Why is NH3 (Ammonia) soluble in water?
Solubility of a substance in water depends on its polar nature. Since water is polar molecule, it dissolves readily polar molecules. As it is said in chemistry that ‘like dissolves like,, polar compounds dissolve polar molecules. The solubility of gas in water depends also on the temperature and pressure of the gas. Gases dissolve in water at high pressures and low temperatures. Ammonia is a gas and it is soluble in water. The solubility of ammonia in water determines the polarity of ammonia. Ammonia will be more soluble when the temperature of the solution is lower and the partial pressure of ammonia on water is higher. Hydrogen bonding aids in solubility of ammonia in water.
The water has two hydrogen atoms and one oxygen atom. The oxygen is more electro negative than hydrogen and hence the electrons are pulled towards the most electro negative element oxygen. This makes the oxygen to have partial negative charge and the hydrogen with partial positive charge. The same thing happens in the case of ammonia formation. The most electro negative element is nitrogen in ammonia compared to hydrogen. So the shared electrons are pulled towards the nitrogen and hence it possesses partial negative charge. The hydrogen possesses partial positive charge. This similar type of distribution of electrons in water and ammonia determines their polar nature.
As both water and ammonia are Polar, ammonia can get dissolved in water. As Oxygen is more electro negative than nitrogen, water acts as solvent here and not ammonia. Moreover, water is a universal solvent. The reason behind the solubility of ammonia in water is the ease in the formation of hydrogen bonds between them. The oxygen atoms in water and nitrogen atoms in ammonia form hydrogen bonds with the hydrogen in the opposite molecule. The formation of hydrogen bonds between these two molecules is responsible for the ammonia getting dissolved in water.