Water’s freezing point on the four scales
Pure water freezes at:
- 0°C (Celsius)
- 32°F (Fahrenheit)
- 273.15 K (Kelvin)
- 491.67 °R (Rankine)
One event, four anchors
That single physical event anchors the scales: Celsius defines it as zero, Kelvin sits exactly 273.15 above its absolute-zero origin, and Fahrenheit reaches it 32 degrees above his brine-based zero. Only Rankine treats it as an unremarkable 491.67.
Strictly speaking this is the melting point too - freezing and melting are the same equilibrium crossed in opposite directions. Jump between any pair of scales with the Fahrenheit to Celsius and Kelvin to Celsius converters.
Freezing and melting points across scales
The table runs from cryogenic gases that only solidify in a lab to metals that stay solid inside a furnace - the same physics, five hundred degrees of Kelvin apart.
| Substance | Celsius | Fahrenheit | Kelvin | Rankine | Notes |
|---|---|---|---|---|---|
| Nitrogen | -210°C | -346°F | 63.15 K | 113.67 °R | Solid nitrogen exists only in cryogenics labs |
| Ethanol | -114.1°C | -173.38°F | 159.05 K | 286.29 °R | Stays liquid in any household freezer |
| Mercury | -38.83°C | -37.89°F | 234.32 K | 421.78 °R | Mercury thermometers fail below this |
| Water | 0°C | 32°F | 273.15 K | 491.67 °R | The anchor point of the Celsius scale |
| Table salt | 800.7°C | 1,473.26°F | 1,073.85 K | 1,932.93 °R | |
| Gold | 1,064.18°C | 1,947.52°F | 1,337.33 K | 2,407.19 °R | Goldsmiths have relied on this point for millennia |
| Iron | 1,538°C | 2,800.4°F | 1,811.15 K | 3,260.07 °R | |
| Tungsten | 3,414°C | 6,177.2°F | 3,687.15 K | 6,636.87 °R | Highest melting point of any metal |
Why freezing points shift in the real world
Dissolve anything in water and its freezing point drops - that is freezing point depression, and it is why road salt keeps highways ice-free down to about 15°F (-9°C) and why seawater freezes near 28.4°F (-2°C) instead of 32°F. Antifreeze pushes a car radiator’s freezing point below -30°F the same way.
Perfectly clean, still water can also supercool: with nothing for ice crystals to grow on, it stays liquid well below freezing - cloud droplets routinely ride along at -40° before flashing to ice. That number is the one temperature where Celsius and Fahrenheit agree: -40°C = -40°F. For what your appliances should read, see the refrigerator and freezer temperature guide, or head to the other anchor point in boiling points across temperature scales.
Why -40 is the magic crossover number
Celsius and Fahrenheit are straight lines with different slopes and offsets, so they must cross exactly once. Set °F = °C in the conversion formula and the algebra lands on a single point: -40°C = -40°F. No degree symbol dispute survives at that temperature - polar researchers just say “minus forty.”
It is also a physically meaningful marker: near -40, supercooled cloud droplets freeze spontaneously without any nucleus, mercury has long since frozen solid (at -38°F), and standard automotive antifreeze mixtures reach their design limit. Check the math yourself with the °C to °F converter.
Freezing point, melting point and a real sample can differ
For a pure substance at a stated pressure, freezing and melting describe the same solid-liquid equilibrium temperature. A real sample can behave less neatly. Dissolved material changes the transition, mixtures can freeze across a range, and a liquid can remain below its usual freezing point until crystals begin to form.
That distinction explains why the water row is a scale anchor but not a promise that every outdoor puddle, freezer tray or cloud droplet changes phase at precisely the same instant. Road salt lowers the freezing point of water; food contains dissolved material; supercooled cloud droplets can remain liquid below 0°C.
Convert the reference temperature only after choosing the correct material and condition. Celsius and Fahrenheit are convenient for everyday observations, while Kelvin and Rankine are absolute scales useful in physical calculations. Preserve source precision rather than presenting a rounded reference as a laboratory constant.