While doing certain laboratory tests for qualitative identification of cations, I found that on passing $\ce{H2S}$ in the solution, the temperature of the solution increased considerably. When the temperature decreased due to addition of $\ce{NH4Cl}$, the solution again attained room temperature in passing $\ce{H2S}$.
This rouses the question: is the dissolution of $\ce{H2S}$ in water exothermic? Or does it react with other components of solution to make an exothermic reaction?
The Wikipedia article stated only this:
Hydrogen sulphide is slightly soluble in water and acts as a weak acid (pKa = 6.9 in 0.01–0.1 mol/litre solutions at 18 °C), giving the hydrosulfide ion $\ce{HS^−}$. Hydrogen sulphide and its solutions are colourless.
The full experiment can be found here.