Why is a relatively large amount of energy required to turn liquid water into water vapor?

Water can exist in three states.

1) Solid State: Called Ice.

2) Liquid State: Called Liquid Water.

3) Gas State: Called Steam.

Remember:
                  The physical states of a matter depends upon the interactions between the particles of that substance. The interactions are very strong in solid state, strong in liquid state and very weak or negligible in gas state. 
                  If you want to change the state from solid to liquid, or from liquid to gas you will have to provide energy in order to break the interactions between the molecules. Stronger the interactions, the more is energy required to break the interactions.
                  Water need more energy to convert from liquid to gas phase because hydrogen bond interactions are present among the molecules of water. And the hydrogen bonds are strong enough. Hence in order to break these interactions high energy is required.

Answer///an early form of the atomic theory; dalton atomic theory;