I want to use lab-grade calcium nitrate in my hydroponic system (as a fertilizer) to grow lettuce, is it safe?
Usually you can buy chemicals in different purity grades. If you buy something that has the grade purum >95% will be your desired ingredient. However there is not usually any information about what the other <5% will consist of.
Personally I would never ingest (or use in food production) anything that is not food grade. Food grade does not necessarily mean very pure, it just means that there is nothing in the other x% that is potentially harmful. Look out for an FCC number, that is the Food Chemical Codex, when you order it and you should be fine.