# How could gas-density measurements help to establish that PCl5 vapor is decomposing?

## Phosphorus pentachloride is a white solid that sublimes (vaporizes without melting) at about 100 degrees C. At higher temperatures, the PCl5 vapor decomposes to give phosphorus trichloride and chlorine. How could gas-density measurements help to establish that PCl5 vapor is decomposing?

The average gas-density of the air(?) around the reaction will increase as $P C {l}_{3} \mathmr{and} C {l}_{2}$ are added to it.
Of course, that will not necessarily be conclusive evidence that the $P C {l}_{5}$ is decomposing , instead of just subliming. A specific test for chlorine in the surrounding gas would be more conclusive.