Pouring grain alcohol into water results in a single liquid phase. No meniscus forms between the alcohol and the water, and the two liquids are considered "miscible". Nearly any pair of liquids are miscible if only a trace amount of one of the liquids is present.
Many liquid mixtures fall between these two extremes. Two liquids are "partially miscible" if shaking equal volumes of the liquids together results in a meniscus visible between two layers of liquid, but the volumes of the layers are not identical to the volumes of the liquids originally mixed. For example, shaking water with certain organic acids results in two clearly separate layers, but each layer contains water and acid (with one layer mostly water and the other, rich in acid.)
Liquids tend to be immiscible when attractions between like molecules are much stronger than attractions between mixed pairs. Thermodynamics can provide a more complete explanation of why liquids sometimes won't mix completely .
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/liquids/faq/print-miscible-immiscible.shtml