Why would Italian dressing be separated into two layers after a while?

Italian dressing separates because it is basically a mixture of oil, vinegar and water with flavorings. Oil does not mix easily with water and vinegar; after a few moments, the oil always rises to the top. This is why you must shake most kinds of dressing before pouring them onto food.

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Italian dressing would separate in two layers because of the difference in densities. The substance on the bottom (water & vinegar) has a higher density than the substance on the top (oil.)