The Water Molecule

All substances on Earth consist of molecules. The water molecule is the most abundantly available molecule on Earth and is also one of the most unique ones, as will be made apparent as you read on. Molecules consist of smaller parts, called atoms. The water molecule contains three atoms: one oxygen atom and two hydrogen atoms. Oxygen, the substance we all breathe, consists of two oxygen atoms. The chemical symbol for water is H2O, where H2 stands for two hydrogen atoms and the O for one oxygen atom. Very strong electromagnetic forces attach atoms in one molecule to each other.

A parallel electromagnetic attraction exists between individual molecules. Molecules are constantly moving and continually forming and breaking bonds with other molecules. Under high pressure and low temperature, molecules don’t travel as much and usually form a solid substance. In this state, the distance between individual molecules is relatively small and the density of the substance is relatively high. As temperature increases, or pressure decreases, molecules travel faster and bonds weaken. At this point the solid substance will melt and form a liquid. At even higher temperatures/lower pressures the bonds weaken even more and the molecule’s state changes to gas. This is called the boiling point. In this stage, the distance between molecules is large and intermolecular bonds are almost non-existent, therefore gases have a very low density. The boiling and melting points are different for every substance on Earth. Under normal pressure and temperature levels, for example, oxygen is a gas, while mercury is a liquid and gold, a solid. However water is the only substance that can naturally be found in all three states: ice (solid), water (liquid), and vapor (gas).

Many of the unique qualities of water are due to strong hydrogen bonds between water (H2O) molecules. The hydrogen (H2) atom has a slight negative charge while the oxygen (O) atom has a slight positive charge. Thus, this molecule has polarity – that is, one side is positive and the other, negative, and is called a dipole. Similar to a magnet, the hydrogen of one water molecule is attracted by the oxygen atoms of another water molecule. As water molecules are constantly moving, they perpetually form and break hydrogen bonds, as fast as every 200 femtoseconds, which is 0.0000000000002 seconds or 5 trillion times per second. The hydrogen bond is weaker than bonds between atoms within a molecule, but stronger than any other intermolecular bond.

When water becomes ice it forms a crystalline structure, which prevents further compression of the molecules. Because of the hydrogen bonds, however, the density of water can reach a higher level in its liquid state than in solid state: which makes it possible for ice to float on water. The highest density of water molecules is at about 4°C (39.2°F). The water molecule is one of the few molecules that has its highest density in the liquid state. If water were not denser than ice, all the lakes would freeze solid in the colder regions of the world, as ice would sink to the bottom. In the summer only the top layer of these lakes would thaw. However, since the highest water density is at 4°C, only the top layer freezes, the water below the ice drops to 4°C and then remains liquid. It can not become ice because there is no space to expand with the layer of ice on top. This gives water plants and animals an opportunity to survive under the ice.

Other special features of water related to the hydrogen bonds are its solvent?  capacity and its high surface tension. Water is known as one of the most universal solvents and can dissolve almost anything except “hydrophobic” (literally, “water-fearing”) substances such as oil. Substances that are “hydrophilic” (literally, “water-loving”) dissolve easily in water. Because water molecules are very small, and dipolar, any hydrophilic substance that comes into contact with water and has either a positive or a negative charge will dissolve. The positively charged molecules will form bonds with the oxygen element of the water molecule, while negatively charged molecules will form hydrogen bonds with the hydrogen part of the water molecule’s hydrogen atoms. However non-polar, hydrophobic molecules like oil will not dissolve in water.

The high surface tension is essential for the action known as “capillary rise”. Capillary rise is the process in which water moves up a narrow tube against the force of gravity.  It occurs because water adheres to the sides of the tube, and then surface tension tends to straighten the surface making the surface rise, and more water is pulled up through cohesion. The process is repeated as the water flows up the tube until there is enough water that gravity counteracts the adhesive force. This same cohesive process is used in plants and trees to extract water and nutrients from surrounding soil. The high surface tension also allows light objects to stay on top water. A graphic illustration of this is the pond skater. This insect is literally able to walk on water!