][5]

[!WARNING] Do not log the 99.61°C one-bar value as normal sea-level boiling; normal sea-level work requires 1 atm (101.325 kPa). ^[11][9]

Abstract

Taken together, ^[11] and ^[7] indicate water boils around 100°C at sea-level air pressure ^[11][7], actually 99.97°C following the thermodynamic definition of the Celsius scale based on the kelvin at a pressure of 1 atm (101.325 kPa) ^[11].

The IUPAC-recommended standard boiling point of water at a standard pressure of 100 kPa (1 bar) is 99.61°C (211.3°F) ^[11].

The NIST Chemistry WebBook lists normal boiling point entries of 373.15 K, 373.16 K and 373.2 K ^[1], while Vaia's solution states the standard boiling point of water is 100°C (212°F) at sea-level atmospheric pressure (1 atm or 101.325 kPa) ^[7], and the high-altitude reference lists 100°C (212°F) at sea level ^[8].

The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor ^[11].

The actual 'pressure of the atmosphere' at sea-level varies above and below that according to location and countless weather influences ^[9], and at 2,438.4 metres (8,000 ft) in elevation, water boils at just 92°C (198°F) ^[8].

Salt water boils at a higher temperature than pure water ^[11][5], with the increase in the boiling point calculated as Delta Tb equals kb times m times i ^[5].

Table of Contents

• Key Takeaways
• Abstract
• 1. Introduction
• 2. Background
• 3. Findings
  • 3.1 Water Boiling Point at Sea Level
  • 3.2 Altitude Pressure and Solute Effects on Water Boiling Point
  • 3.3 Primary Standards and Reference Sources
• 4. Discussion
• 5. Conclusion
• References

1. Introduction

This introduction frames one question: what Celsius reading marks pure-water boiling at sea level? Taken together, ^[4][8][11] indicate pressure governs the answer. See Background and Findings for the boiling definition, pressure dependence, pressure-range behavior, altitude effect, cooking consequence and scale history.

Scope stays narrow. This report interrogates pure water at defined sea-level pressures. Normal and standard definitions and publication pressures are detailed elsewhere in this report; see Background. Solution elevation and altitude effects are detailed elsewhere in this report; see Altitude Pressure and Impurity Effects. Solute chemistry and mountain cooking therefore sit outside the core question.

The report moves from Background through Findings and Discussion to Conclusion.

2. Background

The boiling point of a substance is reportedly the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor ^[11]. The normal boiling point reportedly is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, one atmosphere, while the standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of one bar ^[11]. Boiling points may reportedly be published with respect to the NIST, USA standard pressure of 101.325 kPa (1 atm), or the IUPAC standard pressure of 100.000 kPa (1 bar) ^[11].

Taken together, ^[8] and ^[11] indicate pressure affects boiling. One Omnicalculator description suggests boiling temperature tracks pressure, which tracks altitude ^[4].

Boiling point reportedly increases with increased pressure up to the critical point and decreases with decreasing pressure until the triple point is reached ^[11]. Actual sea-level pressure reportedly varies above and below the defined value with location and countless weather influences ^[9].

This background outlines temperature scales. The International Committee of Weights and Measures reportedly adopted ITS-90 at its 1989 meeting ^[2]. Historically, the Celsius scale reportedly anchored 0°C to freezing and 100°C to boiling at standard atmospheric pressure until 1954 ^[11]. In 2019, the definition of the kelvin scale reportedly changed to be based on the Boltzmann constant rather than depending on the triple point of water ^[9].

Dissolved substances raise boiling ^[5][11][13]. One LibreTexts page reports pure water boiling at 100°C at 1.0 atm while 2% saltwater boils at about 102°C ^[5].

3. Findings

3.1 Water Boiling Point at Sea Level

• Pure water boils at 100°C (212°F) at sea-level atmospheric pressure in Vaia's chemistry textbook ^[7] and Wikipedia's high-altitude cooking entry ^[8].
• Water remains at 100 degrees Celsius while boiling at 1 atm or 101.3 kPa at sea level in TerpConnect chemistry material ^[3].
• 212°F or 100°C holds at sea level in EBSCO's boiling-points primer ^[10].

Start there.

See the sea-level baseline at the start of this section.

Sea-level pressure fixes the 100°C point at 1 atm or 101.325 kPa in Vaia's textbook ^[7]. Atmospheric pressure is defined to be exactly 101,325 Pa, yet actual sea-level pressure varies above and below that defined value with location and countless weather influences, in Webel's pressure explainer ^[9]. 101.32 kPa was the pressure for a vapor-liquid equilibria study cited in the NIST Chemistry WebBook ^[1]. Definitions set the mark.

The normal boiling point ties the vapor pressure of a liquid to the defined atmospheric pressure at sea level, one atmosphere ^[11]. The normal point is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, one atmosphere, in Wikipedia's boiling-point entry ^[11]. Water boils at 100°C at 1 atm of pressure, but a solution of saltwater does not, in LibreTexts introductory chemistry ^[5].

Water boils at 100°C, rounded from scientific precision of 99.97°C (211.95°F), under standard pressure at sea level ^[11].

The normal boiling point is commonly given as 100°C (212°F) (actually 99.97°C (211.9°F) following the thermodynamic definition of the Celsius scale based on the kelvin) at a pressure of 1 atm (101.325 kPa) ^[11].

CoolProp gives 99.9743°C at 1 atmosphere (101325 Pa) ^[9].

Celsius is defined by t/°C = T/K minus 273.15 with the degree Celsius equal in magnitude to the kelvin ^[2].

At t90 of 100°C the tabulated t90 minus t68 equals minus 0.026°C ^[2].

The standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of one bar ^[11].

The IUPAC-recommended standard boiling point of water at a standard pressure of 100 kPa (1 bar) is 99.61 °C (211.3 °F) ^[11].

At 100,000 Pa, CoolProp gives a standard boiling point for water of 99.6059 °C ^[9].

Boiling points may be published with respect to the NIST, USA standard pressure of 101.325 kPa (1 atm), or the IUPAC standard pressure of 100.000 kPa (1 bar) ^[11].

The Celsius temperature scale was defined until 1954 by two points: 0 °C being defined by the water freezing point and 100 °C being defined by the water boiling point at standard atmospheric pressure ^[11].

Normal versus standard boiling conditions for water.

Water boils at 212°F at sea level ^[12], but changes in elevation and atmospheric pressure alter that temperature ^[12].

The normal boiling-point data were compiled by the Thermodynamics Research Center, NIST Boulder Laboratories, in the NIST Chemistry WebBook ^[1].

• Molecular weight of water is 18.0153 in that WebBook ^[1].
• Melting- and freezing-point reference pressure is standard atmosphere p0 equal to 101325 Pa in NIST's ITS-90 document ^[2].
• Freezing point at sea level is a constant value, 0°C or 32°F, in OmniCalculator ^[4].
• Vapor pressure follows log10(P) = A − (B / (T + C)) with P in bar and T in K in the NIST Chemistry WebBook ^[6].
• Coefficients calculated by NIST from Stull, 1947 cover 255.9 to 373. in that WebBook ^[6].
• Ptriple is 0.0061 bar from Sato, Watanabe and colleagues in that WebBook ^[6] and Pc is 220.64 bar from the same author group in that WebBook ^[6].
• Tc is 647. ± 2. K averaged from 7 values in NIST's entry ^[6].

3.2 Altitude Pressure and Solute Effects on Water Boiling Point

Water boils when vapor pressure equals surrounding pressure and liquid changes into vapor, as Wikipedia's boiling-point entry defines ^[11]. OmniCalculator calls this liquid-to-gas change ^[4].

The temperature at which water starts boiling is not constant and depends on pressure, which in turn is dependent on altitude ^[4]. OmniCalculator ties altitude to pressure ^[4]. OmniCalculator states the boiling point of water depends solely on pressure ^[4].

The boiling point of water is lower at higher altitudes due to the decreased atmospheric pressure ^[8]. Water boils at lower temperatures at higher elevations ^[13]. Under these circumstances, water boils at temperatures significantly below 100 °C ^[8].

Added pressure raises boiling up to the critical point where gas and liquid properties become identical, as Wikipedia's boiling-point entry reports ^[11]. Wikipedia notes vacuum lowering ^[11]. Wikipedia's boiling-point entry states boiling decreases with decreasing pressure until the triple point is reached ^[11]. A liquid in partial vacuum boils lower than at atmospheric pressure, according to Wikipedia's boiling-point entry ^[11].

Effects bite above about 2,000 feet (610 m), as Wikipedia's high-altitude cooking entry reports the effect starts to become relevant there ^[8]. Wikipedia marks that relevance line ^[8]. Each 152.4-metre (500 ft) gain cuts boiling by approximately 1°F, according to Wikipedia's high-altitude cooking entry ^[8].

At 2,438.4 metres (8,000 ft) in elevation, water boils at just 92 °C (198 °F) ^[8].

For the location described as located at 2430 meters (7970 ft) above sea level with pressure calculated as 29.921 × (1 - 0.0000068753 × 7970)^ 5.2559 = 22.25 inHg ^[4], the boiling point is calculated as boiling point = 49.161 × ln(22.25) + 44.932 = 197.44°F ^[4].

At an elevation described as probably not under 11,000 feet, potatoes, after remaining for some hours in the boiling water, were nearly as hard as ever, Wikipedia's high-altitude cooking entry recounts ^[8]. Food will take longer to cook, or cannot be prepared at all depending on temperatures required, according to Wikipedia's high-altitude cooking entry ^[8].

Means of compensation include extending cooking times or using a pressure cooker to provide higher pressure inside the cooking vessel and thus higher temperatures ^[8].

Solute effects on boiling ^[4][5] are covered in the section on solution types, concentrations and magnitudes.

Boiling-point elevation due to the presence of a solute is a colligative property, with the amount of change related to the number of particles of solute in a solution and not related to the chemical composition of the solute ^[5].

Free Science Project describes boiling-point elevation as a "colligative property" that depends on the number of dissolved particles, not their identity ^[13].

Free Science Project reports the temperature increase is roughly proportional to the amount of dissolved salt, confirming that this is a colligative property dependent on the concentration of dissolved particles ^[13].

Sugar, baking soda, or other soluble substances will also raise the boiling point, though the exact amount may differ ^[13].

The increase in the boiling point is calculated as Delta Tb equals kb times m times i, where kb for water is 0.515°C/m ^[5].

Adding enough NaCl solute to a solvent to produce a 0.20 m solution will have twice the effect of adding enough sugar to a solvent to produce a 0.20 m solution ^[5].

Salt dissolves into sodium (Na+) and chloride (Cl-) ions that interfere with water molecules escaping as vapor, and more energy (higher temperature) is needed to overcome this interference ^[13]. Vapor pressure is lowered by dissolved particles ^[13]. Higher temperature compensates for lower vapor pressure ^[13].

The boiling point of water is no longer exactly 100 °C ^[9]. In 2019 the definition of the kelvin scale (and resulting impact on the Celsius scale) was changed to be based on the Boltzmann constant, rather than depending on the solid-liquid-gas triple point of water ^[9].

3.3 Primary Standards and Reference Sources

• Formula: H2O ^[1] and CAS Registry Number: 7732-18-5 ^[1] are listed.
• Formula: H2O is listed ^[6].
• The document states Symbols used in this document: Tboil Boiling point ^[1].

The International Committee of Weights and Measures adopted the International Temperature Scale of 1990 at its meeting in 1989, in accordance with the request embodied in Resolution 7 of the 18th General Conference of Weights and Measures of 1987 ^[2]. According to the ITS-90 definition document, that scale supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 Provisional 0.5 K to 30 K Temperature Scale ^[2]. Both predecessors are named ^[2].

The kelvin is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, according to the ITS-90 definition document ^[2].

Table 1 in the ITS-90 definition document lists the triple point of water as a defining fixed point at T90 273.16 K and t90 0.01 °C with Wr equal to 1.00000000 ^[2]. T denotes the triple point at which the solid, liquid and vapour phases are in equilibrium ^[2].

The ITS-90 definition document relates International Kelvin Temperatures T90 and International Celsius Temperatures t90 by t90/°C = T90/K minus 273.15 ^[2]. That relation is equation (2) ^[2].

Practical reach runs from 0.65 K to the highest temperature practicably measurable in terms of the Planck radiation law using monochromatic radiation, according to the ITS-90 definition document ^[2].

Throughout its range, for any given temperature the numerical value of T90 is a close approximation to the numerical value of T according to best estimates at the time the scale was adopted, and measurements of T90 are more easily made, are more precise and are highly reproducible by comparison with direct measurements of thermodynamic temperatures, according to the ITS-90 definition document ^[2].

Ease and precision are claimed ^[2].

It also depends on exactly which calibration standard you use for your measurement equipment ^[9]. The freezing point of tin (231,9681 °C) became a permitted alternative to the boiling point of water ^[2]. The International Temperature "Scale" of 1990 is not a scale, it's an equipment calibration standard ^[9].

After 2007 it was defined to be based on Vienna Standard Mean Ocean Water (VSMOW), a precisely defined water standard, according to Webel ^[9].

Each sample of clear water will start boiling at the same temperature at a given terrain elevation, according to OmniCalculator ^[4]. Elevation dependence belongs to Altitude Pressure and Impurity Effects.

Reliable readings require a stable rolling boil and a fixed thermometer position, according to the Free Science Project protocol ^[13].

• That protocol states to always wait for water to reach a stable rolling boil before measuring and to keep the thermometer in the same location for consistent readings ^[13].
• A good digital cooking or candy thermometer that reads to 0.1°F is ideal for that measurement, according to the Free Science Project protocol ^[13].
• Sea salt, rock salt, or kosher salt will show similar results, though the exact temperature change might vary slightly due to different mineral content, according to the Free Science Project protocol ^[13].

Full magnitudes belong to Altitude Pressure and Impurity Effects.

4. Discussion

For publication pressure bases, see Background ^[11]. For the sea-level baseline and precise values, see Findings and below ^{[8][7][3][9][11]}. Taken together, ^[11] and ^[9] indicate the different publication pressure bases correspond to different boiling-point values.

At 2,438.4 metres (8,000 ft) in elevation, water boils at just 92 °C (198 °F) ^[8].

The general solution effect is covered in Altitude Pressure and Impurity Effects. The boiling-point elevation due to the presence of a solute is a colligative property related to the number of particles of solute in a solution and not related to the chemical composition of the solute ^[5][13]. The temperature increase is roughly proportional to the amount of dissolved salt ^[13]. One calculation shows the boiling point increased by 1.78°C to 101.78°C ^[5]. Taken together, ^[5] and ^[13] indicate larger amounts of dissolved particles produce larger increases in boiling temperature.

Why retain 100°C alongside IUPAC and CoolProp figures (3.1) ^[9][11]? See Background and earlier in Discussion for the pressure definitions and precise values. It also depends on exactly which calibration standard you use for your measurement equipment ^[9].

Water boiling at 99.3 °C (210.8 °F) at 215 m (705 ft) elevation ^[11].

5. Conclusion

Water boils at 100 °C, rounded from scientific precision of 99.97 °C, under standard pressure at sea level ^[11].

Taken together, ^[5][7][11] indicate water boils at 100°C at 1 atm (101.325 kPa) at sea level ^[5][7][11].

• Boiling points may be published with respect to the NIST, USA standard pressure of 101.325 kPa (1 atm), or the IUPAC standard pressure of 100.000 kPa (1 bar) ^[11].
• The IUPAC-recommended standard boiling point of water at a standard pressure of 100 kPa (1 bar) is 99.61°C (211.3°F) ^[11], with the standard boiling point defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of one bar ^[11].
• The actual 'pressure of the atmosphere' at sea-level varies above and below that according to location and countless weather influences ^[9], the boiling point of water is lower at higher altitudes due to the decreased atmospheric pressure ^[8], and when anything is dissolved in water, the solution will boil at a higher temperature than pure water would ^[5].

Name the pressure and water obliges: 99.97°C at one atmosphere, 99.61°C at one bar ^[11].

References

^[1] Water — https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Type=TBOIL · government ^[2] https://www.nist.gov/system/files/documents/pml/div685/grp01/ITS-90_metrologia.pdf — https://www.nist.gov/system/files/documents/pml/div685/grp01/ITS-90_metrologia.pdf · government ^[3] Boiling Point — https://terpconnect.umd.edu/~wbreslyn/chemistry/pressure/boiling-point.html · academic ^[4] Boiling Point at Altitude Calculator — https://www.omnicalculator.com/chemistry/boiling-point-altitude · general ^[5] 13.9: Freezing Point Depression and Boiling Point Elevation — https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(LibreTexts)/13%3A_Solutions/13.09%3A_Freezing_Point_Depression_and_Boiling_Point_Elevation · academic ^[6] Water — https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=4 · government ^[7] 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 ^[8] High-altitude cooking — https://en.wikipedia.org/wiki/High-altitude_cooking · general ^[9] Fun fact: The normal boiling point of water isn't exactly 100 °C (at least not since 2019 when the definition of the Kelvin scale was changed to use the Boltzmann constant and decoupled from the triple point of water) — https://www.webel.com.au/node/3553 · general ^[10] Boiling points | Science | Research Starters | EBSCOhost — https://www.ebsco.com/research-starters/science/boiling-points/ · general ^[11] Boiling point — https://en.wikipedia.org/wiki/Boiling_point · general ^[12] The Boiling Point of Water Calculator — https://www.thermoworks.com/blogs/learning-center/boiling-point-calculator?srsltid=AfmBOoo3rZnIPmcbvUPuJWzlSlpfMqfNqYTjmpIlRB9hc7-1V-uAWsK9 · general ^[13] Salt Water Boiling Point - How Does Table Salt Affect Water's Boiling Temperature? — https://freescienceproject.com/projects/Saltwater/ · general

Source quality: 2 academic, 3 government, 8 general.