How can we determine if the enthalpy of solution of a compound is positive or negative?
In order to make a clear idea about what I mean, take the following problem:
A 3.5 g sample of $\ce{NH4NO3}$ was added to 80. mL of water in a coffee cup calorimeter of negligible heat capacity. As a result, the temperature of solution decreased from $21.6 \ ^\circ\text{C}$ to $18.1\ ^\circ\text{C}$. Calculate the enthalpy change of the solution ($\Delta H_{\text{sol}}$ ). (specific heat of the solution = $4.18 \frac{\text{J}}{\text{g}\cdot ^\circ\text{C}}$; density of water $1.00 \frac{\text{g}}{\text{mL}}$)
Answer: $+28 \frac{\text{kJ}}{\text{mol}}$
Why it is positive despite there is a decrease in temperature?