In radiology, water is everywhere. It is one of the four basic radiologic densities, along with air, fat, and bone. It represents the fulcrum of the Hounsfield scale of computed tomographic densities. In magnetic resonance imaging, we rely on its signal characteristics to determine the sequence that we are looking at. Finding that a lesion has the same imaging characteristics as water generally indicates that it is benign and requires no further imaging evaluation. Water is such a common feature of our daily practice that many of us hardly take any notice of it.
This is a mistake, because water is one of the most extraordinary substances known to man. Though colorless, odorless, and tasteless, it is utterly necessary for life, in large part because virtually all of the chemical reactions on which life depends take place within it. From a human point of view, fresh water is absolutely necessary for irrigating crops, cooking, and drinking. It constitutes only ∼2% of the water on earth, and in the decades to some, it will likely become a scarcer and more highly valued resource than carbon-based fuels.
Yet water is not in short supply. It is probably the second most common molecule in the universe, after hydrogen. It covers ∼71% of the earth’s surface. The total amount of water on earth is about 330 million cubic miles. For comparison purposes, it would be possible to fit nearly every human being on the planet into a space of approximately one cubic mile. Moreover, we are made up largely of water, which constitutes between 70% and 80% of our body mass. In short, the universe, our planet, and we are floating in it.
The ancients thought that water is one of the four fundamental substances, along with air, fire, and earth. The notion that water is made up of two different elements, hydrogen and oxygen, was first proposed by the father of modern chemistry, Antoine Lavoisier in the 1780s. In fact, it was Lavoisier who first coined the name hydrogen, which means “water former.” Yet he did not know that the ratio of hydrogen-to-oxygen is 2:1. Unfortunately, Lavoisier’s immense scientific contributions could not protect him from the French reign of terror, and he was guillotined in 1794.
Water is not only ubiquitous but also extraordinary. Many of its most remarkable properties can be traced to the form of chemical attraction known as the hydrogen bond. Hydrogen bonding is possible because the water molecule is dipolar, meaning that it is slightly negatively charged at its oxygen end and slightly positively charged at its hydrogen end. Each individual hydrogen bond is relatively weak, but many of them exert a powerful force. This means that water molecules attract one another more strongly than the molecules of virtually any other substance.
This hydrogen bonding gives water a very high specific heat, the amount of energy required to change its temperature. It takes a lot of energy to heat it, and a large amount of energy is released as it cools. As a result, water can store enormous amounts of energy and release it with very little change in temperature. This means that as the seasons change, people living near a body of water, such as a lake or ocean, are likely to experience smaller fluctuations in temperature than someone surrounded by dry land.
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