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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
Water is vital for life. In the search for extraterrestrial life, the absence of water is generally considered an exclusion criterion for life as we know it. However, for most people, water is nothing special. We use it daily for drinking, cooking, cleaning, and washing. We just have to open the faucet and it’s readily available in abundance. H2O is pretty simple stoichiometrically, but from a chemical and physical point of view, it is an extraordinary substance. This molecule possesses many of the fascinating and outstanding properties that are essential to life.
A water molecule consists of one atom of oxygen and two atoms of hydrogen. The bonding of the atoms takes place via their electrons and can be compared to a handshake: some people have a strong handshake, and some people a weak one. Similarly, various elements attract electrons to different degrees, creating bonds with different strengths. One measure of this attraction is called electronegativity. Oxygen has the second highest electronegativity, with a value of 3.4, whereas hydrogen has an average value of 2.2. To stay with the metaphor of the handshake, the oxygen atom pulls so hard that it attracts the two electrons of the hydrogen atoms. As a result, the oxygen atom is partially negatively charged, and the hydrogen atoms are partially positively charged. Opposing charges attract each other, therefore a three-dimensional network of hydrogen bonds forms between water molecules. This causes water to have many extraordinary and interesting properties, such as a relatively high melting and boiling point.
In the periodic table of the elements, oxygen is part of the chalcogens. The other chalcogens—sulfur (S), selenium (Se), tellurium (Te), and polonium (Po)—are relatives of oxygen. Among relatives, there are similarities and sometimes shared characteristics. So it is with chemical compounds. In fact, H2S, H2Se, H2Te, and H2Po have similar melting and boiling points. The melting points rise from -86 to -35°C, and the boiling points from -60 to 36°C. But water, at 0 and 100°C, does not fit into this pattern. Water should actually have significantly lower melting and boiling points. The reason for these relatively high temperatures is that a lot of energy has to be applied to break the hydrogen bonds. However, because of its melting point at 0°C and boiling point at 100°C, water is the only known substance that occurs in all three states of aggregation on Earth, and each state of aggregation fulfills essential functions on our planet.
Water vapor enables precipitation and the distribution of moisture over the planet. In addition, it causes ~60% of the natural greenhouse effect.[1] Without water vapor in the atmosphere, it would be 21°C colder on average,[2] making life possible only on a narrow strip along the equator, if at all. It is amazing to think that all these essential functions are accomplished by an evanescent vapor representing only 0.001% of the total budget of water on the earth.[3]
Ice also plays an important role in global climate regulation because of its high albedo—reflection of sunlight. Moreover, ice is an important drinking water reservoir. Usually, the solid phase of a substance is denser than the liquid phase. But the negative thermal expansion of waters allows ice to float on water, a phenomenon called density anomaly. This special feature is essential for aquatic life. Without this property, lakes, rivers, and seas would gradually freeze from bottom to top at negative temperatures, killing bottom dwellers and, if the whole water column froze, even swimming organisms.
Liquid water is the habitat for many plants and animals. It acts as a catalyst in many fundamental chemical reactions and can behave as both acid and base. Water is by far the most common component of the human body, its content varying according to age, gender, and physical condition, but being typically between 70% and 80% in adults. Water enables heat and mass transport on our planet because it is an excellent solvent and has by far the highest specific heat capacity of all liquids. The specific heat capacity is a measure of how much energy has to be applied to heat one gram of a substance by one degree. Just as electric energy can be stored and transported in a battery, so water stores and transports thermal energy between oceans and continents, the equator and the poles, summer and winter, day and night. Without this compensatory effect, there would be bigger and life-damaging temperature differences on Earth. Less than 2% of all water on Earth is groundwater and fresh water.[4] However, this seemingly negligible portion supplies almost all of humanity with drinking water.
There are a few more special features that make water extremely valuable for life, such as it having the second highest surface tension of all known liquids, the highest specific enthalpy of vaporization, as well as a low viscosity.
Finally, I would like to mention a property that often receives little attention in a scientific context: aesthetics. Ice crystals and snow crystals usually form in the highest symmetry of the hexagonal point symmetry group. The hexagonal axis gives the snow crystals their characteristic shape. Wilson Bentley (1865–1931) photographed more than 5,000 different snowflakes and in 1922 formulated the thesis: “No two snowflakes are alike.” Under the microscope, there are similar or similar-looking crystals, but at the atomic level, every ice crystal is indeed unique.
In summary, ice, water, and steam have many remarkable properties. Without these qualities, life on Earth is hardly imaginable. Either these are huge coincidences and a happy interaction of different laws of nature, or an intelligent Being acted as engineer, coder, and designer and has projected, constructed, and programmed H2O molecules genially and incomparably purposely. The latter possibility seems the more reasonable one, because it explains the observed water properties in the simplest and most elegant way.
NOTES
[1] KY Kondratyev, NI Moskalenko. The role of carbon dioxide and other minor gaseous components and aerosols in the radiation budget. In: JT Houghton, editor. The global climate. New York: Cambridge University Press; 1984, pp. 225–233.
[2] Ibid.
[3] T Oki, S Kanae. Global hydrological cycles and world water resources. Science 2006; 313(5790):1068–1072. doi:10.1126/science.1128845; R. Schäffer. Hydrochemische Methoden zur geothermalen Erkundung und Charakterisier-ung von Thermalwässern [PhD dissertation]. Darmstadt (Germany): Technische Universität; 2018, pp. 4–5. http://tuprints.ulb.tudarmstadt.de/7502/ [accessed June 25, 2020].
[4] Ibid.
Rafael Schäffer works as postdoctoral scientist and lecturer at the Technical University of Darmstadt, where he also earned his PhD in Geosciences. His research areas are geothermal, thermal, and mineral groundwaters, hydrochemistry, as well as alpine hydrogeology. He enjoys giving popular scientific talks on science and faith and on creation and evolution.