You find a melting point for a substance in an encyclopedia, which includes a reference to a journal article. Do you cite the encyclopedia, or the journal article?
In situations like this, the best best approach is "Cite What You Read." If you get the value from a handbook, encyclopedia, database, or other source, cite the primary source article if you actually read the article. Otherwise, cite the secondary source where you find the data.
"The instructor performs a sample search for the melting point of aspirin using Reaxys, which reports more than fifty reported values for this property from a variety of journal articles and patents. She chooses one value and directs the students’ attention to the reference to the primary literature beside the value in the database; then, she asks the students how many of them would cite Reaxys. A few will raise their hands. She then asks how many students would cite the primary article. More students tend to raise their hands, and she asks them to keep their hands up. Then, she asks any student who has read the primary article to keep his or her hand up, and all of the students lower their hands. She tells them that the students who still have their hands read are allowed to cite the primary article; everyone else must cite Reaxys. The key take-away is that students may only cite information that they themselves have read. This portion of the class closes with a brief discussion of the reasons that one might choose to cite the primary article over the database and vice versa." (1)
1. Currano, J. N. Teaching Students Where Credit Is Due: Two Lesson Plans for Teaching Documentation and Assignment of Credit. In Credit Where Credit Is Due: Respecting Authorship and Intellectual Property; Mabrouk, P. A., Currano, J. N., Eds.; American Chemical Society Symposium Series, Vol. 1291; American Chemical Society, 2018; pp 85-99. DOI: 10.1021/bk-2018-1291.ch009