Why is it strange that water's densest point is 4 degrees Celsius?

Answer 1

Short Answer:

Water gets more dense as it cools until it reaches 4 degrees C. At that point, because water molecules (and the forces between them) are not spherical, they begin to exhibit a tendency to hold particular orientations at preferred separation distances that keep them further apart than was possible above 4 C. Cooling from 4 C towards 0 C removes more kinetic energy allowing the preferred orientations and separations to become more orderly, further decreasing density. At freezing, 0 C, the nonspherical forces between molecules lock them into the preferential orientations and separations of the ice crystal arrangement with a sharp increase in average separation and decrease in bulk density.

Full answer:

Most substances expand when heated and most substances have a higher density in a solid state than in a liquid one.

Water is different. Water becomes slightly lighter when it freezes. That is why ice floats.

As ice is made colder though, it follows the usual rule and gets more dense.

Water, as a liquid follows this rule - most of the time. For a small range of temperature, between 0 degrees Celcius and 4 degrees Celcius, water does the opposite and water is 0.013% denser at 4 C.

Both of the peculiar behaviors can be traced to the same origin. When any fluid freezes heat energy goes out of the fluid as the solid forms. The temperature remains constant during freezing, so the kinetic energy of the particles does not change. All the energy (heat of fusion) is a change in potential energy as a consequence of forces between particles. Usually forces between particles want to pull them closer together, but in water the forces are more complex.

Because water has a pretty strong electric dipole moment, orienting the water dipoles just right releases a lot of energy. To get them oriented "just right" involve a complex interplay of forces between each dipole and several nearby dipoles. This is further complicated by the tendency of water molecules to form hydrogen bonds.

As water cools below 4 C, kinetic energy decreases enough to allow the water molecules to spend more and more time in a preferred orientation of lower potential energy, but in order to get into this arrangement the the positions of the molecules have to move towards a regular geometric structure and that takes up more space that just being disarrayed and pushed tightly together. As the local geometric order increases and kinetic energy decreases from 4 C to 0 C, the local ordering increases and the molecules move further apart and density goes down.

When water gets to 0 C, the local geometric arrangement locks in and molecules form the regular periodic structure of ice crystals with a corresponding jump in space between molecule or rather a sudden decrease in density. Ice crystals grow as heat leaves the ice/water mixture at 0 C until the entire volume is solid ice.

If ice did not float, it would sink and the bottoms of oceans and lakes would be permanently frozen. There are planets and moons in our solar system covered in materials that do not float when frozen. We are lucky.

Data:

Water reaches it maximum density at 3.98 °C (39.16 °F).

The density of ice 0.9168 g/cm3 at 0 °C .

The density of water, 0.99984 g/cm3 at 0 °C and 0.99997 g/cm3 at 4 °C.

Answer 2

This strange phenomenon cause is the formation of water molecules clusters depending on temoerature.