volatile

Synonyms: airy buoyant capricious effervescent elastic elusive ephemeral erratic expansive fickle fleeting flighty flippant frivolous fugacious fugitive gaseous gay giddy impermanent imponderable inconsistent inconstant light lively lubricious mercurial momentary playful resilient short-lived sprightly subtle temperamental ticklish transient transitory unsettled unstable unsteady up-and-down vaporous variable whimsical

Antonyms: calm firm stable steadfast