Do you make students memorize solubility rules? Why or why not?
I did make my general chemistry students memorize solubility rules the first couple times I taught. I don't force them to memorize it anymore. Too many points lost on exams due to the exceptions.
I tell them it will help them if they memorize it because then they won't waste time in an exam trying to figure out if something is aqueous or a solid - but the rules are there in case they want to double-check themselves.
I'll admit that this is something to which I haven't given much thought. I've always asked my general chemistry students to memorize the major solubility rule with a few exceptions (e.g. chlorides are always soluble except for Ag+ and Pb2+). As time goes on, I've become less worried about the exceptions and more concerned about the trends... how do +1 vs. +2 vs. +3 compounds behave? ...does the charge of the counterion influence the solubility of the salt? I think it's a good oopportunity to pick apart trends.
The students do a "chemical reactions" lab at about the same time that we cover solubility rules in lab so they have something tangible to go along with this discussion.
I think we / I have always given them the solubility rules table from the textbook. Sometimes just understanding the table is enough of a challenge.
But by doing this, we don't spend any time talking about why some combos are soluble and others aren't, which is too bad.
PS I'm talking about general chemistry here.
In gen chem, we have them learn them to be able to predict precipitation reactions.
I've always had my Gen Chem I students memorize them (maybe because I had to? I hadn't really thought about it) but given them to my prep chem students.
Not sure if others do it this way, but this works well for me. It's admittedly simplified and missing the subtleties, but I think it works great for gen chem.
1. Memorize the strong acids and bases.
2. Salts containing anions of the acids (such as NO3- from HNO3) and cations from the bases (Na+ from NaOH) are ALWAYS soluble. Everything else precipitates.
3. EXCEPT: Ag and Pb halides, Ba and Pb sulfate.