Which of these compounds does not liberate heat when water is added to it?

Which of these compounds does not liberate heat when water is added to it: $\ce{KNO_3}$, $\ce{NaOH}$, $\ce{CaO}$, $\ce{H_2SO_4}$, $\ce{Na}$.

I know from experience that dropping sodium tablets into water has explosive results, but I am clueless about the method to solve this problem. I know the answer is $\ce{KNO_3}$, but I don't know why. If someone can provide the method, I think I can do the problem.

It is important to note that if a molecule or formula unit is capable of undergoing hydrogen bonding ($\ce{NaOH}$, $\ce{H2SO4}$), then the formation of this hydrogen bond will be extremely exothermic.
Also, if a substance can react to form a substance that will undergo hydrogen bonding ($\ce{CaO + H2O -> Ca(OH)2}$), this species will subsequently form these hydrogen bonds and liberate heat.
Lastly, as you have stated you know from personal experience, the reaction between a alkali metal ($\ce{Na}$) and water is extremely exothermic— so much so that many of these metals will melt from the heat liberated.
• Hydrogen bonding is a special type of electrostatic attraction that occurs between $\ce{H}$ bonded to $\ce{O}$, $\ce{N}$, or $\ce{F}$ and the unbonded electron pairs of a highly electronegative atom. In the case of dissolving $\ce{NaOH}$, the $\ce{OH-}$ that dissociates and the $\ce{H2O}$ participate in hydrogen bonding. In the case of dissolving $\ce{H2SO4}$, the $\ce{HSO4-}$ and the $\ce{H2O}$ participate in hydrogen bonding. These bonds are the strongest intermolecular bond, and are very stable. The formation of these bonds is therefore extremely exothermic. – ringo Apr 29 '15 at 15:28