Question
Is straining a liquid to remove dissolved impurities permitted on Shabbos?
Answer
Straining a liquid would be problematic (the melacha is called meshamer) if the straining removed perceptible impurities from the liquid. Thus there would be a problem if the impurities were in suspension, making the liquid cloudy; but if they are in solution, having no visible effect on the liquid, the straining should be permissible.