The words that and which illustrate this concept very well. Many writers believe these two words may be used interchangeably. After all, they function in much the same way, right? Well, not quite.
Effective writers use that in phrases and clauses essential to meaning and which in phrases and clauses that are not. Often commas will signal the difference between what is essential to meaning and what is not, but that might not always be true.
Consider the example sentence in the graphic above. The phrase in question is which appears in the micrograph as thin, gray elongations and globules. Is this phrase essential to the meaning communicated in Fe3C? Not really. For this case, iron carbide always appears the same for the material and preparation conditions used to produce the micrograph. Thus, which appears in the micrograph as thin, gray elongations and globules is side information and not essential to meaning. Therefore, the writer used which correctly. The use of commas is also appropriate.
But suppose for a moment that the writer did not use commas to separate out the which phrase. The sentence would then appear like this:
What about using that instead of which? Could the commas be omitted then? That sentence would look like this:
Scientists and engineers can easily comprehend the need for precision in many of their endeavors. Writing is no different. Writers who bring precision to their writing efforts yield commensurate results. So be one of those writers! Learn what you need to learn to improve not only the effectiveness of your communications but also the professionalism of the image that those communications project.