點解NH4Cl溶於水會係吸熱反應?

The dissolution of NH4Cl is a endothermic reaction because energy is required to break down the ionic bond between NH4+ and Cl-. It is a spontaneous reaction, i.e. soluble in water because the entropy gain (1 molecule divide to 2 ions) is greater than the enthalpy loss and hence the final Gibbs free energy is negative. i.e. DH > 0, DS > 0, but DG = DH-TDS < 0.