Ammonium hydroxide or solution of ammonia (NH4OH) is weak alkali because it produce the low concentration of hydroxyl ions in the aqueous solution( which dissolved in water) .
Sodium chloride stimulates the taste buds of our tongue that detects the salty taste. So, sodium chloride is salty
Acetic acid has a very low concentration of hydrogen ions ( H+) in its aqueous solution. So, acetic acid is called weak acid.
We know that alkalis are water soluble but bases are metallic oxides and hydroxide which may or may not be soluble in water( such as NaOH, KOH, Mg(OH) etc...) and they are alkalis where as other most of the bases are insoluble in water( such as HgO, CuO, etc...) which are only bases but not alkalis. Hence, all alkalis are bases but all bases are not alkalis.
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