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5.3.2Chemical Energetics

Absolute Zero — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 12 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Absolute Zero and the Kelvin Scale

5.3.2 Absolute Zero

The kelvin scale is built on the lowest temperature that matter can reach.

Absolute zero
  • The lowest possible temperature, at which particles have their minimum possible energy.
Value
  • Absolute zero is 0 K, which is equal to −273 °C on the Celsius scale.
Kelvin scale
  • The kelvin scale starts at absolute zero, so temperatures on it are never negative.
Size of a unit
  • One kelvin is the same size as one degree Celsius, so temperature changes are equal on both scales.

Part 2 of 5Particle Behaviour on Cooling

Cooling removes kinetic energy from particles, which has a natural limit.

Particle Behaviour on Cooling
Cooling
  • As the temperature falls, the average kinetic energy of the particles decreases.
Gases
  • At low temperatures a gas condenses into a liquid before absolute zero is reached.
Limit
  • Particles cannot have negative kinetic energy, so there is a lowest possible temperature.
Practical limit
  • Scientists have cooled matter to within a tiny fraction of a kelvin of absolute zero, but never reached it.

Part 3 of 5Converting Temperatures

Use these rules for every conversion between the two scales.

1
To convert from degrees Celsius to kelvin, add 273 to the temperature in degrees Celsius.
2
To convert from kelvin to degrees Celsius, subtract 273 from the temperature in kelvin.
3
Always use kelvin in gas law calculations that involve ratios of temperatures.
4
Check that the kelvin value is never negative, because a negative value shows a conversion error.

Part 4 of 5Worked Example

Question Convert 25 °C into kelvin.

  1. T (K) = θ (°C) + 273.
  2. T = 25 + 273.
  3. T = 298 K.

Answer 25 °C is equal to 298 K.

Part 5 of 5Exam Checklist & Tips

5.3.2 – Key Points for the Exam

  • Absolute zero The lowest possible temperature, equal to 0 K or −273 °C.
  • Kelvin The SI unit of temperature, with a unit size equal to one degree Celsius.
  • Celsius A temperature scale with 0 °C at the freezing point of water and 100 °C at its boiling point.
  • Minimum energy The lowest energy that particles can have, which is reached at absolute zero.

Common Mistakes

  • Subtracting 273 when converting from Celsius to kelvin instead of adding 273.
  • Writing a kelvin temperature with a degree symbol, for example 298 °K instead of 298 K.
  • Writing 0 °C as 0 K; 0 °C is equal to 273 K.
  • Stating that a temperature below 0 K is possible; no temperature lower than absolute zero exists.

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