Use 'gaseous' to describe one of the main forms matter takes, different from solids and liquids. Oxygen is typically in a gaseous state.
Pro Tip 2/3
Science Context
The term 'gaseous' is often used in scientific contexts to discuss chemistry and physics topics. In our science class, we learned how gaseous nitrogen is used in food packaging.
Pro Tip 3/3
Visibility
'Gaseous' doesn't mean the substance is always visible; many gases are clear. The gaseous vapors escaping from the tube were invisible.