what causes water to evaporate every fluid (pure) has a definite vapor pressure at a fixed temperature , now if any how the partial presuure of the fluiid is less then the the vapor pressure of the fluid then the fluid starts evaporating to attain the partial pressure equal to the vapor pressure of the fluid, and then the equllibrium is reached and evaporation stops. basically at the top of the fluid there is a random motion of the fluid molecule due to the difference between the adhasive and cohesive forces, some molecule say 'n' goes into the vapur phase and some strikes back say m such that n>m then after the above explained condition is achived then the constraint m=n. conclusion whenever there is the fluid in liquid state is present then in that system partial pressure is exactly to the vapor pressure. and whenever there is no fluid in liquid state is present then partial pressure is always less then the vapor pressure. geo