When we mix water with salt, there is no chemical reaction as such, there is simply a dissolution of the salt ions that are attracted by the water molecules. But water retains its chemical structure just like salt.
When we begin to heat the water and the boiling temperature of the water is reached, it begins to evaporate, changing from liquid phase to gaseous phase. On the contrary, salt needs a much higher temperature to change its phase to gas, that is why the crystals of salt that were dissolved in the water do not evaporate and remain in their initial solid state separated from the water that evaporated.