And if more molecules evaporate, more molecules will be present in the space above, more gas molecules. And if more gas molecules are present in the space above, the vapor pressure is higher. Therefore, compared to ideal situations for each case in a non ideal situation that's endothermic the pressure for each will be higher and that's why they curve upward. Now, in an exothermic, the opposite holds. In an exothermic reaction, the bonds formed are stronger than the bonds broken. Therefore, the final solution will hold its molecules very tightly and will not let them go.