is a countable noun. Also, it is assumed that the reader knows
which reaction the writer is referring to, as in the Methods
section of a paper, and thus the definite article the precedes
The 8% gels were
placed in [a or the] cold room overnight.
Cold room is a countable noun. Either a or the could
precede this noun, depending on the meaning of the sentence.
If the cold room is familiar to the reader, then the article
the is used. If the writer means to use the word cold room
generically--referring to any cold room (as in the Methods section
of a paper)--then the article a is used.