why volume of ice decrease on melting and ortho nitro phenol is more volatile than para nitro phenol ?

Why volume of ice decreases on melting ? 

In ice , the water molecules are tetra hedrally oriented with respect to one another.At the same time each oxygen atom is surrounded tetra hedrally by four hydrogen atoms, two of these are bonded covalently and the other two by hydrogen bonds. 

The hydrogen bonds are weaker and therefore longer than the covalent bonds.This arrangement gives rise to an open cage-like structure. There are evidently a number of holes or open spaces in this structure . 
These holes are formed because the hydrogen bonds holding the H2O molecules in ice are directed at certain definite angles . In liquid water such hydrogen bonds are fewer in number.

Therefore, when ice melts, a large number of hydrogen bonds are broken. The molecules , therefore, move into the holes or open spaces and come closer to one another than they were in the solid state. 
This results in a sharp increase in the density. The density of liquid water is therefore, higher than that of ice.
Hence, the volume of ice decreases on melting.

Why ortho nitro phenol is more volatile than para nitro phenol ? 

On account of larger distance between the two groups , para nitro phenol do not form inter molecular hydrogen bonding. It forms inter molecular hydrogen bond. 
As a result of inter molecular hydrogen bonding para nitro phenol undergoes association, resulting in increase  of molar mass and hence an increase in boiling point.

On the other hand, ortho nitro phenol forms intro molecular hydrogen bonding , so no such type of association among the molecules is possible. 
Consequently, the ortho nitro phenols are more volatile than the para isomers.

Summary :  


What is hydrogen bonding ?
Why volume of ice decrease on melting ? 
Why o-nitro phenol is more volatile than p-nitro phenol ?

Click here: hydrogen bonding and its important in life.