In the pure water solvent, the molecules present on the surface are only of water and the vapour pressure exerted is from the solvent molecules. When a non-volatile solute is dissolved in water such as glucose, the vapour pressure exerted on the solution is only from solvent molecules. Due to the glucose dissolving in water, a fraction of molecules covering the surface of water are occupied by glucose molecules. The presence of the solute molecules prevents the escape of solvent molecules from the surface into the vapour phase, hence the vapour pressure of an aqueous solution of glucose is lower than water.