Barium Oxide

Barium oxide, BaO, is a white hygroscopic compound formed by the burning of barium in oxygen, although it is often formed through the decomposition of other barium salts.

2Ba + O2 → 2BaO
BaCO3 → BaO + CO2

It reacts with water to form barium hydroxide.

BaO + H2O → Ba(OH)2

Read more about Barium Oxide:  Uses, Preparation, Safety Issues