Miscible Definition in Chemistry – What Is Miscibility?

Miscibility is the property of two substances to completely mix to form a homogeneous solution. Usually the term is used to describe liquid mixtures, but it applies to solids and gases, too.

Two substances are miscible if they mix in all proportions or concentrations to form a solution. In other words, it doesn’t matter whether you mix them equally or one component is present in a greater amount than the other.

Two substances are immiscible if they don’t completely mix to form a solution. When combined, immiscible substances separate into layers or form a heterogeneous mixture.

Examples of Miscible Mixtures

Ethanol and water are miscible liquids. No matter what proportions are mixed, they form a solution. Benzene and acetone are miscible. Hexane and xylene are miscible.

All gases are miscible with each other at normal pressures. For example, helium and nitrogen gases are miscible. Air and argon are miscible. Ethanol vapor and water vapor are miscible.

Miscible solids work a bit differently because they form from liquid melts and then solidify. Elements that form alloys are miscible. So, iron and carbon are miscible (to make steel). Copper and zinc are miscible (to make brass). Miscibility also produces minerals. For example, olivine [(Mg,Fe)₂SiO₄] is a solid solution formed by forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄).

Examples of Immiscible Mixtures

Oil and water are a classic example of immiscible liquids. You can mix oil and water, but they will separate. Other immiscible liquids are water and benzene, water and toluene, and methanol and cyclohexane.

While all gases are miscible at normal pressures, gas-gas immiscibility can occur at high temperatures and pressures. Under these conditions, the compressed particles behave more like liquids, but the temperature exceeds the critical temperature. For example, benzene vapor and water vapor become immiscible at high pressure.

Solids that don’t form alloys are examples of immiscible solids. They may mix as liquids, but separate upon solidification. For example, copper and cobalt are immiscible solids.

Partially Miscible Mixtures

Technically, miscibility is black-and-white. Two substances are either miscible or they aren’t. But, there are levels of immiscibility. Some solvents are soluble in each other in certain proportions. In other cases, very little of one component remains unmixed. For example, butanone (methyl ethyl ketone) and water are immiscible because butanone isn’t soluble at all proportions, even though it is largely soluble in water.

Identifying Miscibility

Usually, you can tell whether two liquids are miscible just by looking at the result. Miscible liquids produce a clear liquid, while immiscible liquids yield a cloudy or layered mixture. However, if the two liquids have the same color and similar indices of refraction, it may be difficult to see layers. Miscible solids form a homogeneous solid. Immiscible solids separate completely or else appear heterogeneous.

For solvents, it’s easiest to simply look up whether the liquids are miscible.

Factors That Determine Miscibility

Several factors affect miscibility. Substances with similar polarity tend to be miscible. In other words, “like dissolves like.” Nonpolar solvents, held together by van der Waals forces, can’t overcome the stronger bonds of polar solvent molecules to get between them and mix. So, polar solvents typically mix with other polar solvents, while nonpolar solvents usually mix with other nonpolar solvents. There are exceptions, so other factors come into play.

The percent weight of the hydrocarbon chain determines whether organic compounds are miscible with water. Ethanol only has two carbon atoms and is miscible with water. In contrast, 1-butanol has four carbon atoms and is immiscible with water.

Polymers tend to be miscible with one another if the mixture has a lower configurational entropy than its components.

Difference Between Miscibility and Solubility

Miscibility and solubility are related concepts. The biggest difference between them is that miscibility describes a mixture of two components in the same phase, such as two liquids or two gases. Solubility is a more general concept that can describe what happens in a mixture of two different phases, like sugar (a solid) and water (a liquid). Solubility is the ability of one component (the solute) to dissolve in the other component (the solvent). Of course, solubility can be applied to mixtures where both the solute and solvent are the same phase. Miscible liquids are soluble at all concentrations.

References

• Gilbert, John C.; Martin, Stephen F. (2010). Experimental Organic Chemistry: A Miniscale and Microscale Approach. Cengage Learning. ISBN 978-1439049143.
• Rowlinson, J. S.; Swinton, F. L. (1982). Liquids and Liquid Mixtures (3rd ed.). Butterworths Monographs in Chemistry.
• Stephen, H.; Stephen, T. (2013). Binary Systems: Solubilities of Inorganic and Organic Compounds. Volume 1P1. Elsevier. ISBN 9781483147123.
• Wade, Leroy G. (2003). Organic Chemistry. Pearson Education. p. 412. ISBN 0-13-033832-X.