system
Given the following post, respond as if you're commenting on Reddit:
human
Why is water to steam always the source to turn a turbine to generate power ? Why is it not the liquid that expands the most when vaporized?

I was just thinking that in energy generation methods such as geothermal & nuclear they always heat up water to produce steam in order to turn the turbine. Wouldn't it have greater energy yield if it was a liquid that expanded more than water upon vaporization and had a lower boiling point/ heat capacity than water?
gpt
As you allow any vapor to expand as it does work, it cools down. As the vapor cools, the amount of 'usable work' that can be done decreases. So we want a substance that not only expands a lot when heated but also carries away a great deal of thermal energy so it can be used to do the greatest amount of usable work. 

Water has an extremely high heat of vaporization and specific heat capacity (it is the reason why steam burns a lot worse than hot water, trust me I've been burnt by an autoclave before) which means water must absorb a lot of thermal energy before it vaporizes. Which also means steam can do a lot of work before it cools too much to be unusable. This work can be extracted from the steam in an 'adiabatic expansion' process. (Its hard to make anything perfectly adiabatic, but you get the idea)

Add to it the fact that water is easily available, free, can be released into the environment, and not corrosive, its a win win.