The simple answer is that ice is less dense than water. The question then becomes: Why is ice, which is water in solid form, lighter than water in its liquid form? Something must be happening to water when it freezes.
One molecule of water has two hydrogen atoms and one oxygen atom, or H2O. The hydrogen atom is slightly positive, and the oxygen atom is slightly negative. At room temperature, these water molecules are loosely linked and free to move around easily, which is what defines the liquid state of matter.
Once the temperature hits 32°F, or 0°C, the molecules line up in a rigid six-sided honeycomb structure, with water molecules—the more positively charged hydrogen attracting the more positively charged hydrogen—linking up with each other in a process called hydrogen bonding.
This causes the molecules to move more slowly and take up more space. You have ice. Because the same weight or mass now takes up more space, it is less dense and floats on water.
We can put salt on ice to melt it. Salt is sodium chloride, or NaCl. (Technically, most salt for sidewalks is potassium chloride, or KCl, but the chemical process works the same way.) The sodium atom in a salt molecule has a slight positive charge, and the chloride atom has a tiny negative charge.
A water molecule has one atom of oxygen bound to two atoms of hydrogen. The hydrogen atoms are “attached” to one side of the oxygen atom, resulting in a water molecule having a positive charge on the side where the hydrogen atoms are and a negative charge on the other side, where the oxygen atom is.
The sodium chloride ions pull and tug on the water’s two hydrogen atoms (unlike charges attract) and tear the bonds apart. The ice no longer has a honeycomb structure. It has melted. In order for the melted ice to freeze again, it has to go much lower than 32°F; the salt has lowered the freezing point of the water.
Most of Earth’s ice is packed into huge sheets called glaciers. The largest glacier covers most of the Antarctic continent and holds about 90 percent of the world’s ice. If all of it were to melt, the oceans would rise 187 feet.
You might want try this science experiment. Fill a drinking glass with water, and then put an ice cube in the water. You can see that the ice cube floats. Now, using isopropyl rubbing alcohol that you can buy in any drugstore, fill a glass with alcohol and carefully drop in an ice cube.
What happens to the ice cube? Alcohol is less dense than water, less even, than frozen water, so, since the ice cube is denser than the alcohol in the glass, it sinks.
From the book, "Ask a Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works"; Copyright © Larry Scheckel, 2013. Publishes December 17; available wherever books are sold.