What is a "chemical substance"..

After a hard day, I came back home, happy to see my parents. I sat and started my dinner, talking with them about my day. As you know, I am having chemical studies. The discussion went on, until I asked them: "what is a chemical substance, according to you?"
They gave me the answer a lot of people would have said, too. According to them,

a chemical substance is each artificial, contaminant substance.

Just today, we were talking about the same topic with a professor of our university. Soon, I'm gonna publishing an article provided by Laura (Ringhio here, that I wanna thank), article that underlines the deep split between the public known chemical's meme , and what it really is. (and thanks to Anna if I get to know what a meme is!)
Wow, I thanked too many people tonight, maybe too much. Don't want to be too kind, so I soon close my post!