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There is no denying the fact that water is the most commonly used solvent and substances like salt and sugar readily dissolve in water. But then there are substances like oil, fat, wax, etc. which do not dissolve in water.
According to the scientific principle of ‘like dissolves like’, only such substances make solutions which have similar types of molecules. Thus, for a substance to be able to get dissolved in water, its molecules must form electrostatic bonds with the water molecules. Molecules of water are polar, which implies that one side of each water molecule contains a partial positive charge and the other side contains a partial negative charge. Thus, when the molecules of another substance like water and oil molecules common salt (sodium chloride) containing ionic or polar characters are put in water, then they are said to get dissolved in it. As a matter of fact, the polar molecules of water are drawn to the charged sodium and chloride ions, thereby creating a cluster around them. Because of this, the attraction between sodium and chloride ions gets reduced. They then get separated and are spread through the water in the form of a solution.
The bonds of oil molecules are different from those of the water molecules; this is the reason why oil molecules do not get dissolved in water. Also, the molecules of oil are much bigger and consist of many more atoms than what water contains. Other than this, oil molecules also have covalent bonds, so oil droplets float on water as the force of attraction between (adhesive force) is less than the force of attraction (cohesive force) between oil molecules. Thus, the oil molecules do not spread in water and lie on top of it, meaning that they are unable to get dissolved in water.