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Q & A: Densities of Solids and Liquids

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Most recent answer: 12/27/2013
Q:
What other substances besides Water, will in their solid state float in their liquid state?

Example, Ice is a solid and floats in its liquid state water.
- Johann Muckler (age 15)
Kearney HS, San Diego,CA
A:
Hi Johann,
Ice floats in water because it is less dense than water. So any substance that has a lower density in its solid state than in its liquid state will float. While a number of compound materials (like salts and alloys) have this property, only 5 elements on the periodic table are known to have a density that is greater in their liquid phase.

Element____Solid Density____Liquid Density____Melting Temp.
___________(g/cc) __________(g/cc)__________Celsius

Arsenic______4.70 ___________5.22___________817
Bismuth______9.80 __________10.07___________271
Gallium_______5.90 ___________6.09____________30
Germanium____5.32 ___________5.60___________940
Silicon________2.33___________2.51__________1410


Thanks for the question,
James

(published on 10/22/2007)

Follow-Up #1: Molecular distances in liquid and solid phases

Q:
Many text books show molecules in the liquid state to be significantly further apart than in the solid state. Am I right in thinking that this is an error? The change of density on melting does not appear to be huge for most materials, and liquids are incompressible.
- David Thomas
Aberystwyth, Wales
A:
Bingo! Youíve got the right idea in focusing on the density.   Probably the text book illustrations are trying to emphasize that the molecules in a liquid state are not aligned in rigid crystalline arrays.  Curiously, ice, the solid state of water, is less dense than the liquid state.  Donít ask me why, youíll have to ask Mike.

LeeH

Beats me. /Mike

(published on 12/04/2007)

Follow-Up #2: ice less dense than water

Q:
Ice is less dense than liquid H2O because of the geometry of the ice crystals. The partially negative oxygen's will interact with the partially positive hydrogen's of neighboring water molecules, and if the number of hydrogen bonds is maximized the H2Os form tessellations of hexagons (DRAW IT!). These hexagons have a lot of space between them, which makes ice less dense than irregular water molecules, because the stability from the hydrogen bonds as a hexagon makes up for the lack of entropy in this ordered configuration.
- Trevor Zandi (age 22)
La Jolla
A:

Agreed. That specific explanation only applies to water. The other materials for which the solid is less dense than the liquid have different detailed explanations for their optimum crystal structures. 

Mike W.


(published on 12/27/2013)

Follow-up on this answer.