All living organisms depend on water. The characteristics of water make it a very unique substance. The polarity of water molecules can explain why certain characteristics of water exist, such as its ability to dissolve other substances, its density and the strong bonds that hold the molecules together. These characteristics not only maintain life through biochemical processes, but also create the hospitable environments that sustain life.
A water molecule consists of one oxygen atom and two hydrogen atoms. Oxygen has a very high electronegativity, meaning it has a very high affinity for electrons. The oxygen in water molecules pulls the electrons from the hydrogen atoms closer to it, creating two poles in the molecule, where the hydrogen end is partially positive and the oxygen end is partially negative.
Dissolving Other Substances
The polarity of water gives it the ability to dissolve other substances. Sodium chloride, or table salt, is an example of a substance that dissolves in water and is composed of sodium and chloride ions. The positively charged ends of water molecules are attracted to the negative chloride ions, and the negatively charged ends of water molecules are attracted to the positively charged sodium ions. When salt is submerged in water, the water molecules surround the ions and separate them, causing the salt to dissolve.
Density when Frozen
Ice floats in water because ice is less dense than water. However, ice is water, and there is no difference between the two substances. This phenomenon can be explained by the polarity of water. When ice is frozen, the water molecules extend themselves as far as they possibly can but are held firmly together by hydrogen bonds. Water expands when it is frozen, but is still composed of the same number of molecules, thus decreasing its density and allowing it to float in water.
The hydrogen bonds that hold water molecules together in water's liquid and solid form give the substance high boiling and freezing points and strong surface tension. Because water molecules are held so tightly together, it takes a large amount of heat for water to boil. Furthermore, when you fill water to the top of a bottle, you can see that some of the water hangs off the top of the bottle because the molecules adhere to each other.