Question: I read that:

> 
> Take a cupful of water in a beaker and add a few drops of dilute
>  sulfuric acid. Heat the water. When it starts boiling add copper
>  sulfate powder slowly while stirring continuously.
>  Continue adding copper sulfate powder till no more powder can be
>  dissolved. Filter the solution. Allow it to cool. Do not disturb the
>  solution when it is cooling. Look at the solution after some time. Can
>  you see the crystals of copper sulfate? If not, wait for some more
>  time.
> 
> 
> 

So why is sulfuric acid required in this experiment?

Answer: 
The reason behind this is the orientation of the overlapped orbitals. Sigma bonds result from head-on(co-axial) overlapping while pi bonds are outcome of lateral(para-axial) overlapping. Here is a pictorial representation of ethene(sp2 hybridized C atoms) : 

![enter image description here](https://i.stack.imgur.com/g7mqW.jpg) 

***The greater the extent of overlapping, the higher the probability of finding the valence electrons in between the nuclei and hence the bond will be stronger & shorter.*** 

In MOT, this can be explained using [Overlap Integral](http://en.wikipedia.org/wiki/Orbital_overlap). This is how Atkins depicts it :

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

***In simple terms, after forming a sigma-bond (a pre-requisite for pi-bonds), the two atoms get locked along the inter-nuclear axis. As a result, the orbitals available for pi-bonding can only partially overlap, thus forming a weaker bond.***
Unsaturated compounds (due to the existence of a double bond) are more reactive than saturated compounds, because the electron density at the internuclear axis is zero in a pi bond (in contrast to the sigma bond).