Final answer:
The chemical reaction between barium hydroxide and ammonium chloride is an endothermic reaction, as it absorbs heat from the surroundings causing water to freeze.
Step-by-step explanation:
The reaction between barium hydroxide and ammonium chloride is an endothermic reaction. This is evident from the described scenario where water freezes into slush due to the reaction, indicating that heat is absorbed from the surroundings. An immediate consequence of this absorption of heat is that the temperature of the surroundings decreases, causing water to freeze, which is a classic characteristic of an endothermic reaction.
For example, in a reaction where barium hydroxide is mixed with ammonium thiocyanate, which is similar in type, the solution becomes so cold that it can freeze water placed under the flask. This behavior is consistent with the definition of an endothermic process where the system absorbs heat, causing the surroundings to cool down.