what does pressurized water look like when you open or turn on the hydrant?
What happens if there is NO pressure in the hydrant?
how do you pressurize the water in the hydrant?
can you change the PSI coming out of a fire hydrant?
And if you can change the PSI, How can change the amount PSI of water coming out of a hydrant?
Water, in its liquid state is not really pressurized, as liquids have a high degree of incompressibility. The properties of water under pressure don't really change that much, other than a lack of gases dissolved in it, which makes it appear transparent rather than opaque white like sprays. Pressure only makes water move in a specific direction.
If there is no pressure in the hydrant, no water comes out. Water comes out of hydrants, faucets, and other fixtures due to pressure from pumps attached to the water supply, this pressure primarily just makes the water "want to flow" in a certain direction because of it's incompressibility.
Technically you can change the PSI on the water coming out of a fire hydrant, because the PSI changes to standard atmospheric pressure the instant it exits the hydrant. Tanks with pumps (like on fire engines) can put the water back under pressure again, but only so much due to the incompressibility.
To better understand what I'm saying about incompressiblity, take a plunger syringe, block the end and see how far you can push the plunger, then fill the syringe with water and try again. You should find that you can't push it anywhere near as far as you could with air.