A burbon on the rocks exists because ice is less dense than water

[sanjuro]

I am fairly new to chemistry, so this might seem like a stupid question, but I am still interested nonetheless. A burbon on the rocks exists because ice is less dense than water, but how can this be the case when the molecules in a solid are closer together?

 

[Monique]

Hi Sanjuro :)

You are true for most substance: when the temperature of a substance increases, the density decreases due to the violent motion of the molecules. When the temperature decreases, the density increases, since molecules are more closely packed.

This doesn't hold true for water, where the density will actually DECREASE when the temperature DECREASES.

The reason for this is that at a lower temperature, the water molecules are allowed to form hydrogen bonds, which allows it to form a hexagonal crystalline structure with many open spaces.

To illustrate: in liquid water, each molecule is hydrogen bonded to approximately 3.4 water molecules. In ice this is 4 other molecules.

This page has a nice figure: http://tidepool.st.usm.edu/pix/ice.gif You can see that the lowest density (at low temperature) is at 0 oC and the highest density is at 3.97 oC, that is when the hydrogen bonds are broken, and the water 'collapses' on itself.