Water’s Liquid and Solid Phases Are Vastly Different
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The molecular nature of water causes molecules to attract and repel each other and to form links between them. These features affect the functional properties of water.
Water is the most common compound on Earth. But it has rather uncommon physical properties that are important to live organisms. These properties are largely based on the chemical structure of the water molecule and how water molecules interact with each other and other molecules.
Chemical Structure of a Water Molecule
A water molecule is composed of two atoms of hydrogen attached to a single atom of oxygen. In an atom, the number of electrons equals the number of protons and the atom has a neutral charge. But in the water molecule, the oxygen atom has a stronger attraction for electrons than the hydrogen atoms do. This strong attraction means the oxygen atoms pull an electron from each of the hydrogen atoms causing the hydrogen atoms to take on a positive charge and the oxygen atom to take on two negative charges. Thus, each molecule of water has two positive poles (one on each of the two hydrogen atoms), and two negative poles (on the single oxygen atom). These opposite charges cause water molecules to attract each other.
Interaction of Charges on Water Molecules
As water molecules move and twist around, the oppositely charged poles of neighboring water molecules attract each other, bringing these molecules to close together, while similarly charged poles of other neighbors repel each other, pushing these molecules apart.
Water molecules are spaced differently in water and ice, and this spacing causes a difference in the density of these two phases. In liquid water, the molecules are free to move past each other. In ice, they are held apart by rigid links that keep the molecules at a longer distance from each other than the average molecule in liquid water.
In liquid water, the molecules are free to slip apart and bounce around and between neighboring molecules. Neighboring water molecules often form and break temporary hydrogen bonds between them where each molecule holds the hydrogen atom with an ice-like bond that pushes the two neighbors apart. This freedom to slip around and to form links causes some molecules to be closer together and others to be further apart.
Different Physical Properties of Water and Ice
As liquid water cools, the amount of movement between molecules slows down and the molecules do not come in contact as often as they do when the water is warmer. The molecules in cold water are not able to move as rapidly or far as they do in warm water, the attractions and repulsions between slowly moving neighboring molecules become stronger, and the molecules become pulled into a tight orientation that reduces the spacing between the molecules. Thus, cold water becomes denser than warmer water. Water has its greatest density at a temperature of 4 degrees Centigrade (39 degrees Fahrenheit) shortly before it freezes. This water sinks below the surface.
As the water temperature drops below 4 degrees Centigrade, ice links form between more and more molecules locking them into position and pushing them slightly apart. The water becomes less dense and rises to the surface. In ice, almost all of the molecules are linked at a distance and no longer able to slip past each other. This gives crystalline water a lower density than liquid water.
Water is Denser than Ice
Ice floats because its molecules are further apart and solid water (ice) is lighter (less dense) than liquid water. As ice forms, it floats on top of the liquid water, forms a solid sheet that insulates the liquid water from colder air temperatures, and prevents the water from freezing solid.
These properties of water are responsible for important changes in the ecological dynamics of large bodies of water in the spring and fall.