Dilute hydrochloric acid was added to substance X. There was a vigorous action and a gas was produced which turned limewater milky. After the reaction stopped, an excess of aqueous sodium hydroxide was added to the solution and a blue precipitate formed. What was X?

Hydrochloric acid is HCl.

Acids can react with a number of things to produce gases, but there is really only one common gas that will react to turn limewater milky. Limwater is Ca(+2) and Cl-. Something reacts with Ca+2 to form a precipitate (limewater turns milky). What's that something (take a guess. If you get it wrong, I'll tell you the answer). Hint: The gas is CO2. What does that come from and what reaction produced it?

Sodium hydroxide precipitates a lot of metals (forming their hydroxides). There aren't very many blue hydroxides -- only one common one that I can think of. Hint: Think of the color of the dome on the outside of the capitol building in Washington D.C.