We all know that oil and water can never mix. Water is the most commonly used solvent. Substances like salt and sugar readily dissolve in water but oil, fats, wax, and many other substances do not. According to a scientific principle, ‘like dissolves like’, only those substances make solutions, which have similar types of molecules. For a substance to dissolve in water, its molecules must be capable of forming electrostatic bonds with the water molecules. Molecules of water are polar. That means one side of each water molecule has a partial positive charge and the other side has a partial negative charge. When the molecules of another substance such as water and oil molecules common salt (sodium chloride), having ionic or polar character are put in water, they get dissolved in it. Polar molecules of water are drawn to the charged sodium and chloride ions and form a cluster around them. This reduces the attraction between sodium and chloride ions. They separate and spread through the water in the form of a solution. Oil molecules do not dissolve in water because the bonds of oil molecules are different from those of the water molecules. The molecules of oil are much bigger and contain many more atoms than those of water. Oil molecules have covalent bonds. Oil droplets float on water because the force of attraction between water and oil molecules (adhesive force) is less than the force of attraction (cohesive force) between oil molecules. So, the oil molecules do not spread in water. That means they do not dissolve in water.
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