I found this: https://www.physicsforums.com/showthread.php?t=105106 But it the question was never really answered there. So I'd like to ask a *similar* question again. Are there any gases at STP, (without being "excited" or "liquefied"--see other topic, above) that are visible to the naked eye? My high school chemistry teacher told me that one of the defining properties of gases is that they are invisible, but I can't find any corroborating information. Google coughs up surprisingly little on this subject. Thank you.