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Before this: Unit conversions

Pressure and temperature

Gas particles move fast in straight lines and collide with each other and with the walls of their container. This picture is the kinetic molecular theory.

Pressure is the force of those collisions per unit of area. More collisions, or harder ones, mean higher pressure.

1 atm = 760 mmHg = 101.3 kPa

Temperature measures the average kinetic energy of the particles. Gas calculations need the Kelvin scale, which starts at absolute zero, the lowest possible temperature.

K = °C + 273

Standard temperature and pressure (STP) is 273 K and 1 atm.

Worked example: convert 25 °C to kelvins

25 + 273 = 298 K

Worked example: convert 1.25 atm to mmHg

1.25 atm × (760 mmHg / 1 atm) = 950 mmHg

Common mistake

Using Celsius in gas laws. A gas at 20 °C is not twice as hot as one at 10 °C. In kelvins those are 293 K and 283 K, a difference of only a few percent.

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Watch Why heating raises the pressure
  1. Gas particles hit the walls. That is pressure: 1.00 atm at 300 K.
  2. Heat the gas to 450 K with the piston locked. Faster particles hit harder: 1.50 atm.
  3. At 600 K, twice the starting temperature in kelvins, the pressure has doubled to 2.00 atm.
  4. That only works in kelvins. 300 K is 27 °C and 600 K is 327 °C, which is not double.

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Practice

1. Convert 100 °C to kelvins.

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2. Convert 0.500 atm to mmHg.

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3. Convert 202.6 kPa to atm.

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4. What happens to the particles of a gas when its temperature rises?

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