When it comes to room temperature, it returns to a gaseous state. Water has nothing to do with this. The release of CO2 when immersed in water causes it to appear to boil. This is exactly the same CO2 coming off of the frozen piece as when it's surrounded by only air.
In a bucket, over the plants is the best positioning.
CO2 is heavier than air. It will "fall" towards the plants from above.