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Does water decompose?
Yes, at 1,000C.
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.