The Unusual Expansion of Water

1. The Unusual Expansion of Water Done by Li Weijun (3S416)

2. Water • Nobody can live without water • Nothing we see today will exist without the unusual expansion of water. • Chemical formula: H2O • Bonding between molecules: Hydrogen Bonding

3. The unusual expansion • When cooled from room temperature liquid water becomes increasingly denser, just like other substances. • But at approximately 4 °C, water reaches its maximum density. In other words, when it cools, it expands to become less dense. Thus, ices are floating on the water. • This unusual negative thermal expansion is attributed to strong, orientation-dependent, intermolecular interactions

4. The molecular structure of water • form four hydrogen bonds; tetrahedral • In water local tetrahedral order due to the four hydrogen bonds gives rise to an open structure and a 3-dimensional bonding network, resulting in the anomalous decrease of density when cooled below 4 C.

5. The procedure from water to ice • the 'cooling' of intermolecular vibrations allowing the molecules to form steady hydrogen bonds with their neighbors • thereby gradually locking into positions reminiscent of the hexagonal packing achieved upon freezing to ice

6. The structure of Ice • The accepted crystal structure of ordinary ice was first proposed by Linus Pauling in 1935. • The structure of ice Ih is roughly one of crinkled planes composed of tessellating hexagonal rings, with an oxygen atom on each vertex, and the edges of the rings formed by hydrogen bonds.

7. The structure of Ice • The distance between oxygen atoms along each bond is about 275 pm • The angle between bonds in the crystal lattice is very close to the tetrahedral angle of 109.5°

8. The Key Point! • This tetrahedral bonding angle of the water molecule essentially accounts for the unusually low density of the crystal lattice -- it is beneficial for the lattice to be arranged with hexagonal angles even though there is an energy penalty in the increased volume of the crystal lattice. • As a result, the large hexagonal rings leave almost enough room for another water molecule to exist inside. This gives naturally occurring ice its unique property of being less dense than its liquid form.

9. Conclusion • The tetrahedral bonding structure of water is the key. • It is beneficial for water molecules to be arranged in hexagonal structure though there are energy penalties for increased volume • Thus, water expands from 4oC to 0o C • Owing to this special property of water, we exist.

10. Bibliography • http://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/3D_model_hydrogen_bonds_in_water.jpg/180px-3D_model_hydrogen_bonds_in_water.jpg • http://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Cryst_struct_ice.png/250px-Cryst_struct_ice.png • http://en.wikipedia.org/wiki/Ice_Ih • http://en.wikipedia.org/wiki/Water_(properties)

11. Thank you!