Is exploding of potassium metal when placed in water a physical or chemical change?

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

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it forms an anion that has a larger radius. anions have a negative charge and are larger than the original atom.