An excellent demonstration of an endothermic reaction is the reaction between powdered barium hydroxide and ammonium chloride.
Mix equal quantities of ammonium chloride powder and barium hydroxide. Mix the reactants thoroughly with a spatula.
The reaction proceeds according to the reaction below. The temperature drops to -10 oC indicating the reaction to be an endothermic reaction.
Ba(OH)2 (s) NH4Cl(s) => H2O (l) + NH3(g) + BaCl2(aq)
The ammonia can be easily identified by smell but its presence can also be demonstrated by placing HCl next to the reaction vessel. Clouds of solid ammonium chloride will form(as shown on the left). The formation of water is obvious as the two powders form a wet paste.