This latter property is very important for organisms in the oceans and/or freshwater lakes. For this reason, ice floats on the water (like an ice cube in a glass of water). In fact, its volume is a little over 9% greater (or density ca. Water expands when it freezes making it less dense than the water from which it freezes. Water, however, is unique in that it bonds in such a way that the molecules are held farther apart in the solid form (ice) than in the liquid. For nearly every known chemical compound, the molecules are held closer together (bonded) in the solid state (e.g., in mineral form or ice) than in the liquid state. When water freezes, however, bonds are formed that lock the molecules in place in a regular (hexagonal) pattern. This effect also helps explain why you are supposed to add salt to ice when making ice cream or to add salt to water when cooking spaghetti (although, in this case, the effect on boiling point is minor and the added salt is mainly for flavor). Increasing salinity also lowers the temperature of maximum density. Normal seawater freezes at -2˚ C, 2˚ C colder than pure water. Incidentally, increasing salinity increases the boiling point and decreases the freezing point. As you add salts to seawater, you also change some other properties. The density of average seawater with a salinity of 35 o/oo (35 g/kg) and at 4˚ C is 1.028 g/cm3 as compared to 1.000g/cm3 for pure water. However, as dissolved solids are added to pure water to increase the salinity, the density increases.
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