What liquid can knock a person unconscious?

Water, if frozen into a block, and thrown really hard can sometimes have this effect. If you mean injested liquid, there's a problem: The line between unconscious and dead is actually a very fine line. Contrary to what you see on the TV, if someone is unconscious, they're in real trouble. This is why there are anesthesiologists. Chloral Hydrate, combined with Ethanol was used in the old days of "Mickey Finn's" bar to knock out customers who could then be robbed. However, the number of customers who actually (a) became unconscious and (b) then regained conciousness is speculative. This was also easily identified, even back then (1800's). In summary, there is no safe way to render a person unconscious, liquid or otherwise, and not risk killing them.