Miscible
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The chemistry term miscible refers to the property of various substances, particularly liquids, that allows them to be mixed together and form a single homogeneous phase. For example, water and ethanol are miscible in all proportions.
By contrast, substances are said to be immiscible if they cannot be mixed together, for example, oil and water.
Some combinations of substances are only somewhat miscible. Adding more than a certain amount of table salt to water will leave some salt in the solid phase, though this is more properly a solution, one specific type of mixture.
In organic compounds, the length of the carbon chain often determines miscibility relative to members of the homologous series. For example, in the alcohols, ethanol has two carbon atoms and is miscible with water, whereas octanol has eight carbon atoms and is not miscible with water. 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.
Miscibility can arise for a number of reasons. In the alcohol examples above, the OH group can form hydrogen bonds with water molecules. In aldehydes and ketones the hydrogen bond can form with a lone pair of electrons on the carbonyl oxygen atom.
In metals, immiscible substances are unable to form alloys. Typically, a mixture will be possible in the molten state, but upon freezing the metals separate into separate phases. This property allows solid precipitates to be formed by rapid freezing of 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. Gasses mix quite readily, but solids only rarely display complete 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.
