Well the simplest answer is..."because God wanted in that way.." As you know, the normal boiling points of water, 100 ""^@C, and ammonia, -33.3 ""^@C..are EXCESSIVELY HIGH....especially given these are molecules of paltry molecular mass, the which gives rise to negligible dispersion force. So why so? Clearly, there is an intermolecular force operating between the water and ammonia molecules ... More @Wikipedia
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