When a salt solution undergoes complete evaporation, the liquid water component of the solution is converted into water vapor, leaving behind the dissolved salt in a solid form. The process of evaporation causes the water molecules to escape from the solution, leaving the salt ions behind. As more and more water evaporates, the concentration of the salt in the remaining solution increases. Eventually, when all the water has evaporated, the salt will be left behind in a solid state, forming salt crystals or a solid residue.