Reaction
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 noun.
The reaction was acidified,
causing the release of carbon dioxide.
Countable
Noun
Release is
a countable noun. Also, it is assumed that the writer is referring
to a specific release (the one occurring in this experiment)
and thus the definite article the precedes the noun.
Next, the carbon
dioxide was evacuated from the flask.
Countable
Noun
Flask is
a countable noun. Also, it is assumed that the writer is referring
to a specific flask (the one in this experiment) and thus the
definite article the precedes the noun.
The gels were composed of two different concentrations:
8% and 12%.
Countable
Noun
The noun gel is a countable
noun and has already been identified to the reader in the previous
sentence. Thus, the definite article the precedes the
noun.
The 8% gels were placed in ___ cold room overnight.
Countable
Noun
The noun phrase 8%gel
is countable and has already been identified to the reader
in the previous sentence. Thus, the definite article the
precedes the noun.
The 8% gels were
placed in [a or the] cold room overnight.
Countable
Noun
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.
Some investigators have not yet enrolled enough patients
in ___ clinical trial.
Countable
Noun
The noun investigators
is a countable noun. The investigators have not been identified
yet for the reader. Thus, some is used instead of the
before the noun.
Some investigators
have not yet enrolled enough patients in the clinical
trial.
Countable
Noun
The noun clinical trial
is a countable noun. It is assumed that the writer is referring
to a specific trial, so the article the is used before
the noun.
The tardy investigators will be notified by ___ fax.
Countable
Noun
The noun investigators is a countable noun. This noun has already been referred tos in the paragraph (and thus the identity is known to the reader). The definite article the precedes the noun.
The tardy investigators
will be notified by ___ fax.
Trick
Question: No Article!
Although the noun fax
is a countable noun, in this example it does not require an article.
For certain idiomatic expressions, like by mail, by boat,
by plane, no article is used before the noun.