Miscibility

Miscibility means how completely two or more liquids dissolve in each other. It is a qualitative rather than quantitative observation—miscible, partially miscible, not miscible. (To state exactly how miscible two liquids were, a scientist would use the larger concept of solubility, usually in a specific weight or volume per liter of solution.) Two completely miscible liquids will form a homogeneous (uniform) solution in any amount. Water and ethyl alcohol, for example, are completely miscible whether the solution is 1% water and 99% ethyl alcohol, 50% of both, or 1% ethyl alcohol and 99% water. When first mixed, miscible liquids often show oily bands—called striations—in the bulk of the solution; these disappear when mixing is complete.

Like any other solubility phenomenon, miscibility depends on the forces of attraction between the molecules of the different liquids. The rule of thumb "like dissolves like" means that liquids with similar molecular structures, in particular similar polarity, will likely dissolve in each other. (Polarity means the extent to which partial positive and negative charges appear on a molecule, because of the type and arrangement of its component atoms.) Both water and ethyl alcohol have very polar hydroxyl groups (-OH) on their molecules, and therefore both undergo the strong intermolecular attraction known as "hydrogen bonding." Hexane, on the other hand, is not miscible with water because its molecular structure contains no polar groups of any kind that would be attracted to the water molecules.