How do I turn room temperature water into boiling water for drinking using an chemical/additive?
Adding something to water that heats it to 100°C and leaves only water is of course nonsense.

A self-heating cup is easy: You take a double-walled beaker, fill the space between with two compounds that react with great heat but no gas formation, separated by an extra wall with a valve.

Google the self-cooling beer keg. It works very similar, only it gets hot outside, not inside. 

Or just google "self-heating coffe cup".