Why is CO2 liquid less dense than CO2 solid

[bobsmith76]

I can sort of understand why water is denser than ice, but for CO2 I cannot understand why it is the other way around. Here is my best shot at understanding it: I imagine H2O's solid structure as a hexagon due to the hydrogen bonds, where each point represents one atom. When these bonds are broken the six line segments separate from each other and now the liquid is just a compilation of these six line segments which is more dense. I imagine a CO2 solid as a set of squares fixed together. When it turns into a solid those squares remain in tact thus the substance does not become more dense.

Does this sound right?

 

[mathman]

As a practical matter, water is the exception. Almost all substances are denser as solids in comparison to liquids.

 

[pallidin]

From: http://wiki.answers.com/Q/What_subs...a_solid_state_than_they_are_in_a_liquid_state


In addition to water, for which the solid form (ice) is less dense than the liquid form, the element Gallium is also less dense in the liquid state. Gallium can be solid at room temperature but will melt in the hand.
== Less Dense in Solid Form ==

confirmed as solid less dense than liquid:


gallium - 5.91 (solid) vs 6.095 (liquid)
bismuth - 9.78 (solid) vs 10.05 (liquid)
germanium - 5.323 (solid) vs 5.60 (liquid)
silicon - 2.3290 (solid) vs 2.57 (liquid)
water - 0.917 (solid) vs 0.998 (liquid)

claimed but probably false:


acetic acid - 1.266 (solid) vs 1.049 (liquid)
antimony - 6.697 (solid) vs 6.53 (liquid) (this "error" is repeated in many places, inc wikipedia)
Water is not always less dense in solid form. Depending on how the water crystals are formed, it may actually be more dense. Examples include HDA and VHDA.



Read more: http://wiki.answers.com/Q/What_subs...than_they_are_in_a_liquid_state#ixzz1BFSglL6f

 

[bobsmith76]

I'm not worried about the statistics of density, I want to know why.