Why is the vapour pressure of an aqueous solution of glucose lower than that of water ?

Explanation

Vapour pressure of any solvent or water is caused due to escaping tendencies of the water molecules from the liquid level / surface. In pure water, only water molecules are present at its surface, but when a non - vilatile solute like glucose is dissolved in it, certain number of non - volatile glucose molecules, with non escaping tendency are also present at the surface of aqueous solution. Thus the number of water molecules at the surface is correspondingly decreased, due to which relatively lesser number of molecules of water can escape out as vapours. This results in lowering * of vapour pressure of water in its glucose solution as compared to that of pure water / solvent. Such a relative lowering of vapour pressure is termed as its colligative property.