Briefly describe the importance of critical temperature. How can it be correlated with van der Waalas constant ‘$\mathrm{a}$’ ?

Give reason for the following:

Liquids have a definite volume but no definite shape.

Give reason for the following :

Liquids have higher density than gases.

Give reason for the following :

Liquids are much less compressible than gases.

Give reason for the following:

Liquids possess fluidity.