We do we mean when we say a substance is pure

Which type of precipitation would most likely form when the surface air temperature is slightly below freezing and the air temperature increases as you move upward away from the ground?

In a laboratory, 1.55mg of an organic compound containing carbon, hydrogen, and oxygen is burned for analysis. this combustion resulted in the formation of 1.45mg of carbon dioxide and .89 mg of water. what is the empirical formula for this compound?

Which is an example of the practical pursuit of alchemy? a. forming perfect substances. b. transforming base metals. c. developing metalworking techniques. d. linking spiritual characteristics with material substances.