When potassium is dropped into water, it reacts with the water to form potassium hydroxide and hydrogen gas. The reaction is highly exothermic, meaning that it releases a lot of heat. This heat causes the hydrogen gas to ignite, which is why the potassium ignites and burns.

The overall chemical equation for this reaction is:

2K + 2H2O → 2KOH + H2

This reaction is a chemical change because the starting materials (potassium and water) are transformed into new products (potassium hydroxide and hydrogen gas).