Phosphorous is an incendiary weapon, used for marking targets and smokescreens.  You can use it against personnel and it is extremely terrifying.  Chemical weapons are ussually considered the nerve agents blister agents, choking gases and a few others.  The concept is they don't harm you through kinetic or thermal energy but through interruption of organ or cellular sytems at the chemical level.  Willy Pete is a nasty SOB though, and I am not advocating its use on civilians only arguing the difference.

Although did the British have bombers in 1920-22?

Wikipedia is written by anyone with an internet hookup.  Lots of great information marbled with disinformation and mistakes.

http://www.abc.net.au/news/newsitems/200511/s1508598.htm

here is an article on WP use.

White phosphorous is often used as an incendiary weapon, but  it serves well enough as a lethal purely chemical burn agent by creating phosphoric acid in the skin, and the smoke is used as a primarily nonlethal chemical weapon at high concentrations - it's used in Iraq to flush out tunnels.

They had these aircraft before 1918.

If Phosphorous isn't bad enough, mustard gas was also used on the Kurds by the RAF during the 20's - 30's.
"Although did the British have bombers in 1920-22?"

I've read accounts of the Brits experimenting with bomb throwers in 1914 (and they were very pleased with the results).  I have no idea how the Brits delivered whatever chemical weapons they may have used, but it doesn't seem like planes were necessary to accomplish the task.