Dry ice is solid carbon dioxide.

When dropped into warm water, it sublimes (It does this in air anyway) rapidly and produces ‘clouds’ of CO2 and water vapour that sinks due to CO2 being denser than air.

If you put universal indicator in the water, you can see it changing colour as the CO2 dissolves in the water to produce carbonic acid, a very weak acid.

Dry Ice in Indicator Solution

Dry Ice in Indicator Solution – Risk Assessment

Image: Still taken from video by Jobjabramon –  under the Creative Commons Attribution-Share Alike 4.0 International license