Why will water in a flask begin to boil at room temperature as air is pumped out of the flask?

The boiling point of a liquid is the temperature at which the vapour pressure of the liquid is equal to the environmental pressure surrounding the liquid.

Since there is no environmental pressure on the liquid in vacuum, its boiling point is directly above its freezing point. In other Words, as soon as the liquid unfreezes from the solid state, it turns to steam because there is no outside pressure to hold the molecules in liquid state.