How nuance will change if "like this chemical thing" replace to "like a chemical thing"?
Native speaker’s answer
Rebecca
Both "this" and "a" actually work in this context. You can swap out "this" for "a" without changing the nuance of the sentence. Many people use "this" when describing things in conversation, but "a" also works well here. Ex: He's like this great big teddy bear. = He's like a great big teddy bear.