Ken's intuitions are sound. This usually indicates that the writer intends to pursue the topic, while that suggests that the topic is finished. Which form is correct in his example, then?
"Heat the water to 100 degree C. At (not in) (that/this) temperature water starts to boil"
This passage suggests that the writer is giving instructions for a process involving making the water boil., since it tells the reader, "Heat the water..." It is very likely that the instructions will say something further about the boiling temperature of water. Therefore the only appropriate form is this: At this temperature water starts to boil. The following sentence could be something like
Let it boil for at least twenty minutes
On the other hand, if the writer does not want the reader to let the water boil, the pronoun will be that:
Do not heat the water to 100 degrees C. At that temperature it would start to boil, and it must be kept below the boiling point...
This is not the whole story, however. Several more distinctions can be made.
The demonstratives this, that, these, and those are used as both determiners and pronouns. The principles underlying their use are the same whether we are talking about demonstrative determiners or demonstrative pronouns. We will confine these remarks to the two singular forms, this and that, but the principles apply to the plural forms as well.
Not surprisingly, the central distinction is one of literal or figurative distance from the speaker or writer. The most common and basic kind of distance is spatial, this being used for things that are physically close to the speaker, and that for things that are distant. These demonstratives also signal figurative distance (or lack of distance).
The closeness can be temporal, for example, referring to an imminent situation or event:
A: I'm not sure I want to go through with this interview I have today.
B; Nonsense, it's your big chance!
An event or situation that is over and done with, on the other hand, is represented by that:
A: You used to be an avid ice skater, didn't you?
B: Yes, but that's ancient history now. I haven't skated in years.
This is used to create psychological closeness to the speaker:
A: What do you think of this idea? We buy a fishing boat and convert it to a tour boat.
That, in contrast, is used to reject or dismiss an idea. B could answer, for example,
B: We couldn't possibly afford to do that. We don't have the capital.
Kaar* shows how the two demonstratives are often used in expository discourse. This is used when the reasoning for a statement is yet to come, while that is used when the reasoning has already been presented. Compare:
1) The correct object itself might still be selected...by the left hand using touch alone. This is because... the right brain is in control of...the left hand
2) The Soviets also know we would keep a second-strike insurance, and that is why these so-called defensive antiballistic weapons would be perceived by them as offensive
So the demonstrative this in discourse can be considered "open-ended," leading to further development, while that can be seen as "closed," something no longer in focus. This is also a signal of acceptance by the writer, while that signals dismissal or rejection.
*Kaar, Kathleen A. "This and That: Implications of Native Usage for the Teaching of English as a Second Language" (Unpublished master's thesis, Cornell University, 1984)