Why is aqueous ammonia alkaline

All aqueous solutions naturally contain hydroxide ions in consequence of the auto-protolytic reaction in Equation (6.2). As we have seen, there will be equal numbers of solvated protons and solvated hydroxide ions unless we add an acid or base to it. A solution containing more solvated protons than hydroxide ions is said to be an acid within the Lowry-Brpnsted theory, and a solution comprising more hydroxide ions than solvated protons is said to be a base. 

But a word of caution species other than metal hydroxides can act as bases. Ammonia is such an example, since it can abstract protons in aqueous solution according to 

To abstract a proton is to remove only the proton. The substantial extent of dissociation in Equation (6.11) helps explain why aqueous ammonia is more properly called ammonium hydroxide , NH4OH. We generate the solvated hydroxide ion OH (aq) by abstracting a proton from water. The OH (aq) ion in Equation (6.11) is chemically and physically identical to the solvated hydroxide ion generated by dissolving NaOH or KOH in water. 

We say a proton is abstracted when removed selectively. Similarly, we call a selective summary or precis of a piece of prose an abstract.  

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