| Themes > Science > Chemistry > Organic Chemistry > Physical Properties of Organic Compounds |
In general, covalent organic compounds are distinguished from inorganic salts by low melting points and boiling points. The ionic compound sodium chloride (NaCl), for example, melts at about 800° C (about 1470° F), but the strictly covalent molecule, carbon tetrachloride (CCl4), boils at 76.7° C (170° F). Between these temperatures an arbitrary line may be drawn at about 300° C (about 570° F) to distinguish most covalent from most ionic compounds. A large fraction of organic compounds melt or boil below 300° C, although exceptions exist. Organic compounds generally dissolve in nonpolar solvents (liquids that do not have localized electric charges) such as gasoline or carbon tetrachloride, or solvents of low polarity such as alcohols, acetic acid, and acetone. Organic compounds are often insoluble in water, a strongly polar solvent. Hydrocarbons have low densities, often about 0.8 compared to water, 1.0, but functional groups may increase the densities of organic compounds to 1.0. Only a few organic compounds have densities greater than 1.2, generally those containing multiple halogen atoms. Functional groups capable of forming hydrogen bonds generally increase viscosity (resistance to flow) in molecules. For example, the viscosities of ethanol, ethylene glycol, and glycerol increase in that order. These compounds contain one, two, and three OH groups, respectively, which form strong hydrogen bonds. |
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