5. Density of Water: Water is less dense as ice than as liquid, affects plant life.
At the same temperature of 0°C, the density (mass per volume) of ice is 0.9187 gram per cubic centimeter (g cm-3 or g/cm3) while that of liquid water is 0.9998 g cm-3 (Cohen et al. 2003). The lesser density means that ice contains lesser mass (quantity of matter) per unit of volume. It also means that the molecules of water are less compressed per unit volume of ice.
The above densities indicate that with the same volume under identical gravitational force, ice will be lighter and liquid water will be heavier compared to each other. On Earth, a cubic centimeter of ice will weigh 0.9187 gram (or 918.7 kg per cubic meter) while the same volume of liquid water will weigh 0.9998 gram (or 999.8 kg per cubic meter) at temperature of 0°C.
The difference of 0.0811 gram between the densities of water as ice and as liquid represents about 8.11 percent reduction in the density of liquid water when it transforms to ice or an addition of about 8.83 percent of the density of ice when it melts and changes to liquid water. It also means that theoretically, about 8.83 percent increase in the density of ice is needed if it is to equalize with that of liquid water.
The density of water is at maximum at 1 g cm-3 (or 1000 kg m-3) at a temperature of about 4°C. Above this temperature its density continues to decrease, for example from 0.9997 g cm-3 at 10°C to 0.9982 g cm-3 at 20°C, to 0.99584 g cm-3 at 100°C. Conversely, the density of ice increases from 0.9187 g cm-3 at -10°C to 0.9203 g cm-3 at -20°C (Cohen et al. 2003). Nevertheless, the increase in density as it gets cooler is only minimal and liquid water is always denser than ice.
Because of its lesser density, ice floats on liquid water. Ice on the surface of a body of water serves as insulator and prevents freezing of the water below. This allows fishes and other aquatic organisms to live even during winter in temperate countries.Thus the surface of a lake may become ice but it is still possible to hook a fish by making a hole and dropping a line.
If the density of water as ice is greater than that of liquid water, life will be in peril in places prone to freezing temperatures. For example, if it should happen that ice density is increased by more than 8.83 percent at 0°C, the resultant density will exceed 0.9998 g cm-3. In this case ice, being more heavy than liquid water, will sink to the bottom. Liquid water at the surface, without insulation from the freezing temperature, will keep transforming into ice. It will just be a matter of time and the entire pond, lake, or any body of water will become solid ice.
In liquid state the molecules of water are in motion and they exchange hydrogen-bond partners continuously. At 0°C water freezes, an indication that the molecules have stopped moving. In this solid state (ice), the hydrogen bonds are more rigid and relatively open.
The decrease in the density of water as liquid water changes to ice also involves expansion. Unlike other substances, water expands or increases in volume as it solidifies. It is therefore expected that an ice when completely melted will yield a volume of liquid water that is lesser than the original volume of the ice.
As a consequence, the freezing and expansion of water within the cell can adversely affect plant growth by rupturing the cell walls. However, trees can resist damage during cold winters due to minimal water content in their trunks and branches. Where freezing is slow, it may cause more damage to plants because only a few crystals are formed. These crystals will likely be large and can rupture the cell walls (Devlin 1975; Bettelheim and March 1998).
The expansion of water as it solidifies is the same principle which explains why a bottle that is filled with water with screwed cap will burst when frozen. When water inside changes to ice, it expands. The expansion would create a strong force sufficient to push the walls of the bottle outward until they shatter. This also explains why seeds of plants stored in a freezer for a relatively long period of time can lose their viability.
(Ben G. Bareja March 2013)