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Why are most hydrocarbons, like alkanes and alkenes, less dense than water?

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    $\begingroup$ Mostly because oxygen atom is heavier and also smaller than carbon. $\endgroup$ Aug 20, 2016 at 6:45
  • $\begingroup$ But hydrocarbons have so many carbon atoms. Like pentane quite a large mass number. $\endgroup$
    – Person
    Aug 20, 2016 at 7:17
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    $\begingroup$ So what? True, all these atoms together weigh more than one, but they also occupy more space. $\endgroup$ Aug 20, 2016 at 7:23
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    $\begingroup$ Well, that's like strongest simplification possible. It gets complicated quickly if you delve into it. Starting from hydrogen bonds and VdW interactions. $\endgroup$
    – Mithoron
    Aug 20, 2016 at 12:31

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There is no truly simple explanation for bulk properties of liquids. But the balance of two factors probably goes a long way to explaining the difference between water and simple hydrocarbons.

The density of a liquid is determined by the mass of the molecules and the efficiency of how they pack in the liquid.

Molecules made of heavier atoms will tend to be denser (e.g. chloroform where the heavy chlorines give the liquid a density about 50% higher than water). Water is made of moderately heavier atoms than a hydrocarbon (oxygen is about 30% heavier than carbon).

The other factor is how well the molecules pack in the liquid. This depends on their shape and the molecular interactions among them. Water molecules have very strong hydrogen-bond interactions causing them to, on average, stay closer than molecules of equivalent size that don't hydrogen bond. Hydrocarbons have only weak van der waals interactions so don't, on average, stay so close.

The interaction of the molecular mass and the forces determine the overall density. Water wins on both compared to hydrocarbons.

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Density is not only related with mass, but with volume too. Density of a substance is its mass per volume. It means increasing volume will decrease the density.

The molecules of hydrocarbons are large in size, increasing the length of their chain will increase both their mass and volume. On the other hand, water molecules are smaller and simpler in structure, but it is heavier than its size.

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To extend the answer provided by @Junaid Jamil, you may borrow a concept used in small molecule crystallography. For an estimate, the volume of the individual molecule is approximated by the assumption that each atom with an atomic volume. These values are tabulated (see e.g., earlier answers provided here and here). Because density is the ratio of mass over volume, the second property you need to know for this estimate is the mass of the individual molecule.

If you plug-in the values, you recognize a trend, matching the pattern of densities of these liquids experimentally recorded in comparison to water. Equally note the jump passing for halogenated hydrocarbons:

|--------------------------------+-------+----------+---------+---------+------------+----------------------|
| compound                       | water | n-octane | benzene | ethanol | chloroform | carbon tetrachloride |
|--------------------------------+-------+----------+---------+---------+------------+----------------------|
| formula                        |   H2O |    C8H18 |    C6H6 |  C2H5OH |      CHCl3 |                 CCl4 |
| mass [amu]                     | 18.02 |   114.23 |   78.11 |   46.07 |     119.37 |               153.81 |
| volume [A**3]                  | 21.55 |   203.28 |  114.36 |   69.83 |      96.46 |               117.18 |
| calculated density [amu/A**3]  |  0.84 |     0.56 |    0.68 |    0.66 |       1.24 |                 1.31 |
|--------------------------------+-------+----------+---------+---------+------------+----------------------|
| experimental density [g/cm**3] |  1.00 |     0.70 |    0.88 |    0.79 |       1.49 |                 1.59 |
|--------------------------------+-------+----------+---------+---------+------------+----------------------|
#+TBLFM: @5$2..@5$7=@3/@4;%.2f

Note the limitations: These estimates err since packing in the (ordered crystalline) solid state from which the atomic volumes are derived leaves gaps between the molecules packed. Even the dense hcp packing of spheres occupies only about 74% of the volume available. In many cases, the intermolecular gaps increase further for molecules freely moving in a melt/liquid. This increment system neither accounts for the eventual molecular shape; in the liquid e.g., n-octane might be in a straight extended conformation, or may curl into a more spherical shape. Nor does it discern if $\ce{C8H18}$ refers to n-octane, or any other isomer of octane (18 constitutional isomers); both variations are known to affect density (e.g., n-octane $\pu{0.703 g cm-3}$ ref; 2,2,4-trimethylpentane $\pu{0.692 g cm-3}$ ref).


The experimental densities the table states are densities the English edition of Wikipedia reports for the compound in the liquid state, which were rounded to two decimals. Preference was given to reports for densities at $\pu{25 ^\circ{}C}$: water, n-octane, benzene, ethanol, chloroform, carbon tetrachloride.

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Density depends on mass and volume. Higher mass and lower volume occupied per molecules promote density. Hydrocarbons have high molecular mass (except methane) and molecules take more space due to presence of vander wall force only. In case of water molecules due to presence of hydrogen bond molecules take less space ya they are more compact than hydrocarbons resulting instead of more molecular mass of hydrocarbon they are less denser.

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