Chapter 3 · Matter and Energy
3.10Temperature: Random Motion of Molecules and Atoms
10 min · two checks
Predict
Which temperature is the same on both the Celsius and Kelvin scales’ “size” of one degree?
The idea
- Relate temperature to particle motion.
- Convert among °C, K, and °F.
Temperature measures the average kinetic energy of random particle motion, not the total heat stored in an object. A cup of boiling water and a bath of boiling water can share a temperature and hold very different amounts of energy. The kelvin is the SI temperature. Zero kelvin is the point at which that random motion is at its minimum; you do not reach a negative kelvin temperature in ordinary chemistry. A Celsius degree and a kelvin are the same size. Only the zero points differ: 0 °C = 273.15 K, so T = t + 273.15.
Fahrenheit is still encountered in weather and some kitchens. Water freezes at 32 °F and boils at 212 °F, so the Fahrenheit degree is smaller: a span of 180 °F matches 100 °C. The conversions are t(°F) = t(°C) × 9/5 + 32 and the reverse, t(°C) = (t(°F) − 32) × 5/9. Gas-law work always uses kelvin, because volume is proportional to absolute temperature, not to °C.
Keep these
- Temperature reflects average kinetic energy of particles, not total heat.
- K = °C + 273.15. Degree size is identical.
- °F = °C × 9/5 + 32. Gas laws need kelvin.
Worked path
Body temperature is 98.6 °F. Express it in °C and in K.
Observe
Given Fahrenheit. Want Celsius, then kelvin.