Why does oil not mix well with water? we know that water mixes well with water. if you add hot water to cold water, you soon have lukewarm water. alcohol, too, mixes well with water. alcohol and water are quite similar. water is composed of oxygen and hydrogen, whereas alcohol is composed of oxygen, hydrogen and carbon. why do oll and water not mix well? the reason is that they aeo not similar. the molecules of water, that is, the tiny pieces of water that inciude oxygen and hydrogen, are very small, whereas the molecules of oil are, by comparison, enormous and very different in their composition.