CAN WATER AND OIL BE MIXED TO FORM A SOLUTION-

　　Can Water and Oil Be Merged to Create a Solution?

　　Everyone has seen what happens when you shake vinegar and oil vigorously in a salad dressing; a temporary mixture is formed, but it eventually separates, with the less dense oil rising to the surface and the denser vinegar/water mix settling at the bottom. This demonstrates a fundamental chemical rule: ‘similar substances dissolve in each other.’ This means that when two liquids, whose molecules have comparable sizes and polarities, are mixed, they generally form a single-phase solution, irrespective of their relative molar amounts. This is characterized by the term ‘miscible in all proportions.’ On the other hand, when a highly polar substance like water is mixed with a nonpolar or slightly polar substance, like most oils, they tend to separate into two distinct phases. This is typically explained in basic chemistry texts by stating that oil is hydrophobic and therefore doesn’t mix with water, while polar small organic acids such as acetic acid, present in household vinegar, are hydrophilic and thus mixable with water.

　　This explanation can lead students (and even some professional chemists) to believe that water and oil molecules repel each other or have only a weak attraction. However, this is inaccurate! An oil molecule is actually attracted to a water molecule with a force much stronger than the attraction between two oil molecules. This stronger attraction can be seen when a drop of oil is placed on a clean water surface. The oil forms a spherical droplet due to the mutual attraction of its molecules, which is minimized in a sphere. When it contacts the water surface, the oil spreads into a thin layer because the attractions between oil and water molecules surpass the oil-oil attractions within a larger droplet on the water.

　　If a sufficiently small oil droplet is placed on the surface, it will spread to create a single molecular layer. By calculating this area, one can estimate the size of each oil molecule and thereby determine Avogadro’s number. Despite these strong interactions, oil molecules do not dissolve into the water solution because of the energy required to break the water molecules’ attractions. As a result, most oil molecules remain separate from the water, although some may adhere to the surface water molecules.

　　In the end, water and oil do not mix to form a solution because non-polar substances like oil are not soluble in water. However, with the help of a surfactant, oil and water can be combined to form a stabilized suspension similar to a solution. Surfactants are widely used in applications involving mixtures of water and non-polar substances such as oils.