What will happen if you put hot water in plants?

Neither.

Tepid or room temperature water is generally considered the best for house plants.

The following URL will take you to an article from The New York Times published in 1878 where the author uses very hot water as an unusual plant remedy.

http://query.nytimes.com/gst/abstract.HTML?res=9D03E7D9113FE63BBC4F52DFB4668383669FDE

I'm not sure I'd try it on an expensive or much loved plant until I'd experimented a bit.