Why is the composition of the vapor different from the composition of the solution?

If there is solution with nonvolatile solute (substance that does not readily evaporate into a gas) only the pure vapor of the solvent is present above the solution and solute stays in solution and do not enters vapor above solution. This is because nonvolatile solute has slow rate of evaporation and low vapore pressure.

If solution has two volatile components, composition of the vapor depends on vapor pressures of the components according Raoult's Law.