Why do olive oil and vinegar (a water-based solution) tend to separate in salad dressing? will added salt dissolve in the oil or in the vinegar? explain your answer?

Vinegar (acetic acid solution) is a polar substance, whereas olive oil is non polar in nature. Like dissolves like. A polar substance cannot dissolve in a non polar compound. Therefore, vinegar which is polar is not soluble in olive oil, a non polar substance. Hence, in a salad dressing olive oil separates out from vinegar.

Added salt would dissolve in vinegar solution as salt, a polar substance which is chemically, sodium chloride is an ionic substance. It can dissolve in the polar vinegar solution but not olive oil.