Ammonia is a base by this same criterion, even though the hydroxide ions come from the solvent water molecules and not from the ammonia. Hydroxide ions are reactive, and will attack other polar molecules at positions where they carry a local positive charge. (For this reason, many chemical reactions that take place very slowly in a neutral water solution will proceed quite rapidly in the presence of a base.) Bases have a slippery feel because the hydroxide ions attack the oils of the skin and convert them to soap. Ammonia does the same thing to fats and greases, which is why a weak ammonia solution is a useful household cleaner. Bases turn red litmus paper (a common acid-base indicator) blue.