We all know that it is essential for life. However, probably because of its abundance and simple chemical composition, we often regard this tasteless and odorless substance as being important, but quite simple and ordinary. Scientifically, it is the exact opposite. It appears to show extremely complex and unusual behavior. It is the most studied substance on Earth. Yet, scientists are still puzzled over its strange properties. Even the best computers we have today cannot simulate all of the different properties of water.
Let us look at an example of the surprising properties of water. The strangeness of water starts with the fact that it exists on Earth. Water, being composed of two fairly light atoms (hydrogen and oxygen), should be in the gas phase at the usual temperature ranges that exist in our world. In fact, all compounds that are close to it (i.e. H2S, H2Se, and H2Te) are found mostly in the gas phase. But, compared to similar substances, it melts about 100 degrees above the expected melting point and it boils about 150 degrees above the expected boiling point (see Figure 1). The result is that it is the only material that exists naturally in all three forms (i.e. as ice, liquid, and vapor) on Earth.
In addition to the example given in the previous paragraph, water has at least 40 different surprising properties (See for example the “Forty-one anomalies of water” section in Ref 1). But, what is even more astonishing is the fact that most of these anomalous properties of water are absolutely crucial for life. Simply stated, life on Earth depends on these extraordinary aspects of water. Below we will discuss some of the anomalous properties of water and their importance for life. At the end we will briefly try to explain why water behaves so differently.
1-Water Has an Unusually High Heat Capacity.
Heat capacity is a measure of the ability to store heat. Formally, it is defined as the amount of heat required to raise the unit mass of a substance by one degree of temperature. If the heat capacity of a substance is high, it will store heat well, i.e. its temperature will not rise much for a given amount of heat. Water has the highest heat capacity among common substances. This has a crucial impact on our life.
It is thanks to this fact that living organisms, which are mostly composed of water, can regulate their body temperature easily. For example, the human body needs to keep its temperature between 36.1 and 37.8 Â°C. This is only possible because it is composed mostly of water. Since the heat capacity of water is unusually high, even if the temperature of the environment changes greatly, the heat exchange between the body and the environment does not cause a great change in body temperature.
Another consequence is the moderation of the climate near large masses of water. The heat capacity of land is much less than that of water. This is why the temperatures of oceans tend to vary much less than that of land. The temperatures in the oceans vary between -2 0C and 35 0C. On land, temperatures may vary anywhere from -70 0C to 57 0C. Compare also the Moon, which has no water. Temperatures on the Moon range from -155 0C to 135 0C.
In addition to having a great heat capacity, water conducts heat more easily than any other liquid, except mercury. This makes the temperature quite uniform in living organisms. Also, the vertical temperature profile in oceans and lakes is essentially uniform due to this fact.
2- Water Has an Unusually High Heat of Evaporation.
When a liquid evaporates, it absorbs heat from the environment. This energy is used to transform molecules into gas form. The heat of evaporation is defined as the amount of heat required to convert a unit mass of liquid into gas. Water has an unusually high heat of evaporation compared to most other common substances. It is so great that you need to supply about five times the amount of energy to evaporate water that is needed to heat it from 0 to 100 0C.
This fact is crucial for the evaporative cooling system of the human body and animals. When we sweat, the sweat absorbs heat from the body in order to evaporate. Since water has a very high heat of evaporation, effectively a large amount of heat is removed from the body through sweating. That is why when we engage in physical activity we sweat. Excess heat in the muscles is easily removed through the evaporation of sweat thanks to the high heat of the evaporation of water.
The high heat of evaporation also prevents dehydration. If it were low, water would then evaporate easily from the body and we would quickly dehydrate.
3- The Density of Water Behaves Unusually as a Function of Temperature
The density of almost all other liquids decreases with increasing temperature. Water is an exception to this. Starting from 0 Â°C, the density of water increases, and reaches a maximum at 4 Â°C, decreasing afterwards. Also most other liquids become denser when they condense, but water is an exception to this as well. The density of ice is less than the density of water, which is why ice can float on water.
Both of these exceptions turn out to be extremely important for underwater life. When the weather gets cold near a lake, first the temperature of the lake’s surface starts to decrease. As the temperature of the surface cools to around 4Â°C, it becomes denser and can move downwards, letting the warmer water reach the surface. Therefore, before the lake can start freezing almost all of the water in it needs to be cooled to approximately 0 Â°C. If there was not an anomaly at 4 Â°C then water would begin to freeze from the surface before the entire lake cools to 0 Â°C. Since water has an enormous heat capacity, the necessity for the entire lake to cool to approximately 0 Â°C before any freezing can occur delays the freezing considerably. It is also crucial that the density maximum in water is near freezing point, not at any other point.
When the temperature finally gets to 0 Â°C and the water start to freeze, it will start to freeze on the surface. Since ice is less dense than water, it will float on the surface and will not sink to the bottom. And once a surface layer of ice is formed, it will protect the rest of the lake from the environment and no further freezing will occur. There would not be any underwater life if ice formed on the bottom. It would also take forever for the ice to melt if it was formed on the bottom rather than on the surface.
The fact that water expands upon freezing is also important for the formation of soils. When the water freezes inside a rock, it can easily crack it into pieces, just like a soda placed in the freezer explodes upon freezing. Therefore, one of the most important steps of soil formation is dependent on this exceptional characteristic of water.
4- The Absorption Coefficient Anomaly in the Visible Region:
Another interesting property of water lies in its absorption of light. Every
substance has a characteristic absorption spectrum that shows how much light at a particular wavelength is absorbed. If the absorption coefficient is high at a particular wavelength then the material will look opaque at that wavelength. If it is low, light at that particular wavelength will be transmitted and the material will appear transparent.
In Figure 2, the absorption coefficient of water is plotted as a function of wavelength (red line). The first thing you notice is that water has a very high absorption coefficient, except for a very narrow region around 500nm. In this small region of wavelength, the absorption coefficient is ten million times smaller than the neighboring regions. What is more interesting than this enormous drop in the absorption coefficient is that this dip happens exactly at the visible part of the spectrum. The human eye can only see wavelengths between 400-700 nm. This visible part of the spectrum is indicated by a rainbow colored strip in the graph. It is amazing that this exactly coincides with the region where water is transparent. Adding to this pleasant surprise is the fact that the amount of light emitted by the sun peaks around this dip as well.
Everything is conveniently adjusted for the habitants of this blue planet. The maximum intensity of emitted sunlight happens to be in the narrow range of the spectrum that we can see. And water on the atmosphere lets this part of the spectrum through thanks to the strange dip in the water absorption spectrum. Worried about the dangerous UV radiation from the sun? This is taken care of too. Just below the visible region, the absorption coefficient of water is ten million times higher. So water vapor in the atmosphere very effectively removes most of the dangerous UV light and shields us.
The spectrum of the light from the sun, the absorption spectra of water and the visible region of the spectrum that we can see are all physically independent phenomena. Yet, it is worth noting that each of these phenomena behaves in such a way that it seems they should have a precise knowledge of each other. If you think this is too much of a coincidence, there is even more. Water is also designed to maximize our visual pleasure. You are probably astonished by the lovely color match between the blue sky and the blue sea. Most people assume that this is because the sky is blue and the sea appears to be blue because it reflects the sky. In fact this is wrong. Water is blue since its absorption coefficient is higher in red; therefore it absorbs more red and reflects the blue part of the spectrum. This can be seen in Figure 2, in which the absorption coefficient in the red colored segment of the rainbow strip is more than 100 times greater than the blue part. The sky is blue for an entirely different reason (since it is blue light that is scattered the most by the nitrogen in the atmosphere). Again two very different, independent physical phenomena are at work here, but the result is a pleasant view for us.
Why is Water so Strange?
Most of these unusual properties of water are the result of the collective behavior of water molecules. That means that one cannot understand them just by thinking about a single H2O molecule. A single H2O molecule is a polar molecule: the two H atoms are slightly positive and the O atom is slightly negative. So when you put these molecules close to each other, the positively charged H atoms are attracted to the negatively charged O atoms of the neighboring water molecules. This is called “hydrogen bonding.” Because of this, molecules tend to order themselves rather than moving randomly, even in liquid water.
Hydrogen bonding is thought to be responsible for most of water’s strange properties. This is why, for example, water has a high boiling point and a high heat of evaporation. Extra energy needs to be supplied to break the hydrogen bonds before boiling and evaporation can occur. Another example is the high heat capacity. As water absorbs heat, it stores this as potential energy by breaking hydrogen bonds without considerably increasing its kinetic energy. This leads to a small temperature increase, therefore a high heat capacity is created for a given amount of heat.
But not all of the anomalous properties of water are that simple. Some of them are not even understood today, despite a considerable amount of current research. For example, water seems to play a crucial role in protein folding. Protein folding is the process by which each protein acquires a unique three-dimensional shape. And it can only function effectively in this particular shape. Despite millions of different possible folding configurations, a certain protein will always fold into its unique structure within milliseconds. But how can a protein always find its way to the same configuration? Water comes into play at this point. It is thought that the hydrophobic (water repelling) interactions between water and protein molecules and the hydrogen bonding interactions in water are major driving forces in protein folding. The exact details of this are still not known. Understanding them is the key for understanding many diseases and developing drugs.
In summary, water is central to our lives. It accounts for a large proportion of our bodies, we drink it, fish in it, and wash and swim in it. But we usually are unaware of how remarkable it is. Here, we have given only couple of examples of the anomalous properties of water that are also crucial for life. Scientifically, our understanding of water is far from being complete. It seems, as the research continues, that the already long list of mysterious aspects of this miraculous substance will get even longer as we learn more about it. On the philosophical side, it is interesting to note that a very simple molecule (H2O) has been selected as a means to do all of these vital, complicated, and unrelated jobs, all at the same time through its unexpected and surprising properties.
“Water Structure and Behavior,” Martin Chaplin,
Segelstein, D., 1981: “The Complex Refractive Index of Water”, M.S. Thesis, University of Missouri-Kansas City.
“Water - The Marvellous Molecule”, BBC TV program.