Miscibility is a term in chemistry that refers to the property of liquids to mix in all proportions, forming a homogeneous solution. In principle, the term applies also to other phases (solids and gases), but the main focus is on the solubility of one liquid in another. Water and ethanol, for example, are miscible in all proportions.
By contrast, substances are said to be immiscible if in any proportion, they do not form a solution. For example, diethyl ether is fairly soluble in water, but these two solvents are not miscible since they are not soluble in all proportions.
In organic compounds, the weight percent of hydrocarbon chain often determines the compounds miscibility with water. For example, among the alcohols, ethanol has two carbon atoms and is miscible with water, whereas octanol with a C8H17 substituent is not. Octanol's immiscibility leads it to be used as a standard for partition equilibria. This is also the case with lipids; the very long carbon chains of lipids cause them to almost always be immiscible with water. Analogous situations occur for other functional groups. Acetic acid is miscible with water, whereas valeric acid (C4H9CO2H) is not. Simple aldehydes and ketones tend to be miscible with water, since a hydrogen bond can form between the hydrogen atom of a water molecule and the unbonded (lone) pair of electrons on the carbonyl oxygen atom.
Immiscible metals are unable to form alloys. Typically, a mixture will be possible in the molten state, but upon freezing the metals separate into layers. This property allows solid precipitates to be formed by rapidly freezing a molten mixture of immiscible metals. One example of immiscibility in metals is copper and cobalt, where rapid freezing to form solid precipitates has been used to create granular GMR materials.
Miscibility is partly a function of entropy, and so is seen more commonly in states of matter that are more entropic. Gases mix quite readily, but solids only rarely display miscibility. Two useful exceptions to this rule are solid solutions of copper with nickel (the cupronickel used in coins and specialty plumbing), and of silicon with germanium (used in electronics). Substances with extremely low configurational entropy, especially polymers, are unlikely to be miscible in one another even in the liquid state. Perdeuteropolyethylene is immiscible in polyethylene.
Miscibility of two materials is often determined optically. When the two miscible liquids are combined, the resulting liquid is clear. If the mixture is cloudy the two materials are immiscible. Care must be taken with this determination. If the index of refraction of the two materials are similar, an immiscible mixture may be clear and give an incorrect determination that the two liquids are miscible.