Why Water Freezes at 0°C and Boils at 100°C — It's Not a Coincidence

It's no coincidence that water freezes at exactly 0°C and boils at exactly 100°C. The temperature unit itself was built around water.

1. Does water really freeze at 0°C and boil at 100°C?

At 1 atm (101.325 kPa), almost exactly yes. Pressure changes that.

Boiling Point by Altitude (Natural Environments)

Location	Altitude	Pressure	Boiling point
Dead Sea	-430m	1.06 atm	101.4°C
Death Valley	-86m	1.01 atm	100.3°C
Seoul, Jakarta (sea-level cities)	0~50m	1.00 atm	100.0°C
Mexico City	2,240m	0.76 atm	92°C
Lhasa (Tibet)	3,650m	0.65 atm	88°C
Himalayan base camp	5,000m	0.50 atm	83°C
Mount Everest summit	8,848m	0.33 atm	71°C

Key: Low-altitude changes are tiny; high-altitude changes are dramatic. Atmospheric pressure decreases exponentially with altitude due to the weight of air above. Even at the Dead Sea (Earth's lowest natural point), boiling point rises only +1.4°C. But climb 5,000m and it drops 17°C.

Boiling Fast ≠ Cooking Fast

"Boiling" just means bubbles form — not that the water is hot. Water boiling at 83°C cooks less effectively than at 100°C (reaction rates roughly halve per 10°C). Noodles at altitude won't cook well no matter how long you boil them — starch doesn't gelatinize properly. See Boiling ≠ Hot for details.

Artificial Pressure — Much Bigger Differences

Natural altitude variation tops out around 30°C, but artificial environments push much further.

Environment	Pressure	Boiling point
Vacuum chamber (lab)	0.023 atm	20°C (room temp!)
Mount Everest summit	0.33 atm	71°C
Sea level	1 atm	100°C
Pressure cooker	1.5~2 atm	120~127°C
Industrial boiler	80+ atm	300°C+
Supercritical power plant	220+ atm	("boiling" ceases to exist — supercritical state)

• Pressure cooker: 121°C is the standard for autoclave sterilization. Cooks 1.5-2× faster

• Vacuum chamber: room-temp water can be made to boil. Basis of freeze-drying

• Supercritical state: above 374°C and 220 atm, liquid/gas distinction vanishes. Used in some power plants and chemical processes

Freezing Point Also Shifts (a Bit)

Freezing point is far less sensitive to pressure. Going from 1 atm to 100 atm drops it only ~-0.7°C. But this small effect lets glacier bases melt slightly under their own weight and flow. That's why glaciers can be said to "flow" at all.

2. The History of the Celsius Scale

1742, Swedish astronomer Anders Celsius (1701-1744) published a scale using water's freezing and boiling points as anchors, divided into 100 parts.

Fun fact: Celsius originally defined it backwards:

• 0°C = boiling point of water

• 100°C = freezing point of water

The "0 = ice, 100 = boil" we use today was flipped by his colleague Carl Linnaeus in 1745. Yes, that Linnaeus — the famous botanist also dabbled in thermometry.

So the unit itself was invented using water, not the other way around.

3. Other Scales Are Also Arbitrary

Scale	0 anchor	100 anchor	Inventor
Celsius (°C)	Water freezing	Water boiling	Celsius (1742)
Fahrenheit (°F)	Lowest-freezing brine	Near human body temp (as measured then)	Fahrenheit (1724)
Kelvin (K)	Absolute zero (-273.15°C)	(N/A — 1K = 1°C step)	Kelvin (1848)
Rankine (°R)	Absolute zero	(N/A — Fahrenheit-step)	Rankine (1859)

Fahrenheit was almost ad hoc. He supposedly set 0°F as the lowest brine freezing temperature he could measure, and 100°F as a healthy person's body temperature (his measurement was actually 96°F; later standardized at 98.6°F).

4. The 2019 SI Redefinition — No Longer Water-Based

Big news: On May 20, 2019, Kelvin was redefined based on Boltzmann's constant (k = 1.380649 × 10⁻²³ J/K). The water triple point is no longer the reference.

Celsius is now a derived unit: °C = K − 273.15.

So in modern SI, water doesn't freeze at exactly 0°C — it freezes "very close to it". The error is around 10⁻⁷, so daily use is unaffected.

Why change?

Water's freezing and boiling points shift slightly with isotopic composition (heavy water content), impurities, and pressure. For industrial and scientific precision measurements, that wobble is a problem. Boltzmann's constant is a universal physical constant — same everywhere in the universe.

Same trend as redefining the second from "1/86400 of an Earth day" to "9,192,631,770 cycles of cesium". Moving from human-scale anchors to universal ones.

5. Triple Point — A More Precise Water Anchor

Before 2019, Kelvin was defined by the triple point of water — the single pressure/temperature where water coexists as solid, liquid, and gas:

• Pressure: 611.657 Pa (~0.006 atm)

• Temperature: 0.01°C = exactly 273.16 K

The old Kelvin was defined as "1/273.16 of the triple point temperature." Note the triple point (0.006 atm) differs slightly from the standard 1-atm freezing point — hence the 0.01°C gap.

6. What About Absolute Zero (-273.15°C)?

-273.15°C = 0 K. Theoretical temperature where molecular motion stops. Unreachable experimentally (quantum zero-point energy).

The lowest temperature humans have achieved is around 38 picokelvin (38 × 10⁻¹² K) — MIT, 2003, using a Bose-Einstein condensate. Practically touching absolute zero, but never reaching it.

One-Line Summary

Water doesn't elegantly align with 0 and 100 — Celsius divided water's two phase transitions into 100 parts and called it "Celsius".

If he'd used 200 parts, water would boil at 200°C today. With 50 parts, 50°C.

And since 2019, even that anchor has shifted to Boltzmann's constant. Strictly, water now freezes "very close to" 0°C, not exactly. In daily life — close enough.