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Physical Properties of Alkanes


The following are the general physical properties of alkanes.
  1. State
    The lower members in the alkane series are gases (methane, ethane, propane and butane). Alkanes containing 5 to 17 carbon atoms are liquids, while higher members are waxy solids. Thus alkanes change from the gaseous state to the solid state with increase of molecular mass.
  2. Nonpolar Nature
    Alkanes are nonpolar in nature. Therefore, alkanes are soluble in nonpolar solvents, such as benzene, ether, chloroform, carbon tetrachloride, etc. Liquid alkanes themselves are good solvents for other nonpolar substances.
  3. Boiling Point
    Lower alkanes have a lower boiling point. The boiling point gradually increases with an increase in the molecular mass (Fig. 17.7). The reason for an increase in the boiling point is that the alkane molecule is nonpolar in nature. These are attracted to each other by weak van der Waals forces. The extent of van der Waals forces in the case of alkanes with higher molecular masses is more because of the larger surface area of the alkane.
Moreover, straight-chain alkanes have higher boiling points than the corresponding branched-chain isomers.

 Boiling Point of alkanes increasing with increase in their molecular masses

  1. Melting Point
    In general, the melting point increases with an increase in the chain length of the carbon atoms in alkanes. However, an alkane with an odd number of carbon atoms has a lower melting point than an alkane with an even number of carbon atoms . This is because in the solid state alkanes with an even number of carbon atoms fit more closely than the molecules of alkanes having an odd number of carbon atoms. This accounts for the alternation of melting point in alkanes (also in alkenes and alkynes).

 Melting point of alkanes shows alternation Density of alkanes increases   with increase in their molecular masses
  1. Colour
    Alkanes are colourless gases, liquids or solids.
  2. Density
    The density of alkanes increases with an increase in the chain length .




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