The Short Answer: A Difference in Polarity
Oil and water do not mix because their molecules have different electrical properties. Water molecules are 'polar,' meaning they have a slight positive charge on one end and a slight negative charge on the other. Oil molecules are 'nonpolar,' meaning their charge is evenly distributed. Polar molecules are strongly attracted to other polar molecules, effectively pushing nonpolar molecules away.
Section 2: Polar vs. Nonpolar Molecules
A polar molecule, like water (H₂O), has an unequal sharing of electrons among its atoms, creating a tiny molecular magnet with positive and negative poles. Nonpolar molecules, like the hydrocarbons that make up oil, share electrons evenly and have no charge separation. This fundamental difference means water molecules would rather stick together via strong hydrogen bonds than allow oil molecules to come between them.
Section 3: A Practical Example in the Kitchen
Think about oil and vinegar salad dressing. Vinegar is mostly water, which is polar. When you shake the bottle, you are temporarily forcing the oil and vinegar together into an emulsion. However, as soon as you stop shaking, the powerful attraction between the water molecules pulls them back together, squeezing out the oil molecules and causing the distinct layers to reform.
Section 4: Importance of Polarity
This principle, often summarized as 'like dissolves like,' is crucial in science. Substances that are polar and mix with water are called 'hydrophilic' (water-loving), while nonpolar substances that repel water are 'hydrophobic' (water-fearing). This concept explains how soap works to clean grease and is fundamental to the structure of cell membranes in all living organisms.