The current homework policies of Chemistry and Physics were adapted from the FAQ of Mathematics. The first rule about answering homework questions is: you do not provide a complete answer to homework questions.
The rationale behind this rule is
Providing an answer that doesn’t help a student learn is not in the student’s own best interest, and if a solution complete enough to be copied verbatim and handed in is given immediately, it will encourage more people to use the site as a free homework service.
The policy of Chemistry even includes drastic measures:
Watch out for answers that provide a full solution. Downvote, comment, flag.
Hence, homework questions usually just get hints and comments. And many homework questions remain technically unanswered.
However, this conduct contradicts the general philosophy of this site, which is to build a library of detailed answers to every question about chemistry. In particular, such unanswered questions are unlikely to help future visitors of this site. The significant number of unanswered questions might leave a bad impression; the site might seem not really useful. Furthermore, the restriction might be frustrating for users who like to answer homework questions. Also note that duplicate homework questions cannot be properly closed as duplicate when no reference answer is available.
That’s not the way it should be. Actually, a lesser-known rule of all three above-mentioned homework policies expressly permits complete answers – after a suitable amount of time:
In the spirit of creating a lasting resource of mathematical knowledge, you may come back after a suitable amount of time and edit your response to include a more complete answer. Or even better, the student can post his own correct answer!
However, it remains unclear how long a suitable waiting period should be. Users may be hesitant to give any answer at all.
Therefore, it seems advisable to try to formulate a community guideline that clarifies when a complete answer to a homework question may be provided and encourages users to do so.
 Note that this part is not included the policies of Physics and Mathematics. The FAQ of Mathematics read: “Don’t downvote others who provide complete answers to questions just because you think it might be homework. It’s not always obvious at first glance that a question is homework, especially when you’re not expecting to see it. Instead, suggest editing the response in a comment.”
 Note that “mathematical knowledge” is a remnant of the original FAQ of Mathematics.