When water and ammonium nitrate are mixed, the temperature decreases because the reaction is endothermic. This means that the reaction absorbs heat from the surroundings in order to proceed. The heat is absorbed to break the bonds in the reactants (water and ammonium nitrate) and form the new bonds in the products (ammonium nitrate solution). Overall, the reaction results in a decrease in temperature.

Here is the chemical equation for the reaction between water and ammonium nitrate:

NH4NO3 (s) + H2O (l) → NH4NO3 (aq)

As you can see from the equation, the reactants (ammonium nitrate and water) are on the left side of the equation, and the products (ammonium nitrate solution) are on the right side. The heat is absorbed from the surroundings in order to break the bonds in the reactants and form the new bonds in the products.

The enthalpy change for this reaction is positive, which means that the reaction is endothermic. The enthalpy change is the amount of heat that is absorbed or released by a reaction. In this case, the enthalpy change is positive, which means that the reaction absorbs heat from the surroundings.

The decrease in temperature that is observed when water and ammonium nitrate are mixed is due to the fact that the reaction is endothermic. The heat that is absorbed from the surroundings in order to break the bonds in the reactants and form the new bonds in the products results in a decrease in temperature.