Asubstance spontaneously explodes when exposed to other substances.
is this a chemical or physical reaction

Chemical Reaction

Explanation:

This is an example of a chemical reaction because it involves the rearrangement of the molecular structure of the substance.

fire could start, buildings collapsing, and ash falling from the sky.

explanation:

this is because the buildings or plants could catch on fire and the buildings would most likely collapse. lastly ash would fall because all the burning would cause ash to fall from the sky.

hope this .