Arrange the following compounds in decreasing order of acidity.
`H_(2)O, ROH, HC -= CH`

Explanation

A stronger acid displaces a weaker aciid fromm its salt, since `H_(2)O` displaces ROH from RONa and both `H_(2)O` and alcohol displace acetylene from sodium acetylide, therefore, water is the strongest, followed by alcohol while acetylene is the weakest acid. in other words, acidity decreases in the order:
`H_(2)O gt ROH gt HC-=CH`
`underset("Stronger acid")(H_(2)O)+underset("Sod. alkoxide")(RONa)to underset("Weaker acid")(R-OH)+NaOH`
`underset("Stronger acid")(H_(2)O)+underset("Sod. acetylide")(HC-=CN a)to underset("Weaker acid")(HC-=CH)+NaOH`
`underset("Stronger acid")(ROH)+underset("Sod. acetylide")(HC-=CN a)to underset("Weaker acid")(HC-=CH)+RON a`