On 37 it asks what solution should be added to Tube 4 to cause a precipitate to fall out of solution. In the tube is aqueous sodium bicarbonate. Obviously to make it precipitate you need to neutralize the solution. The book says use 5% HCl rather than 10% acetic acid. Since the higher pH is due to a weak base (sodium bicarbonate) would you use a weak acid to neutralize it? wouldn't using a strong acid just make the solution acidic?