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I just learnt about acids and bases at class.I noticed that my textbook mentions the term "alkali/alkaline" and sometimes it mentions "base/basic". As far as I know, alkalis and bases are metal oxides hydroxides. But why does my textbook make use of two words that mean the same thing instead of just one of them? Do these terms (alkali and base) mean something different?

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These words are in many cases interchangeably used: an alkaline solution/compound is the same as a basic solution/compound.

But you can specify "alkali" to refer to those bases that are soluble in water and that are salts of alkaline or alkaline earth metals too.

Alkali is like a subset of bases which fulfills these conditions.

E.g.:

  • Sodium hydroxide is a base, and also is an alkali metal salt, and is also soluble in water, so it is also an alkali.
  • Ammonia is a base, soluble in water, but not a salt, so it is not an alkali.
  • Magnesium hydroxide is a base, is a salt of an alkaline earth metal, but it is not soluble in water, so it is not an alkali.
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I think an alkali is a substance that dissolves in water to produce $\ce{OH- (aq)}$ ions. Since the $\ce{OH- (aq)}$ ions can react with acids, that makes alkalis bases as well. Therefore alkalis are soluble bases. Ammonia can also dissolve in water to form ammonium, $\ce{NH4+ (aq)}$ ions and $\ce{OH- (aq)}$, that is, it undergoes hydrolysis, in which case hydroxyl, $\ce{OH- (aq)}$ ions are produced. Therefore in that respect ammonia qualifies as an alkali, and we have to remember the very high solubility of ammonia in water as demonstrated in the fountain experiment. Sodium carbonate is an alkali because it also dissolves in water to form hydroxyl ions. Therefore, from this summary there are two very important criteria for compounds to be considered an alkali:

  1. It must dissolve in water
  2. It must produce $\ce{OH- (aq)}$ ions in the aqueous medium.
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