Student B asked, "Why does the vapor pressure of a substance lower when a nonvolatile solute is placed in it?"
Student A replied, "It must have something to do with a change in the rate of evaporation at the surface. Let us ask our friend on the other side of the
screen.
The solute molecules are strongly bound to the solvent molecules at the surface and do not allow as many to escape. The rate of returning molecules does not change, so the net effect is the lowering of the vapor pressure at equilibrium
• The solute molecules are weakly bound to the solvent molecules at the surface and do allow as many to escape. The rate of returning molecules does not change, so the net effect is the increase of the vapor pressure at equilibrium.
The solute molecules are strongly bound to the solvent molecules at the surface and, yet it allows many to escape. The rate of returning
molecules increases more rapidly, so the net effect is the lowering of the vapor pressure at equilibrium.
The solute molecules are weakly bound to the solvent molecules at the surface and do allow as many to escape. The rate of returning
molecules increases more rapidly, so the net effect is the lowering of the vapor pressure at equilibrium.