Here's why:

* Exothermic reactions release heat into the surroundings, causing the temperature to rise.

* Endothermic reactions absorb heat from the surroundings, causing the temperature to drop.

When sodium reacts with water, it produces a large amount of heat, often causing the sodium to melt and the hydrogen gas produced to ignite. This vigorous reaction is a clear indication of an exothermic process.

The chemical equation for the reaction is:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + heat

This reaction releases a significant amount of energy, making it highly exothermic.