Water is one of the most widespread materials on this planet. Water fills the oceans, sculpts the land, and is a primary component in all living things. For all of its commonness, water is a very unusual molecule whose unique qualities are due to its physical structure.
Water is a compound made up of three atoms: two hydrogen atoms and one oxygen atom. The way these three atoms bond causes one end of the resulting molecule to have a slightly negative charge, and the other end a slightly positive charge. For this reason water is described as a polar molecule.
The positive end of one water molecule is attracted to the negative end of another water molecule. When two oppositely charged ends of water molecules get close enough to each other, a bond forms between them. This kind of bond is a hydrogen bond. Every water molecule can form hydrogen bonds with other water molecules. Even though hydrogen bonds are weaker than the bonds that hold together the atoms within a water molecule, they are strong enough to affect the nature of water and give this unusual liquid some unique characteristics.
Water is the only substance on Earth that exists in all three states of matter: solid, liquid, and gas. Because hydrogen bonds are relatively strong, a lot of energy is needed to separate water molecules from one another. That is why water can absorb more heat than any other material before its temperature increases and before it changes from one state to another.
Since water molecules stick to one another, liquid water has a lot of surface tension. Surface tension is a measure of how easy or difficult it is to break the surface of a liquid. These hydrogen bonds give water’s surface a weak, membranelike quality that affects the way water forms waves and currents. The surface tension of water also impacts the organisms that live in the water column, water below the surface, as well as those on its surface.
Atmospheric gases, such as oxygen and carbon dioxide, are capable of dissolving in water, but not all gases dissolve with the same ease. Carbon dioxide dissolves more easily than oxygen, and there is always plenty of carbon dioxide in seawater. On the other hand, water holds only 1/100 the volume of oxygen found in the atmosphere. Low oxygen levels in water can limit the number and types of organisms that live there. The concentration of dissolved gases is affected by temperature. Gases dissolve more easily in cold water than in warm, so cold water is richer in oxygen and carbon dioxide than warm water. Gases are also more likely to dissolve in shallow water than deep. In shallow water, oxygen gas from the atmosphere is mixed with water by winds and waves. In addition, plants, which produce oxygen gas in the process of photosynthesis, are found in shallow water.