49.5g of water becomes water vapor at its boiling point 100 degrees C. How many kJ of energy is required to evaporate this amount of water?

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

Self-ionizes to form an equilibruium system in which [ h_{3} o^{+} ]=[ oh^{-} ]=10^{-7} m

The law of conservation of mass states that a mass of an object or the amount of matter composed inside an object or thing will not change unless it is added or removed of the matter that is contained in the object itself.