Question

Why vapour pressure low when urea desolve in water under ordinary condition

Answer 1

Vapour pressure is the pressure of a substance with its liquid form.while the top most surface of any liquid is in equilibrium with the vapour phase of that liquid, if you put a different substance in it there will be competition between the molecules of both type of substance. If urea is added it's molecules also occupy the topmost surface of solution. So the equilibrium of vapour liquid phase of water decreases.. Making water's vapour pressure less