why do salt cause so much burn when it get mixed with blood on any wound ? or simply why it creates an intense burning effect on an open wound ?

This is just a guess but i would think the ionic shock to the nerve cells causes the pain...

And maybe osmosis causing cells to rupture releasing chemicals that stimulate the pain nerves. (Or something. IANAB )

If you increase the salt level in a solution, doesn't that cause it to expand, become hypertonic? Might that swelling cause the pain? Notice that I'm asking and NOT telling.

 

This seems more like chemistry than biology. Let me know if you want me to move it there.

I think this is close to the likely mechanism. However, I would assume that not any swelling is the responsible but the cell damage that a highly hypertonic solution may cause. Nerve signalling tend to get sensitized upon tissue damage (i.e. the wound) and I would speculate that the presence of salt or other harmful compounds would exacerbate this. In addition electrolyte sensing receptors may add to the overall response, i.e. further stimulating nociceptors.