On water’s expansion with freezing

On water’s expansion with freezing

On water’s expansion with freezing

09/16/17

“Why does water expand upon freezing?”

The variation of volume with thermal energy for most liquids has a very simple characteristic. When heat is applied, the volume increases, and vice versa for cooling. This is because the added (or subtracted) energy will cause the amplitude of the vibrations of the molecules to change, thereby modifying the volume. For example, when a liquid freezes, the molecules will pack into one another, thereby shrinking the volume.

However, water exhibits a very peculiar phenomenon. When water is cooled to its freezing point, its volume will actually expand. Why does this happen? Well, let’s analyze it using our scientific mindset. Unlike most other molecules, water has a very unusual structure. Specifically, a water molecule’s primary form of bonding is based on hydrogen bonding. When temperature decreases, the strength of a hydrogen bond actually increases (since the lower thermal energy means that the hydrogen bonds will have less vibrational energy, therefore lowering the chance to shake out of position and increasing stability).

Once water is cooled into ice, the only bonding will be hydrogen bonding. Specifically, it will be bonded in a hexagonal structure, which is a much more “open” network than most structures. The tandem of hydrogen bonding and a hexagonal structure vastly decreasing the density (Levine, Scott 2013). And because density is described by the equation, with being the density,  being the mass, and being the volume, and as mass is constant, when the density decreases the volume must increase as a result. Consequentially, the volume of water increases upon freezing! This fact has multiple implications. For example, a lower density of ice means that ice will float in water, which allows for complex structures such as ice glaciers to occur naturally.

 

References

“Why Does Water Expand When It Freezes? .” FAQ: Water Expansion on Freezing, New York University, 3 Dec. 2013, http://www.iapws.org/faq1/freeze.html.

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