There are two parts to this question. First question: Why don’t water and oil mix? Second question: Why does oil stay on top of water?
One liquid will dissolve or mix into another if the molecules of the two liquids have similar electric dipole moments, or strengths. A magnet, for example, has a north and a south pole, with opposite magnetic charges. Two of the same poles, such as a north pole and a north pole, repel. Opposite poles, north and south, attract. Similarly, molecules have electric dipoles, with a positive end and a negative end based on the electric charge each end carries. The charge can be greater in magnitude on one end than on the opposite end. Materials with dipoles of similar strengths dissolve in each other more easily than those with dipoles of different strengths. Water and oil have very different dipole charges, so they don’t stay mixed. Liquids that do not mix are said to be immiscible. Now to the second part of the question. Oil is less dense than water, so it stays on top of, or floats on, water. Almost all liquids made from crude oil float on water, including gasoline, kerosene, turpentine, engine oil, baby oil, mineral oil, diesel fuel, and jet fuel. All oils made from plants, such as linseed oil, castor oil, sunflower oil, coconut oil, and olive oil, are also less dense than water and, therefore, floats on top of water, too.
Both oil and water are liquids, so why don’t they mix?
Answers
Devendra Bahal
There are two parts to this question. First question: Why don’t water and oil mix? Second question: Why does oil stay on top of water?
One liquid will dissolve or mix into another if the molecules of the two liquids have similar electric dipole moments, or strengths. A magnet, for example, has a north and a south pole, with opposite magnetic charges. Two of the same poles, such as a north pole and a north pole, repel. Opposite poles, north and south, attract. Similarly, molecules have electric dipoles, with a positive end and a negative end based on the electric charge each end carries. The charge can be greater in magnitude on one end than on the opposite end. Materials with dipoles of similar strengths dissolve in each other more easily than those with dipoles of different strengths. Water and oil have very different dipole charges, so they don’t stay mixed. Liquids that do not mix are said to be immiscible. Now to the second part of the question. Oil is less dense than water, so it stays on top of, or floats on, water. Almost all liquids made from crude oil float on water, including gasoline, kerosene, turpentine, engine oil, baby oil, mineral oil, diesel fuel, and jet fuel. All oils made from plants, such as linseed oil, castor oil, sunflower oil, coconut oil, and olive oil, are also less dense than water and, therefore, floats on top of water, too.