From Sodium amide, Wikipedia:

Sodium amide can be prepared by the reaction of sodium with ammonia gas, but it is usually prepared by the reaction in liquid ammonia using iron(III) nitrate as a catalyst. The reaction is fastest at the boiling point of the ammonia, c. −33 °C.

Shouldn't it be "at the boiling point of ammonia", since it must have a constant boiling point (for each particular pressure)? Or is "the ammonia" okay here because the author had in mind "that particular ammonia, under the pressure used in that particular reaction"?

Maybe we can either use or omit the definite article here, depending on what we intend to underscore?

  • the there denotes aforementioned, as in an aforementioned ingredient when a recipe is actually followed: "... the chocolate chips may be gooey on a hot day..."
    – TimR
    May 3, 2016 at 11:40
  • Also compare this not so eloquent example: Put 2 quarts of water in a pot. Bring (+) water to almost boiling, because you can't make good tea at the boiling point of (the) water. If you use the you're referring back to the water in the pot. If not, to any water. Meanwhile, the lack of the definite article at (+) is due to condensed writing (aka, I think "note taking English"), such as in a recipe, set of instructions, a to-do list, and, of course, notes on a lecture, etc, when articles are often omitted. May 3, 2016 at 13:40

1 Answer 1


By inserting the word the at that point the sentence is referring to "that particular ammonia". In particular it means "liquid ammonia using iron(III) nitrate as a catalyst", so combined it means: "The reaction of sodium with liquid ammonia using iron(III) nitrate as a catalyst is fastest at about −33 °C". The catalyst is important here, as without it, the sodium would just dissolve in the ammonia to make a solution (if I'm remembering correctly).

The use of the circa is a reference to the boiling point being variable with pressure, as you noted; atmospheric pressure and vessel pressure being a variable.

This reminds me of how much fun I had doing chemistry with anhydrous liquid ammonia as a student!

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