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Claim analyzed
Science“Pure water boils at 100 degrees Celsius at sea level (standard atmospheric pressure).”
The conclusion
Open in workbench →Authoritative reference sources support this statement as the standard boiling point of pure water at sea-level pressure. The exact thermodynamic value at 1 atm is slightly below 100.00 °C, but 100 °C is the accepted rounded value in general science and education. Differences involving 1 bar versus 1 atm are technical convention issues, not a practical refutation of the claim.
Caveats
- The exact boiling point at 1 atm is slightly under 100.00 °C, so the claim is accurate as a standard rounded value rather than an exact laboratory constant.
- Some technical references define a 'standard boiling point' at 1 bar instead of 1 atm; for water, that yields about 99.61 °C.
- Real boiling temperature varies with actual pressure, so altitude and weather can shift it away from 100 °C even near sea level.
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Sources
Sources used in the analysis
The NIST WebBook lists water’s normal boiling point as 373.17 ± 0.04 K. That corresponds to about 100.02 °C at standard atmospheric pressure, which is consistent with the claim that pure water boils at 100 °C at sea level.
The paper uses Po = 101325.0 pascals, which is one standard atmosphere, and To = 373.15 K, which is the absolute temperature assigned to the steam point at one standard atmosphere. This directly supports the standard boiling point of water at 1 atm.
NIST gives a reference value for the boiling point of water: "boiling point of water at 101 325 Pa = 373.1339 K". Converting 373.1339 K to the Celsius scale gives 99.9839 °C, which is often rounded to 100 °C in general use for the boiling point at standard atmospheric pressure.
The normal boiling point of a liquid is the temperature at which its vapor pressure is equal to one atmosphere (760 torr). A liquid boils at a temperature at which its vapor pressure is equal to the pressure of the gas above it. Under one atmosphere of pressure, water’s normal boiling point is 100 °C.
IUPAC defines the standard boiling point: "The temperature at which the vapour pressure of a liquid is equal to 1 bar." For water, IUPAC notes that this standard boiling point at 1 bar is 99.61 °C, which is slightly lower than 100 °C because 1 bar (100 kPa) is slightly less than 1 atmosphere (101.325 kPa).
Pure water boils at 212 degrees Fahrenheit (100 degrees Celsius) at sea level. But the boiling point of water is not a constant; it changes with altitude.
Thermodynamic data for water… Normal boiling point: 373.124 K. This corresponds to the temperature at which the vapor pressure of water equals 101.325 kPa (1 atm).
In listing physical data for water on Earth, NASA notes: "Boiling point of water at 1 atm: 100°C". This value is given as the boiling point corresponding to a pressure of 1 atmosphere, which is the standard atmospheric pressure at sea level used in physical data tables.
Thus, the normal boiling point for pure liquid water is 100 °C (373.16 K). The last statement is in agreement with our common experience in which water only boils when the temperature reaches about 100 °C, if the external pressure is 1 atm or very close to it.
Labster explains: "The normal boiling point is defined as the boiling point of a liquid at an external pressure of 1 atmosphere or 760 mm Hg. This is the boiling point of a liquid at sea level." It then gives the example: "For example, the normal boiling point of water is 100 °C." The page emphasizes that the actual boiling point changes with atmospheric pressure away from this standard.
The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid, and the liquid changes into a vapor. The table shows the water boiling temperature at different vacuum pressure… At 1013.3 mbar (standard atmospheric pressure), the water boiling point is 100 °C (212 °F).
The page states that for water, the boiling point is 100 degrees Celsius at a pressure of 1 atm, and that at sea level water remains at 100 degrees Celsius while boiling.
The boiling point of water is 100 °C (212 °F) at standard pressure. For purists, the normal boiling point of water is 99.97 degrees Celsius at a pressure of 1 atm (i.e., 101.325 kPa). Until 1982 this was also the standard boiling point of water, but the IUPAC now recommends a standard pressure of 1 bar (100 kPa).
Khan Academy notes: "For water, the normal boiling point is 100°C, which is the temperature at which the vapor pressure of liquid water equals 1 atm." The article clarifies that this is called the normal boiling point because it is defined at a pressure of 1 atmosphere, and boiling points change when external pressure changes.
The boiling point of water is 99.97 °C, or 211.95 °F, under standard pressure at sea level. Usually, you'll find that these values are rounded to 100 °C or 212 °F. This calculator has the values of P₁ and T₁ set to 1013.25 hPa and 100 °C, respectively. These values correspond to the normal atmospheric pressure at the sea level and boiling point of water.
Precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering to the current (since 2019) definition of the kelvin. The boiling point of water is no longer exactly 100 °C. The main reason is that 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.
The table shows water’s boiling point at an absolute pressure of 1.013 bara / 14.7 psia / 760 mm Hg / 29.92 in Hg as 100 °C and 212 °F.
At 1 standard atmosphere (101,325 Pa), the normal boiling point of pure water is 100 °C by definition in general chemistry; the exact thermodynamic value is about 99.97–100.0 °C depending on the temperature scale and reference formulation.
The normal boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure. At sea level the atmospheric pressure is one atm or 760 torr, and at 100 degrees Celsius the vapor pressure of water is 760 torr, so the normal boiling point of water at sea level is 100 degrees Celsius.
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The arguments
Two AI advocates debated this claim using the research gathered.
Argument for
Multiple independent authoritative references explicitly state that at standard atmospheric pressure (1 atm = 101,325 Pa, i.e., sea level) pure water's normal boiling point is essentially 100 °C: NIST lists 373.17 K (~100.02 °C) and 373.124 K at 1 atm (Source 1; Source 7), NIST's reference value at 101,325 Pa is 373.1339 K (= 99.9839 °C, routinely rounded to 100 °C) (Source 3), and USGS and NASA both present the sea-level boiling point as 100 °C (Source 6; Source 8). The tiny deviations from exactly 100.00 °C reflect modern scale definitions and rounding conventions rather than a different physical claim, and the core equivalence—water boils at ~100 °C when its vapor pressure equals 1 atm—is directly affirmed by NIST's steam-point assignment To = 373.15 K at 1 atm and standard chemistry definitions of “normal boiling point” (Source 2; Source 4).
The Proponent's argument relies on a fallacy of approximation, attempting to equate routine rounding conventions with scientific fact when precise measurements prove the actual boiling point is not exactly 100 °C. By citing sources like NIST and Webel IT Australia, it is clear that the true thermodynamic boiling point of water under one standard atmosphere is 99.9839 °C, meaning the claim of an exact 100 °C boiling point is false (Source 3, Source 16).
Argument against
The claim that pure water boils at exactly 100 degrees Celsius at standard atmospheric pressure is scientifically outdated and inaccurate. Precise measurements and modern thermodynamic standards demonstrate that the actual boiling point of water at one atmosphere is approximately 99.97 °C to 99.98 °C (Source 13, Source 15), with the National Institute of Standards and Technology specifically defining it as 373.1339 K, which converts to 99.9839 °C (Source 3, Source 16).
The Opponent equivocates between “exactly 100.00 °C” and the motion's ordinary-precision statement “100 °C at sea level,” ignoring that NIST itself reports the boiling point at 101,325 Pa as 373.1339 K (= 99.9839 °C) and also lists “normal boiling point” values like 373.17 K and 373.124 K at 1 atm—figures that are explicitly treated as consistent with 100 °C once rounding and reference conventions are applied (Source 3; Source 1; Source 7). The Opponent's reliance on secondary summaries to label the claim “outdated” (Source 13; Source 15; Source 16) does not rebut the core definition that at 1 atm water's boiling point is ~100 °C (steam point 373.15 K at 1 atm; normal boiling point 100 °C at 1 atm), which is directly supported by NIST and standard chemistry references (Source 2; Source 4; Source 6; Source 8).
Expert review
3 specialized AI experts evaluated the evidence and arguments.
Expert 1 — The Logic Examiner
The evidence from NIST and other scientific bodies demonstrates that the boiling point of pure water at 1 atm is approximately 99.97 °C to 100.02 °C, which is universally rounded to 100 °C in standard scientific and general contexts (Source 1, Source 3, Source 15). The opponent's argument relies on a fallacy of precision, falsely claiming that a standard physical approximation is 'false' because it does not match a hyper-precise thermodynamic value to the hundredth of a degree.
Expert 2 — The Context Analyst
The claim is framed as an exact, universal value, but it omits that the “normal boiling point” at 1 atm is only approximately 100 °C and depends on definitions/standards (e.g., NIST's 373.1339 K = 99.9839 °C at 101,325 Pa) and that “standard boiling point” is sometimes defined at 1 bar (giving ~99.61 °C) rather than 1 atm (Sources 3, 5, 13, 16). With that context restored, the statement is directionally correct for everyday/intro-chemistry rounding at sea level but misleading as written because it implies an exact 100.00 °C at standard pressure rather than ~100 °C under specified conventions.
Expert 3 — The Source Auditor
The highest-authority sources — NIST (Sources 1, 2, 3, 7), USGS (Source 6), NASA (Source 8), Purdue University (Source 4), and IUPAC (Source 5) — all confirm that pure water's normal boiling point at standard atmospheric pressure (1 atm = 101,325 Pa, i.e., sea level) is effectively 100 °C, with the precise thermodynamic value ranging from 99.9839 °C to 100.02 °C depending on measurement convention and scale definition. The claim as stated — '100 degrees Celsius at sea level' — is a well-established, universally accepted scientific fact at ordinary precision; the opponent's argument that it is 'false' because the exact value is 99.9839 °C conflates pedantic precision with the truth of the claim, and the sources supporting that nuance (Sources 13, 15, 16) are of lower authority and themselves acknowledge that 100 °C is the standard rounded value.