In chemistry, any substance that in water solution is slippery to the touch, tastes bitter, changes the colour of acid-base indicators (e.g., litmus paper), reacts with acids to form salts, and promotes certain chemical reactions (e.g., base catalysis). Examples of bases are the hydroxides of the alkali metals and alkaline earth metals (sodium, calcium, etc.; see caustic soda) and the water solutions of ammonia or its derivatives (amines). Such substances produce hydroxide ions (OH) in water solutions. Broader definitions of bases cover situations in which water is not present. See also acid-base theory; alkali; nucleophile.

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