<P> Miscibility / mɪsɪˈbɪlɪti / is the property of substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous solution . The term is most often applied to liquids, but applies also to solids and gases . Water and ethanol, for example, are miscible because they mix in all proportions . </P> <P> By contrast, substances are said to be immiscible if there are certain proportions in which the mixture does not form a solution . For example, butanone is significantly soluble in water, but these two solvents are not miscible because they are not soluble in all proportions . </P> <P> In organic compounds, the weight percent of hydrocarbon chain often determines the compound's miscibility with water . For example, among the alcohols, ethanol has two carbon atoms and is miscible with water, whereas 1 - butanol with four carbons is not . Octanol, with eight carbons, is practically insoluble in water, and its 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 almost always to be immiscible with water . Analogous situations occur for other functional groups . The straight - chain carboxylic acids up to butanoic acid (with four carbon atoms) are miscible with water, pentanoic acid (with five carbons) is partly soluble and hexanoic acid (with six) is practically insoluble . The same pattern exists for other molecules, for example aldehydes and ketones . </P>

What is it called when two liquids separate