Vapor pressure lowering

I have got this problem in my text book,



> 
> Consider these three
>  mercury manometers. One of them
>  has 1 mL of water placed on top of the
>  mercury, another has 1 mL of a 1 m
>  urea solution placed on top of the
>  mercury, and the third one has 1 mL of
>  a 1 m NaCl solution placed on top of
>  the mercury. Identify X, Y, and Z with
>  these solutions. 
> 
> 
> 


[![enter image description here](https://i.stack.imgur.com/JqZGM.png)](https://i.stack.imgur.com/JqZGM.png)


I got the answer in answer sheet is X = ${H\_2}O$ , Y = $NaCl$ and Z = ${(NH\_2)}\_2{CO}$ . Can any one please explain me? Thanks in advance.




The answer hings on the [colligative properties](https://en.wikipedia.org/wiki/Colligative_properties) of solutes. In essence solutes lower the vapor pressure of the solvent. NaCl splits into two ions (i.e. two solute "particles") and lowers the vapor pressure of water more than urea for which the whole molecule dissolves as one solute "particle." 


If there was no liquid the Hg level would be higher than any of the three tubes shown since there would be a pure vacuum above the Hg. 


The scale of the relative differences in drawing is obviously greatly exaggerated. 


 4