Ice floats because it is less dense than the water or other liquid in the drink. Density is a measure of how closely packed atoms are in a space. Density is defined mathematically as the mass of a substance per unit of volume, and is commonly expressed in units g/cm3 (gram per cubic centimeter). Every substance has a specific density at a given temperature and pressure. The amount or size of a substance does not change its density. Even an iceberg floats because it is less dense than water.

Water is made of the molecules of two hydrogen atoms and one oxygen atom. In liquid form, water molecules are packed closer together than in solid ice. Liquid water is denser than ice, so the ice floats.

As water freezes into a solid, it expands. We can observe this expansion when water in a closed container is frozen. As the water freezes, the container will break from the force of the expanding ice. Another way to observe this expansion is to mark the level of water on a paper cup and then place it in a freezer. It is easy to see how the volume of water has increased as ice. There is about a 9 percent increase of volume when water freezes.

Water is unique in the way its density decreases as a solid. Other materials behave in the opposite way. Solids are usually denser than their liquid forms. The water molecules bond differently as water freezes. Each molecule in liquid water bonds to about 3.4 other water molecules. In solid form, each molecule in ice bonds to four other molecules. This change in bonding alters the molecular structure. Greater spaces form between molecules, causing the ice to expand.

The density of water varies with temperature and impurities. At 4°C (39.2° F) pure water has a density of 1 g/cm3. When water turns to ice it has a density less than one (.915 g/cm3). Turning water into a solution with salt or sugar increases its density. An ice cube made of a sugar solution can become dense enough to sink in water.

As you can see, density is a property of solids, liquids and gases. The Earth's density varies as you move around the globe. More tightly packed matter, or matter with greater density, results in more gravitational pull.

For example, gravity's pull above land is usually greater than its pull above water because solid materials are usually denser than liquid water. Some rocks are denser than other rocks, so gravity's pull above one rocky area can be greater than another, if everything else is the same.

NASA has discovered that these density differences can affect spacecraft and satellite orbits. In some locations, an object in low-Earth orbit may move faster because gravity has a stronger pull on it. A satellite's orbital distance may change as it passes over land or water.

The density of Earth's atmosphere also affects satellites. Sometimes, satellites in low-Earth orbit "rub" against the atmosphere. This "rubbing" may make a satellite slow down due to friction with the atmosphere. The satellite may even need a "boost" into a higher orbit where the atmosphere is less dense so that it won't "drag" as much. This happened with the Skylab space station, launched by NASA in 1973, causing the Skylab to fall to Earth in 1979.