Miscibility means how completely two or more liquids dissolve in each other. It is a qualitative rather than a quantitative observation—such as, miscible, partially miscible, or 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.
There are several fundamental rules governing the miscibility of liquids in other liquids. First, the solubility of liquids in liquids increases with increasing temperature. Second, the more similar two compounds are in terms of polarity, the more likely that one is soluble in the other; i.e., polar compounds dissolve in polar compounds, and non-polar compounds dissolve in non-polar compounds. (Polar molecules dissolve in polar molecules because the dipole of one attracts the dipole end of the other.) Thus, benzene and carbon tetrachloride, being both non-polar, dissolve in each other, but neither will appreciably dissolve in water, which is polar.
Both alcohols and ethers with up to three or four carbons are miscible in water because the OH groups in these molecules form hydrogen bonds with the water molecules. Alcohols and ethers with higher molecular weights are not miscible in water, however, because the water molecules cannot surround those molecules. The molecule 1-heptanol, for example, consists of an alkyl chain of seven carbons and an OH group. The OH group forms hydrogen bonds with water molecules, but the alkyl portion of the molecule exerts no attraction on the water molecules. This part of the molecule is called hydrophobic, meaning water-hating. Because this part of the molecule cannot be surrounded by water, 1-heptanol is immiscible in water.
In aqueous solutions, globular proteins usually turn their polar groups outward toward the aqueous solvent, and their non-polar groups inward, away from the polar molecules. The non-polar groups prefer to interact with each other, and exclude water from these regions, leading to immiscibility. This type of interaction is usually weaker than hydrogen bonding, and usually acts over large surface areas.
Many gases are miscible with liquids. The miscibility of gases in liquids usually decreases with increasing temperature, and increases with increasing pressure. For example, more oxygen is dissolved in the blood at higher than normal pressures.