The physical properties of water, an essential part of Sukkot and life
Beyond wet!
By ROY PITTMANWater Drop Illustrative(photo credit: Courtesy)
The significance of water and its relationship to Sukkot could hardly be greater. The world is said to be judged regarding water on Sukkot. When the Temple stood in Jerusalem, the simchat beit ha-sho’evah, or water libation, was performed during the holiday. The sages said those who did not see the rejoicing of that ceremony never saw rejoicing in their lives.At the end of Sukkot – as Jews discard the four species with which they celebrated, all of which were dependent on water to grow – they traditionally begin to pray for rain, on which the Land of Israel depends.The list goes on. However, rather than examine tradition, let’s look at the science of water itself and some remarkable aspects of its physical nature.Water is the most abundant substance on Earth and thought to be the third most abundant molecule in the universe. It covers 70% of the Earth’s surface and is absolutely indispensable to all life. Water is a relatively simple molecule, but it has remarkable properties.According to the periodic table, water has a molecular weight of 18 and boils at 100°C or 212°F. For contrast, methane has an atomic weight of 16 and a boiling point of -161°C or -258°F.Ordinarily, boiling point is related to molecular weight, so one would expect that water would boil at close to the boiling point of methane. However, this is far from what we observe.Something else is going on, and that something is polarity. Water is not only extremely wet, it is extremely polar.What does that mean? Polarity is a simple idea. Anything is said to be polar if the ends are different. A baseball bat is polarized, as is every human body. In chemistry, though, polarity describes an electrical difference. One end of a molecule has an excess of electrons and the other has a scarcity – very much the case with water. Methane, on the other hand, is completely non-polar so it has a very low boiling point.Diagram 1 indicates the polarity of a water molecule:
The oxygen atom is red, the hydrogen atoms are gray and the - and + symbols indicate regions of excess or a scarcity of electrons. This is a pretty good diagram for indicating how water is polarized, but there is more.Diagram 2 shows an oxygen atom has six electrons in its outer valence shell.It will complete that valence shell by sharing one electron from each hydrogen atom. As it does so, the valence octet of electrons form what are called sp3 hybrid orbitals.The result is that the water molecule has two hydrogen atoms projecting from one side, both of which are strongly electropositive, and on the other side two un-shared pairs of electrons that are strongly electronegative. This molecule is very strongly polarized, so much so it can form a new kind of weaker but still significant bond: the hydrogen bond.In diagram 3 the number 1 points to two of the four hydrogen bonds.Hydrogen bonding is a relatively weak attraction compared to the covalent bonds within the water molecule itself. The hydrogen bond is responsible for a number of water’s physical properties. These properties include its relatively high melting and boiling point temperatures, as more energy is required to break the hydrogen bonds between water molecules.One more fun fact: The density of water is about one gram per cubic centimeter. This relationship was originally used to define the gram. The density varies with temperature, although not in a linear fashion.As temperature decreases, density increases until it reaches a peak at 4°C (39°F). At that point if cooled further, water’s density actually begins to decrease.This is unusual – and why ice is less dense than water.When water is held at temperatures below 4°C, the hydrogen bonds break, which allows water molecules to pack closer together.The unusual density curve in which ice is less dense than water is vital to life. If water was most dense at it’s freezing point, ice cold water at the surface of lakes and other bodies of water would sink, the lake would freeze from the bottom up, and all life present would be killed.The writer is a photographer, writer, former chemistry instructor and retired RN who has lived for many years in Arizona. He loves sharing his understanding of how ubiquitous and seemingly simple things like water work. His work can be seen at roypittman.com.
