When we add substances as sugar or salt in water, it causes the water to evaporate more slowly at a given temperature. Theory behind this is not difficult as when you add salt in water then salt molecules form a bond with water molecules. So the movement of molecules decreases and as we know when molecules move fast then water evaporates quickly. Because of the bond between water and salt molecules, It takes extra energy to break this bond and that's slows down the evaporation.
Another reason is that as every one knows water starts to evaporate at high temperature. At 100 degree centigrade water starts boiling and it also starts evaporating. But the cause for boiling of water is the speed of molecule. When heat is provided to water then molecules of water starts moving with great speed and soon their speed come to that point where that is considered enough for water to boil and evaporate.
When we add salt then salt then water becomes heavier and water's molecules make bond with salt's molecules and as a result it affects on thee speed of molecules. So even if we provide heat to water then molecules start moving very slowly and they take more time to evaporate than water without salt.
Another reason is that as every one knows water starts to evaporate at high temperature. At 100 degree centigrade water starts boiling and it also starts evaporating. But the cause for boiling of water is the speed of molecule. When heat is provided to water then molecules of water starts moving with great speed and soon their speed come to that point where that is considered enough for water to boil and evaporate.
When we add salt then salt then water becomes heavier and water's molecules make bond with salt's molecules and as a result it affects on thee speed of molecules. So even if we provide heat to water then molecules start moving very slowly and they take more time to evaporate than water without salt.