Water Density

One way to visualize how water can have different densities is to look at the frozen form of water. Ice 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. The lattice arrangement allows water molecules to be more spread out than in a liquid, and, thus, ice is less dense than water. iced teaThe density of ice is about 90 percent that of water, but that can vary because ice can contain air. 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 denser 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 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 that gallon will weigh more and be more dense—ocean water, which contains dissolved salt, is denser than pure water. 

The instrument to measure the density of a liquid is called a hydrometer. It is one of the simplest of scientific-measuring devices, and you can even make your own out of a plastic straws. More often, though, it is made of glass and looks a lot like a thermometer. It consists of a cylindrical stem and a weighted bulb at the bottom to make it float upright. The hydrometer is gently lowered into the liquid to be measured until the hydrometer floats freely. There are etched or marked lines on the device so the user can see how high or low the hydrometer is floating. In less dense liquids the hydrometer will float lower, while in more dense liquids it will float higher. The hydrometer is measuring the relative density, also called specific gravity, of the liquid. Relative density is the ration of the density of one substance to the density of another reference substance, for liquids the reference substance is typically water at its maximum density (at 39.2 °F or 4 °C). 

Hydrometers have many uses, not the least being to measure the salinity of water for science classes in schools. They are also used in the dairy industry to get estimates of the fat content of milk, as milk with higher fat content will be less dense than lower-fat milk. Hydrometers are often used by people who make beer and wine at home, as it offers an indication of how much sugar is in the liquid, and lets the brewer know how far along the fermentation process has gone.