Chuck is correct. Nitrogen is an inert gas and will not dissolve in liquid beer/water.
There must be CO2 in the beer for the high pressured nitrogen to froth up.
You do not “need” a blended gas to serve the beer. You would need CO2 to carbonate the beer slightly for the nitro to serve up a proper pint. However, if you only serve with nitrogen, the beer will go flat over time as the empty space in the keg expands.
For this you could either just “top up” your keg with CO2 periodically to make sure it stays relatively carbonated - or you can serve on beer gas (usually 70-75% N2 and 25-30% CO2) in order to have enough CO2 available to keep the beer carbed.
I actually tend to go for a higher concentration on N2 to make sure I don’t overcarb.
