1.2. Solubility, Miscibility And Immiscibility
When one liquid dissolves in a second their molecules mix freely together and a solution is formed. Such liquids are said to be miscible. When two liquids will not mix (i.e. their molecules will not interact together freely) they are said to be immiscible; therefore engine oil and water are immiscible. When two immiscible liquids separate the one with the lowest density will rise to the top.
As a rule of thumb like dissolves like. That is to say that polar liquids will mix with other polar liquids but not non-polar liquids, also non-polar liquids will mix well with other non-polar liquids but not polar ones. Liquids that will not mix with water are said to be hydrophobic and those that will mix with water are said to be hydrophilic.
Substances are not necessarily soluble or insoluble, in many cases substances are slightly soluble. For example the solubility of alcohols in water is affected by the size of the hydrophobic portion of the molecule and the number of -OH groups present. Hence ethanol (CH3CH2-OH) is completely soluble in water and butanol (CH3CH2CH2CH2OH) is only partially soluble (7.9 g of butanol per 100 ml of water). Also Butanediol (HO-CH2CH2CH2CH2-OH) has two -OH groups and is completely soluble in water although it has the same number of carbon atoms as butanol.