Miscibility

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Miscibility is the ability of two or more substances to mix, and form a single homogeneous phase. When two substances are immiscible they will form separate phases when mixed; the best known example is oil and water. On the other hand, water and ethanol are miscible in all proportions, and some combinations of substances are only somewhat miscible; for example, adding more than a certain amount of table salt to water will leave some in the solid phase, though this is more properly a solution, one specific type of mixture.

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.