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.

 

[ravenprp]

This is a great question! The reason why ice is less dense than water is because of the unique structure of water molecules. In liquid water, the molecules are constantly moving and sliding past each other, which allows them to pack together more closely and create a higher density. However, when water freezes into ice, the molecules arrange themselves into a crystal lattice structure, with empty spaces between the molecules. This arrangement makes ice less dense than liquid water. So even though the molecules in a solid are closer together, the empty spaces in between them make the overall density lower. This is why ice floats on top of liquid water and why a bourbon on the rocks exists - the ice is less dense and stays afloat on top of the liquid. I hope this helps to clarify things!