The answer is chemical change
That is exploding of potassium metal when placed in water is chemical change.
When potassium is dropped into a beaker full of water then potassium moves about rapidly and then ignites within seconds, that is the metal is gone and all activity ceases.
So it an example of chemical change.
The reaction of potassium with water is as follows:
2K + 2H₂O ---->2KOH + H₂