Comparative Densities

• Density is the amount of material that a given volume of a substance contains. Approximate density values can be found by weighing a substance at sea level and dividing the resultant figure by its volume. A cubic meter of water weighs 1,000 kilograms, while a cubic meter of ice weighs only 917 kilograms, according to "Serway's College Physics." Accordingly, water has a greater density than ice.
  
  

Comparative Volumes

• Since water has a greater density than ice, water will occupy more space after it changes from a liquid to a solid state. In other words, ice will have a greater volume than an equal weight of water. Based on the comparative densities given in "Serway's College Physics," a kilogram of liquid water occupies a volume of 1,000 milliliters while a kilogram of ice occupies a volume of 1,091 milliliters.
  
  

Hydrogen Bonding

• The reason for this unusual behavior of water is a phenomenon called hydrogen bonding. The water molecule consists of two hydrogen atoms and one oxygen atom. The two hydrogen atoms are attached directly to the oxygen atom, and the angle between them is 104.5 degrees. The reason why the hydrogen atoms are crowded on one side of the oxygen atom is because the other side is occupied by two pairs of electrons. As a result, the electrical charge is unevenly distributed in the water molecule. The side containing the electron pairs has a greater share of the negative charge than the side containing the hydrogen atoms. This will result in a weak attraction between each hydrogen atom and an electron pair of a neighboring oxygen atom. Because of the mobility of water molecules in the liquid state, this hydrogen bonding will not establish a rigid lattice, but when the water falls below 4 degrees Celsius, the bonding becomes stronger. When water freezes, the hydrogen bonds form a rigid lattice that takes up more space than the less organized water molecules in liquid water. Lattices are voluminous because of the empty space that exists between its component parts. This is why water occupies a greater volume after it turns to ice.
  
  

Practical Consequences

• Water reaches its greatest density at 4 degrees Celsius. Consequently, when the temperature falls below the freezing point of water for a considerable period of time, the water at the bottom of ponds and lakes will eventually fall to 4 degrees Celsius. In the meantime, the water on top will approach the freezing point, and ice will eventually form on the water's surface. This is important, because if ice were denser and less voluminous than water, ice would form at the bottom of lakes instead of on top. Eventually, entire lakes would become solid masses of ice, and fish and other aquatic creatures would die.