Water: the odd case around us
traduzione: Paola di Cerbo
Whoever would like to learn about the properties of liquids and, more generally, chemicals, taking water as an example, is likely to incur more than a misunderstanding.
The temptation is definitely great: Water is definitely the most abundant virtually pure chemical substance we are daily in contact with, it is essential to life and, the most important thing is that we all have directly and personally experimented its characteristics and its behavior.
Unfortunately, water contains in itself more than just an anomaly, each of which combining to bring about making the properties of this substance different in several respects, starting with chemical-physical one, from those of the majority of liquids and chemicals in general, even considering the tens of thousands chemical species known nowadays. The intersection of these anomalies makes the most abundant and “known” substance on our planet a “bad friend” for those who want to approach knowledge of liquid substances and their properties following an inductive method.
ANOMALIES IN DETAIL
Density of the solid state compared with the liquid state
It is certainly not the only case (for example the cast iron is another one), but there are very few chemicals that, like water, show a density at the solid state lower to that at the liquid state. To be more precise, the temperature at which the water, in all its physical states, presents the maximum density, is +4 ° C.
From here one can understand the reason why the ice floats on the water, a fundamental condition for possibilities of life in the Arctic regions, (which largely depends on the presence of liquid water under the ice blanket) and for the entire ecosystem in general.
The deposition of the gradually forming ice on the sea bottom , in fact, would lead to rapid freezing of a large fraction of oceans, with a dramatic decrease in the availability of liquid water on our planet.
Unusually high melting and boiling points
If one considers the atomic composition of water (1 oxygen and 2 hydrogen atoms), one would expect a boiling point lower than 100°C and a correspondent melting point lower than 0°C. Hydrogen sulfide( an heavier homologous of water with a Sulfur instead of an Oxygen atom) which should get a higher melting and boiling points compared with water, presents a melting point of -86°C and a boiling point of -60°C, and it’s a gas at room temperature and 1 atm pressure. Water itself should be aeriform, if we considered on the base of its constituent atoms. Also the temperature range (26°C) in which hydrogen sulfide exists in liquid form is considerably more narrow compared to “liquidity” range of water (100°C)….good for us…..because water can thus support all the vital functions of different organisms.
Extremely high heat of fusion and vaporization
Suppose to use two different liquids, presumably having different boiling points, and to bring each of them close to boiling point.
So doing, we overcome the “energy gap” from room temperature to boiling point and then we no longer have to consider this factor. To “keep” water boiling , an extra amount of heat, i.e. energy, will be requested; it’s higher compared with almost all known substances. The energy (heat) amount required to bring a single gram of a liquid substance to vapour is called heat of vaporization: it amounts to 2250 J / g for water! This peculiarity greatly influences the dynamics of both heat and water molecules transference in the atmosphere.
Paired with this feature is the equally high value of the ” heat of fusion “: 333 J / g for water. The heat of fusion is the amount of energy required for melting a single gram of ice. This feature is of the utmost importance in climatic and ecological field as it explains the so-called ” thermostatic effect at the freezing point” . In practice the same freezing or melting processes, in the time lapse within they occur, prevent temperature from changing.
Would you like to try? Let’s put a thermometer in a glass of water containing some pieces of ice and expose everything to sun or place on a hot surface : as far as all the pieces of ice melt ( it may take several minutes for this to happen ) the temperature inside the glass will continue to be around 0 ° C corresponding to the ice melting temperature.
The same would happen if we put a thermometer in a glass of water placed in a freezer at -25 ° C : as far as all the water in the glass hasn’t frozen, the temperature in it will be 0 ° C : only after that, the temperature gradually begins to decrease.
Heat of fusion and heat of vaporization together can be described as “latent heat” associated with a thermodynamic transformation, specifically with a phase transition , also called state transition.
Extremely high surface tension
More easily observed than properly understood without going into too much thermodynamic and fluid dynamic details , it is a property connected with separation surface (also called interface ) between a fluid and a different liquid, gaseous or solid phase. We can in the first instance state that it corresponds to the work required to raise the liquid surface by a unit.
A liquid such as water , characterized by high surface tension, opposes high resistance to the increase of its interface.
Even under mechanical stress, such as the attempt to put an object above its surface (think for example to a bug, but also to a metal paper clip), water surface will tend to ” bend ” itself in function of pressure , resisting to the weight force of the object, supporting it within certain limits, rather than” stretching”, extending the initial area.
Similarly, the high value of water surface tension , equal to 7.2E +9 N / m, the highest among known liquid , governs the formation of water droplets , their size and the fact that they can ” lean ” on many types of surfaces (for example on plant leaves) without being immediately ” smeared ” in a thin veil. Needless to say, the same property plays an important role in cell physiology.From the thermodynamic point of view the surface tension is calculated as the derivative of the variation of Gibbs free energy respect to the interface area, at constant pressure and temperature.
Very high specific heat
Imagine we have two pots filled with same amount of a liquid on two identical stoves and a thermometer immersed in each of them in order to check the temperature. In these conditions the same amount of the two liquids heat is supplied to both of liquids.
Yet, if we’re using two different liquids, we’ll see that internal temperature will rise more quickly in one of them, while the other will require a longer time to reach the same temperature. Water would take longer to warm up if compared to the majority of fluids and even of solids. It requires a larger amount of heat per unit mass to raise the temperature by one degree Celsius. or, as the physics would say, it has a high “specific heat.”
The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius, and it is measured in J / gK ( where J indicates the energy deriving from heat and K is a degree Kelvin, but can easily be replaced by the usual degree Celsius).
The water specific heat is equal to 4.18 J / gK , an exceptionally high value.The effects are fundamental for the very existence of life on Earth:
– they are at the base of the tendency for water to prevent rapid changes in temperature caused by external factors , with consequent preservation of the characteristics of living habitats as well as of theinternal temperature of our bodies ;
– they favour heat tranfer through the displacement of large masses of water, directly or by convective motions, with absolutely relevant consequences from climatological point of view to the cooking of pasta!
Excellent solvent capacity
Water is the ideal solvent for solubilizing an extraordinary large amount of solid, liquid and gaseous chemicals,: from polar molecules such as carbohydrates, amines , acids, alkaloids, up to the ionic species such as inorganic like sodium chloride.
Water dissolves both the chemical species able to dissociate into ions , ie acting like electrolytes , and the molecular species, undissociated: in both cases the key factor that allows the solutes to remain in solution are the hydrogen bonds that allow the isolation, molecule by molecule or ion by ion , of the chemical species that gets surrounded, that is ” solvated “, by the aqueous solvent .
Albeit approximatively, we can say that, in the context of organic chemistry , molecules showing minor solubility in water are incapable of forming extramolecular hydrogen bonds, and lack of highly polarized bonds. In other words these ones are the so-called “apolar ” substances.
Extraordinary direct and indirect reactivity
Despite what it looks like, inspiring empathetic feelings of peace and tranquility, water reacts and transforms; it stands in fact at the base of a probably unparalleled number of chemical reactions.
Its role in chemical reactivity can be ascribed ed to 3 different effects:
– Maintenance of the molecules in solution (gas, solids and liquids, especially polar);
– Separation and solvation of electrolytes (eg, inorganic salts, acids, bases, etc.) in ionic species;
– Direct reactivity in the context of chemical reactions where water acts as a reagent.
In all cases the reactions can be redox or acid-base.
The presence of water is at the base of the oxidation of metal (eg the the formation of rust) , of the hydrolysis of esters and ethers in biological complex ( such as polysaccharides and triglycerides ) and even of the hardening of the lime caused by the conversion of calcium hydroxide in carbonate. In many cases the reactions could potentially also occur in the absence of water (eg. the reaction of calcium hydroxide would require carbon dioxide only : water does not even appear in the chemical equation! ) , but the process is incredibly longer because of low reactivity of the substances in the solid state, due both to the reduced surface exposed and susceptible to come into contact with the reagent, and to the limited mobility or molecular ions , which drastically reduces the possibility of successfully collisions among the reagents involved.
We can not forget, as fully described in a following chapter, that the role of water is pivotal in virtually all the biochemical reactions that occur in living beings .
The rather extraordinary fact is that water shows a viscosity coefficient so low ” despite ” his marked ability to form hydrogen bond networks, which are at the base of the majority of the other properties so far described.
Such a viscosity , which sometimes becomes extremely substantial, it’s a common characteristic of substances containing a number of intermolecular hydrogen bonds , such as glycerol , glycols and formamide: the hydrogen bonds “slow” in a sense, the reciprocal movements of the molecules, hampering the movement of the whole mass of the liquid , as a sort of internal friction .
The hydrogen bonds in water, though numerous and strong , are easily eluded; single molecules can quickly modify their relative positions, and allow the mass of liquid to skim , rebalancing in a very short time any eventual difference in pressure … just consider an example on a global scale : the case of tides.
The magnitude of the dielectric constant is proportional to the capacity of material to polarize under the ef fect of an electric field, however generated, “accordingly” reducing the intensity of the field within the material.In addition to combine to bring about the extraordinary tendency of the water to form multiple hydrogen bonds, the existence of an extremely high dielectric constant lets water to solubilize ionic species, separated in their charged particles and carefully surrounded by water molecules suitably oriented to minimize the electric field generated by the ionic charge.
These two features together imply that water displays the characteristics of an electric dipole, and
characterize it as a dipolar molecule.
A Consequence of this is the formation of a partial electrical positive charge on the hydrogen atoms and a negative one on the oxygen, which generates electrostatic interactions between hydrogen atoms of a molecule and oxygen atoms of another, forming what are actually defined as hydrogen bonds. From what has been described so far, the reasons why multiple hydrogen bonds between two molecules of water are highly disfavoured compared to interaction with more than two distinct molecules.
The crosslinking pattern of the hydrogen bonds in a three-dimensional complex system is variable, and to some extent “structured” in space and time, thus forming clusters of molecules or clathrate. This is primarly due to the tendency of the oxygen atom to form two hydrogen bonds with electropositive atoms belonging to different water molecules. This peculiar characteristic implies that each molecule links up to 4 other molecules in the crystalline solid (ice, with hexagonal crystal structure) and on an average 4.7 molecules in the case of liquid water, with the formation of transitory fluctuating “domains” , as we will see later.
The number of hydrogen bonds with which water molecules are in fact “connected” underlies the possibility to investigate different “types” of water present in a sample of a complex material by spectroscopic analysis: we can thus identify (and within certain limits quantify) the presence of water molecules in different states of interaction among themselves or a substrate (eg, carbohydrates, salts, macromolecues), and distinguish between the so-called “bound water” and the total water amount present in our material.
In the video showed below, especially in its second part, we present a simulation that shows the temporary and ongoing reorganization of the hydrogen bonds system in liquid water, with the possibility of changes in their number and their directionality towards different molecules:
Ultimately, as we shall see, some of the properties of water, especially with regard to the interaction with macromolecules, derive from the very small dimension of the molecule, with only 0.9584 Å internuclear distance between oxygen and hydrogen in the same molecule.
Let’s try to “invent” another molecule showing the same few constitutive characteristics of water by
choosing different elements in the periodic table : we will begin to understand that the so peculiar properties (physical, chemical and biological) belonging to water are not obtained by chance, or even worse by a random sequence of events, but are a logical consequence of its structure. It’s disarmingly simple in its constitutive denominator, nevertheless so incredibly complex in its ability to create structures and interactions up to more complex levels of organization of matter.
WATER AND LIFE
It’s even “more essential” than oxygen, if we consider that obligate anaerobes microorganisms do exist, and it’s surely most important of all the individual chemical species that different living beings use to catabolically derive the energy required for the maintenance of their vital function.
Although enzymatic activities within these environments is difficult to conceive, there are many of organisms belonging to different kingdoms perfectly able to produce and accumulate substances in order to create micro-isolated environments and to perform biochemical reactions at the interface of them. Moreover, numerous reaction complexes (e.g. the radicalic oxidation of unsaturated fatty acids or the path of Maillard reactions) can be taken as an example of how structured reactivity can be consistent within virtually anhydrous environments (it has although to be noticed that the complex showed can not considered as part of the biochemistry of a living being since they are essentially extra-biotic). This statement, as mentioned in the introduction, excludes the functionality of two of mainly hydrophilic molecular classes absolutely fundamental for life at least as we conceive it on our planet: nucleic acids and enzymes.
Questioning about the reason why water results essential to life could eventually lead us astray. In fact there are very few water features that make it really indispensable and irreplaceable.
First of all, we’re talking about a molecule that has a really “minimal” size. Periodic Table of the elements in our hands, there are not many molecules (or elements at free state) smaller than water and nevertheless stable under terrestrial conditions. Water can, therefore, easily occupy the interstices in macromolecules, even in extremely coiled ones, without being forced to stop at the surface. Differently from many other small molecules, water is able to establish direct and specific interactions with distinctive parts of these molecules through the formation of hydrogen bonds, both in direction water-site on the molecule, both in the direction water-water.
These two features together make water able to bring some chemical classes of large and small molecules (the so-called “hydrophilic” molecules) to express their full potential and peculiarities. At this point, I’ll take a chance of an hazardous metaphor for explaining what I mean.
Imagine one of those large inflatable silhouettes, several meters high, which are often used as callouts in fairs, shows and amusement parks. When they are deflated, thy’re not very different each other and they look a bit like a heavy and muddled accumulation of rubber resting on the ground, possibly differently colored.
Only getting air in them, these structures rise, their parts distance themselves and place each in a specific position: so an arm appears, an open mouth, the designs become visible and interpretable: now we recognize that it is a man, or a dragon, or maybe an octopus with eight tentacles. The different parts that already constituted the structure of the object, are now effectively able to interact with the observer, showing their function: to describe the nature and the properties of the subject. All this is made possible by the air inlet. In fact, in this case a different fluid would be fine: also a different gas, as long as weight is not overly different from that of the surrounding air. If we had used a much lighter or much heavier fluid, than we’d have had problems: too heavy arms would have been much more cumbersome and would not have been raised with respect to the body ; presumably also the facial expression would have been a bit ‘ “pulled down .”
We could have tried and filled the balloon with gravel but it probably would not even managed to pass throughout the rubber structure, and would not entered small cavities.
In the case of biological molecules the border between ” outside” and “inside ” is extremely labile, often non existent , because the majority of the molecules rather than as a glove ( in function of affinity or repulsive forces between the molecules and the environment where they are placed ) would be more reasonably understood as having a complex and articulated shape, which can assume many conformations in response to the interactions with the environment like, using a metaphor again, an umbrella tipping in one direction or another depending upon the wind.An in depth analysis about the influence exerted by solvation on the conformational states of the macromolecules, up to their collapse or denaturation, can be found in Article “when the matter is
organized in a more complex way: the evolution of the concept of substance. “
So, it is true that the case of liquid water shows numerous anomalies compared to other liquids, and the same happens if we consider the other physical states (solid and vapor), but it is equally true that a good portion of them are “derived” (ie, share a common cause with other features).
Other relevant anomalies show their effect in other fields of application, not referred the biochemical dynamics at the base of life.
AND… MAYBE IS’S NOT ‘EVEN H2O
Title is obviously a provocation, but it hides a substantial truth… more than one, indeed.
In chemistry courses it’s told that ionic species resulting from dissociation (autoprotolysis) of the water are H+ and OH–.
Accurate teachers immediately specify that H+ would represent a free proton, since the hydrogen atom in its most widely spread isotope on the Earth, known as protium, does not even have a neutron so, once lost the electron to justify the positive charge, it would only be a single proton, an elementary particle, able to go for a walk in a liquid solution in a glass … that does not sound very good…
In addition to the hydroxyl anion, we assist at the formation of the so-called hydronium ion H3O+. The cationic species are produced by protonation of the water in a considerable amount, we’d say a “stoichiometric” amount, when a strong acid is introduced into water. The hydroxyl group, anionic, is produced in the same way by the addition of bases, such as ammonia.Whoever would like, might suppose further possibilities for the chemical formula and structure of the hydronium ion: if several water molecules participated to autoprotolysis with the result of producing only a single cation and anion, both monovalent… what kind of chemical species will H5O2+ be? The hydronium ion H3O+ could be described as a sort of pyramid shown in the figure, but what conformation and most of all, what “significance” can the dioxide di penta hydrogen ion, and the more complex structures described by the general formula H(2n +1)On+, effectively have?These structures can be depicted as a group of several water molecules joined together to form a single larger water molecule with a proton in excess giving it a positive charge; Are maybe these structures already present in the water, beyond the autoprotolysis that focuses on their presence as charged species?
We can suspect that the one we are accustomed to, the usual H2O, is something very similar to a “minimum formula”, a way of representing the substance that not only does not highlight the structure of bonds, but either does not indicate the exact number of the constituent atoms in the molecule. In practice, a minimum formula only shows the minimum relative ratios between stoichiometric coefficients of the elements that form the molecule.
A better description of the meaning of minimum and brute formulas and of the various representations of molecular structures can be found in Article “Why do we use the chemical formula? (and how to interpret it).
As a matter of fact, it has been noticed that the velocity of movement of what would be a virtually simple H + is higher one might expect on the base of the mechanisms ruling ions migration in a liquid medium. The movement of Na + ion in a dilute aqueous solution of sodium chloride, under the action of an electric field that attracts it towards the negatively charged cathode, is strongly prevented by a series of factors intuitively connected with a kind of “friction” generated by water molecules surrounding it and the reorganization of water molecules in order to to let the cation pass. It ‘s true that the hypothetical H + (which, is actually not present independently) would certainly have a smaller size compared with Na+, however this fact would not justify its high capacity of diffusion. What actually happens looks more or less like the effect generated by a device consisting of several steel balls held in abeyance and aligned in order that each of them is in contact to the next through thin nylon strings: if one detaches a ball with fingers of a few centimeters and then leaves it to fall in contact with the other aligned, only the last ball on the opposite side wiil be lifted (virtually) with the same force and therefore the same height corresponding to the impact received.
The initial and final balls resemble the electrical charge of the cation, while the central group which is basically stable and transmits the charge is the water clathrate.
How can we understand these complex aggregation states of water molecules that characterize its liquid state?
No one will be surprised in verifying the existence of a stable crystal lattice in water at the solid state , for the accuracy an hexagonal type, justified by the number and directionality of the intermolecular hydrogen bonds that are optimized in this structure. There are other ice crystal structures, but they are more difficult to obtain under the operative conditions we are accustomed to ( for example, their formation requires a spatial reorganization of the molecules too much different from the one in the free state of the water ,while the ice ” Ih ” ( the one we are commonly used to) looks pretty much like it. These alternatives “crystalline ice” are about 11 but, except the hexagonal ” Ih “, they only form under extremely high pressure.
In the figure above the covalent bonds among hydrogen and oxygen atoms that can be formally regarded as a single molecule of water are represented in black, while all of the hydrogen bonds among one molecule and the others are represented in white.
A discussion of the specific crystallization process, including a video with a simulation of formation of the hexagonal crystal lattice from liquid water, can be found in a previous paper “Glasses and crystals: the antipodes of the solid.” Using the model in figure, we can already draw two important considerations: first, the “rarefaction” of water molecules, placed at regular distances and optimized energy in the lattice of crystalline ice, explains the relatively lower density of this material compared to the same substance in the liquid state, where the individual molecules are free to approach closer to each other. Secondly, as shown in the animation, the covalent and hydrogen bonds do not have a fixed role: albeit changing their bond lengths (see images above) they can be considered as fluctuating, rotating and in resonance ……. Formally the same hydrogen atom pass from one molecule to another, while physically remaining roughly in the same position in space.
In the same way in water at the liquid state the continue formation and breaking of hydrogen bonds is at the basis of the formation of “fluctuating” aggregates , also called domains, having a structure similar to to crystalline lattice of ice, albeit much more limited in its size and lifetime.
Thus, we can discover clusters composed of a large number of molecules in liquid water : in the simplest of them we can see a tetrahedral structure , not very different from that of the carbon backbone in adamantane that underlies the crystalline structure of diamond. From a formal point of view, each water molecule is now surrounded on average by 4.7 other molecules of the same type. The growth pattern involves the passage from the single water molecule to a cluster composed of 5 molecules (the central one + 4 molecules linked to this by hydrogen bonds), than to a larger cluster consisting of 14 molecules (see figure), according the criterion that each oxygen atom can bind 2 hydrogen atoms belonging to another molecule and each hydrogen can bind an oxygen the same way.
For energy stability reasons, these clusters, especially the more complex, usually assume the geometrical structure of the icosahedron, a solid with 20 triangular faces.
The union of 20 tetrahedral units consisting of 14 molecules can turn to form an icosahedral cluster consisting of 280 water molecules, although according to some researchers the formation of larger clusters still based on icosahedrons would however be possible. The relative “flexibility” of hydrogen bonds implies that there’s the possibility of conformational modifications of these intermolecular clusters under the effect of external factors, with the result of a relative collapse / expansion of the structures.
The formation of organized complexes involving a large number of water molecules has been certainly observed when a solute, a polar or better an ionic dissociable species, is solved into it with the formation of clathrates involving hundreds of water molecules, all suitably oriented with their dipoles, in the formation of a kind of shell around the ion in solution. It is believed, however, that these clusters also constitute the normal rganizational state of pure liquid water and that magnitude and conformation of these structures can vary under the effect of physical factors, such as heating or repeated cycles of microfiltration.
The lifetime of “individual” aggregative states is currently a key topic in the scientific community : depending on the experimental evidences recorded, and the theories developed, this time might settle on 50 fs (femtoseconds) , 0.1 ns ( nanoseconds) or much longer. The most probable hypothesis predicts different organizational levels , with relaxation times differing in size.
The interest in the subject tends to assume speculative characteristics, fashinating in their potential applications but just as ” slippery ” in the risk of lacking in accuracy.
In fact, some para-scientific theories identify in the existence , complexity and variability under the influence of physical and chemical external factors , of these structures a sort of ” water memory” , as well as a possible interpretive approach , still forward looking, to interpret in a scientific way some bio-functional evidences of wate , in the first place those that are commonly used in homeopathy.