The Coldest Cold: Absolute Zero

Why it matters

A Himalayan winter night touches −30°C; deep space sits near −270°C. But cold has a hard floor. At −273°C molecular motion itself would cease — nature allows nothing colder. That floor is absolute zero, and a whole temperature scale is built on it.

Predict first

What is so special about −273°C?

Cooling a substance slows its molecules down. At −273°C — absolute zero, or 0 K — molecular motion would stop entirely. It is a theoretical concept: all gases liquefy or solidify before reaching it, and temperatures below it are impossible.

• Molecular motion would completely cease there — no lower temperature is possible — correct
• It is just the temperature where all gases freeze; colder temperatures still exist
• It is simply the coldest temperature ever recorded on Earth

What you do

Match each Celsius temperature with its kelvin twin. One rule does all the work: K = °C + 273. (Spot the landmarks: absolute zero, the ice point and the steam point.)

Check yourself

At absolute zero (0 K), the molecules of a substance would…

Average kinetic energy is proportional to absolute temperature, so at 0 K molecular motion would cease entirely. It is theoretical — practically this temperature can never be attained, and every gas liquefies or solidifies first.

Normal body temperature is 37°C. On the Kelvin scale that is…

Kelvin (K) = °C + 273, so 37 + 273 = 310 K. Note that no degree sign is written with kelvin values, and each kelvin is exactly the same size as one Celsius degree — only the zero point differs.

Why are there no negative temperatures on the Kelvin scale?

The Celsius zero (the ice point) is arbitrary, so negatives exist below it. The Kelvin scale plants its zero at −273°C, where molecular motion would cease, so every possible temperature is a positive kelvin value — one big reason gas-law calculations use this scale.

• stop moving altogether — molecular motion ceases (a theoretical concept) — correct
• keep vibrating just as vigorously as at room temperature
• turn into pure energy and vanish
• 310 K — correct
• 236 K
• 37 K
• Its zero is absolute zero, the lowest possible temperature — nothing lies below it — correct
• Negative numbers are simply not permitted in scientific units
• Kelvin degrees are too large for readings to go below zero