I am an engineering researcher at the University of California-Santa Barbara. Thank you for your GREAT question “Why it is that water is less dense in solid form than in liquid form?”

This is a personal favorite question of mine, as it has to do with a lot of the research that I do. Typically, solids are always more dense than liquids. So it is very interesting that water—the most important molecule on our planet—is denser as a liquid than it is as a solid. This occurs because of the type of interactions that water molecules have with each other. When two water molecules come very close together, they can form a special type of bond called a “hydrogen bond”. The hydrogen bond forms between the hydrogen atom of one water molecule and the oxygen atom of the other molecule. These hydrogen bonds are very weak (much weaker than chemical covalent bonds) but they are still important. In the solid ice form, each water molecule will form a hydrogen bond with 4 other water molecules. The 4 hydrogen bonds in ice create a very ordered network of ice molecules. However, when water is in its liquid form, each water molecule can only form a hydrogen bond with 3 other water molecules. Since each water molecule only forms 3 hydrogen bonds, this makes the arrangement of the water molecules a lot crazier and disorganized. The overall result is that, with 3 hydrogen bonds per molecule, the water molecules are able to squeeze together more tightly and create a denser liquid. I have attached a link to an image below which will help illustrate this process.

The red balls represent the oxygen atom in water, the white balls represent the hydrogen atoms in water, and the dashed lines represent the hydrogen bonds between water molecules. In the first picture (a) you can see how solid ice forms 4 hydrogen bonds and is ordered very nicely. In the second picture, you can see how liquid water has fewer hydrogen bonds and the water molecules are no longer ordered in a nice way.

I hope this helps with you question!