Michael S. answered • 08/05/26
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
"The speed of the particles in a gas is directly proportional to the Kelvin temperature."
That is the intended answer — it is the only option that points at a real postulate. The other four each contradict one.
Why each of the others is excluded
"The particles in a gas can be compressed" — read this one carefully. A gas is compressible, but that is because it is mostly empty space, not because the particles themselves squash. KMT treats the particles as having negligible — but fixed — volume. The gas compresses; the particles do not.
"Particles collide and emit energy to the surroundings" — the opposite of postulate 4, which says collisions are perfectly elastic and total kinetic energy is conserved. If energy leaked out at every collision, molecules would gradually slow and the gas would settle to the bottom of its container.
"Particles have a distinct volume measured in mL" — contradicts postulate 2, which assumes particle volume is negligible compared with the container.
"Particles attract one another via intermolecular forces" — contradicts postulate 3, which assumes no attractions or repulsions except during a collision. This assumption is exactly what fails at low temperature and gives rise to non-ideal behaviour.
One honest note on the wording of the correct option
The postulate is properly stated in terms of kinetic energy, not speed:
KEavg = (3/2)kT, so KEavg is directly proportional to T
Since KE = ½mv2, speed actually scales with the square root of temperature:
urms = √(3RT/M)
So "speed is directly proportional to T" is loosely worded — strictly, doubling the absolute temperature multiplies the average speed by √2, not by 2. It is still the answer here, because it is the only choice describing a genuine temperature dependence while the others are flat contradictions. But if you were marked down on a free-response question for writing it that way, that is why.
