IGCSE 0625 / Section 2.1 / Core

Counting from absolute zero.

The Celsius scale was pinned to water: zero for freezing, one hundred for boiling. The kelvin scale starts somewhere more fundamental, at absolute zero, the coldest anything can ever be, where particle motion all but stops.

The key idea

A temperature in kelvin equals the Celsius temperature plus 273. The kelvin scale begins at absolute zero (0 K, about minus 273 degrees Celsius), the lowest temperature possible, where the particles have their minimum energy.

Definition · learn the exact words

A temperature in kelvin (K) equals the temperature in degrees Celsius plus 273. Absolute zero is the lowest possible temperature, 0 K.

T (K) = θ (°C) + 273

There is no temperature below 0 K, so a kelvin value can never be negative.

Section 01

Slide between the scales.

Move the temperature and watch the Celsius and kelvin readings change together.

Stage 1 · Learn

Check temperature scales

Four quick checks. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

To convert a temperature in degrees Celsius to kelvin you...

Quick check+10 XP

Absolute zero is...

Quick check+10 XP

A temperature of 0 degrees Celsius is equal to...

Quick check+10 XP

At absolute zero the particles have...

Section 02

Converting, and the floor.

Going from Celsius to kelvin is a single step, and the scale has a hard lower limit.

The conversion, with a worked example:

T (K) = θ (°C) + 273
Worked example

Convert 25 degrees Celsius to kelvin.

Step 1 · Write the ruleT = θ + 273
Step 2 · SubstituteT = 25 + 273
Step 3 · AnswerT = 298 K
Examiner trap

The kelvin scale starts at absolute zero, so a kelvin temperature can never be negative. After converting, always check your answer is not below 0 K; if it is, something has gone wrong.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

A temperature of 27 degrees Celsius expressed in kelvin is...

Exam style+20 XP

A gas is at 500 K. Its temperature in degrees Celsius is...

Exam style+20 XP

Which of these is the coldest temperature?

Skill unlocked

Temperature scales, mastered.

Particle model nearly complete. Keep the chain going.

-Rank -Level -Score -Topics
Go deeper · practice
Six original Cambridge-style questions
Converting between Celsius and kelvin, the meaning of absolute zero, and comparing temperatures across both scales. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Thermal physics challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just temperature scales.
Start the unit challenge →