Answer to Question #251047 in General Chemistry for umai

Question

Based on the kinetic molecular model, solids usually have higher densities compared to liquids. However, ice, a solid form of H2O floats on liquid water which means ice is less dense than water. What do you think is the reason for this phenomenon? What is the biological significance of this concept?

Answer 1

Ice actually has a very different structure than liquid water, in that the molecules align themselves in a regular lattice rather than more randomly as in the liquid form. It happens that the lattice arrangement allows water molecules to be more spread out than in a liquid, and, thus, ice is less dense than water. The density of ice is about 90 percent that of water, but that can vary because ice can contain air, too. That means that about 10 percent of an ice cube (or iceberg) will be above the water line.

This property of water is critical for all life on earth. Since water at about 39°F (4°C) is more dense than water at 32°F (0°C), in lakes and other water bodies the denser water sinks below less-dense water. If water was most dense at the freezing point, then in winter the very cold water at the surface of lakes would sink, the lake could freeze from the bottom up. And, with water being such a good insulator (due to its heat capacity), some frozen lakes might not totally thaw in summer.

The real-world explanation of water density is actually more complicated, as the density of water also varies with the amount of material that is dissolved in it. Water in nature contains minerals, gasses, salts, and even pesticides and bacteria, some of which are dissolved. As more material is dissolved in a gallon of water then that gallon will weigh more and be more dense—ocean water is denser than pure water.