If I mix two chemicals in a test tube and notice after a few seconds that there are ice crystals forming on the outside of the test tube, I am witnessing a(n) *

endothermic reaction

Explanation:

It simply means that you are witnessing an endothermic reaction.

An endothermic reaction is one that absorbs heat energy from its surrounding, thereby leaving the reaction vessel with a lower temperature as compared to before the reaction.

It is as opposed to exothermic reactions which are reactions that give off energy in the form of heat to the surrounding, thereby leaving a reaction vessel warmer than before the reaction.

In this case, the formation of ice crystals outside the test tube means that heat energy has been absorbed by the reaction which leaves the vessel a temperature cold enough to activate the formation of ice.