Why Does Water Mix With Salt But Not With Oil?

Water

In chemistry, miscibility is the ability of two substances, usually liquids, to mix together in equal proportions to form a homogeneous product. Water is polar, carrying a negative charge at one end of its molecule and a positive charge on the other end. Those charges are attracted to opposite charges. This is why water "sticks together": think of raindrops running together to form a larger drop, or surface tension that allows you to fill a glass with water above the brim. The bonds between the hydrogen atoms and the oxygen atom that make up a molecule of water are strong and difficult to break.

Salt

On the other hand, the bond between the atoms of sodium and chloride that join together to form salt are relatively easy to break. Think of them as fair-weather friends who will separate if a better offer comes along. When water is introduced to salt, the sodium and chloride atoms get a better offer: their bond breaks, and the separate atoms join with the water molecules. Sodium, chloride and water mix together to form a homogeneous product. Salt dissolves in water. Or to be technical, a polar solvent dissolves a polar compound.

Oil

Oil is nonpolar. Its charges are well-balanced, and unlike water, oil is "content" and not looking for something else to grab onto. If sodium and chloride are fair-weather friends, the atoms of oil are the newlyweds who are so happy together that they aren't looking elsewhere, and it will take some rather aggressive intervention to separate them. Oil does not dissolve in water because the water can't find a way between the oil molecules. Oil doesn't dissolve in water, or to be technical, polar solvent and nonpolar compounds are immiscible.

References

• University of Southern Maine: Solvent Chemistry