Does water decompose?

Decompose is different than boil/evaporate:

boil/evaporate: phase change into a gas. Water in gas form is still water (H2O)

decompose: breaks down into constituent smaller part. 2* H2O is broken down into 2* H2 and an O2 molecule.

Basically they're two very different things and it makes sense they happen at different energy levels.

Also: significant water decomposition happens at much higher temperatures. At 2200c only around 3% of water in a given volume will decompose. At 3000c around half will decompose.