Important words and concepts from Chapter 3, Campbell & Reece, 2002 (1/14/2005):
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(1) Chapter title: Water and the Fitness of the Environment
(a) Found at this site are additional pages of possibly related interest including: [water]
(a) Water is the most important substance necessary to the existence of life (Carbon fills the #2 slot).
(b) "Water is so common that it is easy to overlook the fact that it is an exceptional substance with many extraordinary qualities. Following the theme of emergent properties, we can trace water's unique behavior to the structure and interactions of its molecules."
(c) Life evolved in water and all metabolically active life either lives in water or carries water around with it, such that a high fraction of the makeup of an organism ([e.g., ]50%) usually consists simply of liquid water. Water is a chemically unique compound to which life is both fully and irreversibly adapted. Outside of the cell, nutrients are dissolved in water, which facilitates their passage through cell membranes. And inside the cell, water is the medium in which most chemical reactions take place. (note, I'm betting the above is a quote but I'm not-yet certain from where or of whom)
(d) The emergent properties that will concern us include: cohesion, adhesion, surface tension, high specific heat, high heat of vaporization, water is a liquid, evaporative cooling, ice floats, water as a solvent, hydrophilicity, hydrophobicity, hydration shells, hydrophobic exclusion, pH, and pH buffering (the italicized terms are either not found in the chapter or not described by that name in the chapter).
(e) This is the first chapter of your text to begin focusing on the details of life processes, in this case the noncovalent interactions that are exemplified by water. Why water? Water, first, is absolutely essential to the existence of life on earth and understanding water's properties give us a means of starting to understand not just the more-complex molecules of life but also how those more-complex molecules interact with water (as they most-assuredly do) as well as with each other.
(f) By studying water we begin our study of liquids, and life, at the level of molecular (and intramolecular) interactions that are, for the most part, liquid. What does that mean? It means that living things are complex but simultaneously dynamic. In the scheme of the phases of physical matter, it is not difficult to understand that solids are capable of exhibiting significant complexity. Solids pretty much by definition, however, are not terribly dynamic. That is, they tend to stay fixed in their form (though culture, e.g., human technology, employs particularly the solid phase, building dynamic machines mostly from macroscopic, solid-phase components). At the other end of the spectrum are gasses. Gasses are inherently dynamic -- amazingly so. But gasses, in their chaotic disorder, are inherently non-complex especially at microscopic scales. In the middle, of course, are liquids, which can be significantly dynamic (as any sailor may tell you) but simultaneously are capable of significant complexity, especially complexity that exists at the level of noncovalent inter- and intra-molecular interactions. Living things embody the complexity that comes from intermolecular interactions (noncovalent bonds), existing basically within a liquid. Even the non-liquids of living things are forged, molecule-by-molecule, within a liquid milieu. Thus, living things, by and large, are molecular machines that both operate within and interact with a watery environment.
(g) "Your objective in this chapter is to develop a conceptual understanding of how water contributes to the fitness of Earth for life."
(h) "Exactly where all of Earth's water came from is still debated, but in one scenario, some planetary embryos were water-rich, endowing any growing planet they happened to hit with extra water. Because the most important impacts were the last few hits by the largest remaining bodies... a planet's water allotment might be determined by a few very large impacts, making the difference between a wet and a dry planet even more of a roll of the dice... 'Although water is what we would call a minor constituent [of a planet] it seems to play an important role in determining how a planet works.' On Earth, it appears to act as a kind of lubricant. Ocean plates sinking into the mantle carry traces of water that lower the melting temperature of mantle rock, helping to fire overlying volcanoes. The subducted water also seems to soften the layer of mantle rock on which the planets glide. 'If there were no water... you might not have plate tectonics' on Earth. Venus lacks plate tectonics even though it is nearly Earth's twin in size and has similar reserves of internal heat. Its lack of water... may be the crucial difference" Richard A. Kerr, 1999, Making new worlds with a throw of the dice. Science 286:68-69
PROPERTIES OF WATER
(a) Water is a polar molecule by two related criteria:
(i) Water contains two polar covalent bonds (both are O-H bonds)
(ii) See Figure, Polar covalent bonds in a water molecule
(iii) One end of water possesses a partial positive charge (the 2× H end) while the other end of water possesses a partial negative charge (the O end--particularly important are the two electron pairs found on oxygen)
(iv) See Figure, Molecular shapes due to hybrid orbitals (Water)
(b) The attraction of opposite partial charges within liquid (or solid) water combined with the attraction between water's partial charges and the partial or full charges associated with other compounds underlie many of the unusual properties associated with water, including:
(i) High cohesion
(ii) High adhesion
(iii) High surface tension
(iv) High specific heat
(vii) Evaporative cooling
(viii) The fact that ice floats
(c) The dissociation of water that we study as pH is an additional emergent property of water
(4) Hydrogen bonding (see also hydrogen bond)
(a) Each water molecule can hydrogen bond with a maximum of four neighboring water molecules
(b) Liquid water possesses some structure due to this hydrogen bonding
(c) See Figure, Hydrogen bonds between water molecules
(d) Hydrogen bonds in water are ~1/20th as strong as covalent bonds
(e) Hydrogen bonds, in water, have only brief durations
(f) Hydrogen bonds in water, however, are extremely abundant, breaking and reforming continuously
(a) Surface tension is an emergent property of water that results from the tendency of water molecules to stick to each other (by hydrogen bonding) and so so better than they adhere to air molecules
(b) Surface tension "makes water behave as though it were coated with an invisible film."
(c) FAQ: Is cohesion responsible for surface tension? Yes, but also involved is the fact that water molecules don't bond very well with air molecules.
(a) Water is able to absorb heat -- without increasing much in temperature -- better than many substances
(b) This is because for water to increase in temperature, water molecules must be made to move faster within the water; this movement requires breaking hydrogen bonds, and the breaking of hydrogen bonds absorbs heat
(i) This is because for water to decrease in temperature, water molecules must be made to move more slowly within the water; this requires the forming of hydrogen bonds, and the forming of hydrogen bonds gives off heat (hence counteracting cooling tendencies as heat is lost from liquid water)
(d) Note again the concept that if forming something requires energy, then breaking that now-formed thing probably releases energy, in this case as heat
(e) Water's high specific heat serves to buffer the internal temperature of organisms, the temperatures of bodies of water, and the temperatures of the entire biosphere, all things that enhance the ability of life to survive on this planet
(b) This high heat of vaporization contributes to the ability of water to serve as local heat sinks (e.g., organisms, lakes, ponds) and as a global heat sink (i.e., oceans) -- these are regions (volumes) that retain heat for longer than surrounding substances (such as air or rocks)
(a) Water's high heat of vaporization, resulting from hydrogen bonding, also is responsible for water being a liquid at typical ambient temperatures
(b) That is, most molecules which are of similar molecular weight to water are gasses at typical ambient temperatures rather than liquids
(c) The molecular weight (MW) of water is 18, but unlike water, all of the following are gasses at room temperature:
(i) CO2 (MW=44) (carbon dioxide)
(ii) O2 (MW=32) (molecular oxygen)
(iii) CO (MW=28) (carbon monoxide)
(iv) N2 (MW=28) (molecular nitrogen)
(v) CH4 (MW=16) (methane)
(vi) H2 (MW=2) (molecular hydrogen)
(a) The vaporization of water is a consequence of individual water molecules escaping the liquid state for the gas, a.k.a., vapor state
(b) Those water molecules that are most energetic (i.e., moving fastest) are most likely to escape liquid water
(c) Faster moving water molecules carry more heat than slower moving ones (heat actually is simply a measure of degree of molecular motion)
(d) "It is as if the 100 fastest runners at a college transferred to another school; the average speed of the remaining students would decline." (Campbell et al., 1999)
(e) This results in the average temperature of liquid water declining with the loss of each more-energetic water molecule to the vapor phase
(f) Evaporative cooling contributes to water's ability to serve as a temperature buffer
(g) We employ evaporative cooling when we sweat
(h) (Evaporative cooling is an example of a system that is perturbed from a dynamic equilibrium. When the air about water is saturated with water -- 100% relative humidity -- water molecules leave the liquid phase as fast as water molecules in the vapor phase enter the liquid phase. Thus there is no net movement of water molecules in and out of the liquid or vapor phases, but there still is continuous movement between the two phases. At times like this you sweat like a pig but don't cool down at all because there is no net movement of water molecules to effect cooling and, assuming constant temperature between the phases, no difference on-average in the temperature between those water molecules leaving and those entering the liquid phase.)
(a) Unlike most substances, solid water (ice) has a lower density than liquid water
(b) As a consequence, solid water floats upon liquid water, rather than sinking beneath it
(c) The lower density of ice is a result of the water solid phase containing on average more hydrogen bonds per water molecule (i.e., approaching 4) than does liquid water at any given moment
(d) See Figure, The structure of ice
(e) More hydrogen bonds results in more structure which, in water's case, results in more unoccupied space, i.e., a lower density upon freezing
(f) Because ice floats, bodies of water freeze from the top down rather than the bottom up (or, more precisely, when water comes into contact with freezing air, the resulting ice remains on the surface of the water rather than sinking downward)
(g) Since ice serves as an insulator, this property of water assures that the complete freezing of bodies of water is far less likely, thus further explaining why so much liquid water exists on this planet
(11) Water as a solvent (see also polar solvent)
(a) The most important property of water to the existence of life has to do with the ability of water to dissolve some substances and exclude others
(b) Water dissolves substances to which it can readily hydrogen bond (or is otherwise attracted to typically because the substance contains a full or partial charge)
(c) Solute = a substance that dissolves in another substance
(d) Solvent = the substance the solute dissolves in
(e) Solution = a solvent in which solutes are dissolved
(f) Aqueous solution = a solution in which water is the solvent
(12) Hydration shells (see also hydration shell)
(a) For a substance to dissolve in water, the substance must displace water-to-water hydrogen bonds
(b) Consequently, for a substance to readily dissolve in water, it must be something that water will hydrogen bond to at least as well as water hydrogen bonds to itself
(c) Furthermore, the substance must also more-readily hydrogen bond to water than it interacts with molecules of its own kind; that way the molecule tends to leave the solid state and enter into solution
(d) Within solution, such a substance will be surrounded by water molecules which are hydrogen bonded to it
(e) See Figure, A crystal of table salt dissolving in water
(f) This surrounding array of water molecules is called a hydration shell
INTERACTIONS WITH WATER
(a) Cohesion refers to the tendency of water molecules to hydrogen bond to each other
(b) Cohesion contributes to a number of water's properties
(c) These properties include the ability of water to be siphoned as well the related property of transport of water from the roots to the leaves of plants
(14) Adhesion (see also adherence)
(a) Adhesion is the tendency of water to stick to substances other than water
(15) Hydrophilicity (hydrophilic) (see also hydrophilic)
(a) A substance that either readily dissolves in water or, if it is very large, is readily wetted by water is said to be hydrophilic
(16) Hydrophobicity (hydrophobic) (see also hydrophobic)
(a) Many substance don't hydrogen bond well with water
(b) Such substances tend not to enter into water solutions
(c) They are termed hydrophobic
(17) Hydrophobic exclusion (see also hydrophobic exclusion)
(a) Because hydrophobic substances tend to like to interact with one another and not with water, hydrophobic substances tend also to both not dissolve into water and to clump together when placed in water
(i) For example, water and oils don't mix
(b) This clumping actually is a result of the area of contact between the water and the hydrophobic substance being reduced to some small amount
(c) In terms of water's structure, what is going on is that water is arranging itself so that only a minimal number of water-to-water hydrogen bonds are lost to water's interacting with the hydrophobic substance
(d) The more the hydrophobic substance clumps, the lower its surface-to-volume ratio, and the fewer hydrogen bonds that are displaced
(e) See Figure, A water-soluble protein
(h) FAQ: Could you explain what hydrophobic exclusion is? Hydrophobic exclusion is very similar to surface tension. In both cases water molecules prefer to interact with themselves rather than interacting with a less-polar substance. This failure to interact significantly with a less-polar substance, however, reduces the number of hydrogen bonds water molecules are able to participate in. Losing hydrogen bonds is energetically unfavorable, and the number of hydrogen bonds lost is directly proportional to the area of contact between the water molecules and the hydrophobic substance. The smaller the area of contact, the fewer hydrogen bonds lost. Two volumes will have a smaller combined surface area if they become squished together rather than remaining separated. Consequently, hydrophobic substances, suspended in water, tend to pool together, away from water. For example, if you place two drops of oil on the surface of a water solution, the oil drops will tend to coalesce into a single drop (see Figures, above). Why care? Lipid bilayer membranes spontaneous form as a consequence of hydrophobic exclusion. Globular proteins spontaneous fold so that their hydrophobic parts are inside, away from water (i.e., hydrophobic exclusion). Hydrophobic exclusion is also one of things that makes lipids better long-term (and compact) storage molecules than carbohydrates. In fact, to life, the two most important roles played by water are (i) water dissolves hydrophilic substances and (ii) water excludes hydrophobic substances, i.e., hydrophobic exclusion.
DISSOCIATION OF WATER
(a) Water molecules can disassociate into charged compounds (i.e., ions) called H+ (hydrogen ion) and OH- (hydroxyl ion)
(b) There can only be so many of these ions in water (note: do yourself a favor and just accept this latter point)
(i) The more hydroxyl ions there are around, the fewer hydrogen ions there can be
(ii) This combining occurs because excess hydroxyl ions will tend to combine with whatever hydrogen ions are around, forming water, significantly reducing the number of hydrogen ions around--this, by the way, is an example of a dynamic chemical equilibrium
(iii) Similarly, excess hydrogen ions tend to mop up hydroxyl ions
(c) A solution containing an excess of hydrogen ions is said to be acidic and tastes sour
(d) A solution containing an excess of hydroxyl ions is said to be basic and tastes bitter
(e) Note that an excess of hydroxyl ions has a corresponding dearth of hydrogen ions
(19) pH (see also pH)
(a) pH is a measure of hydrogen ion (H+) concentration in water solution
(b) pH actually is equal to the inverse log of the Molar hydrogen ion concentration
(c) The more hydrogen ions present in a water solution, the higher the Molar concentration, and therefore the lower the pH
(d) See Figure, The pH of some aqueous solutions
(e) Solutions containing excessive hydrogen ions are termed acidic and have a pH that is less than 7
(f) Solutions containing excess of hydroxide ions (OH-) are termed basic and have a pH that is greater than 7
(g) (Note that the dissociation of water, H2O <==> OH- + H+, an example of a reversible reaction, as well as the reversible dissociation of numerous other acidic or basic compounds that underlies much of the chemistry of pH)
(20) pH buffer (see also pH buffer)
(a) Various aspects of biological systems work best at well-defined pHs
(c) The reason for the need to keep pHs within well-defined ranges has to do with enzymes typically functioning optimally only within narrow pH ranges, and if enzymes stop working well, whole system falls apart (e.g., organisms die)
(d) (Yes, I know that hydrogen ions really exist as hydronium ions, and that even that is a simplification, but this is biology so don't worry about it)
(21) Vocabulary [index]
(f) Hydration shells
(g) Hydrogen ions
(h) Hydroxyl ions
(i) Hydrogen bonding
(o) Ice floats
(q) pH buffer
(r) Surface tension
(22) Practice questions [index]
(a) Much of the unusual "emergent behavior" of water can be attributed to what?
(b) The technical term for the bulk property of water, its tendency to strongly adhere to itself, is __________. (note: this is not the same term as that for how water molecules bond to each other)
(c) What aspect of water makes it unexpected that it would be a liquid at room temperature?
(d) What is the name of the means by which water molecules bond to one another that is responsible for cohesion, adhesion, water having a high specific heat, a high heat of vaporization, etc.?
(e) Why does ice float? (that is, why is ice less dense than liquid water?)
(f) What is the most important, specific property of water to the existence of life? Answer requires two parts.
(g) Describe how evaporative cooling works. Do not get too detailed, the basic reason can be reasonably given in only a sentence or two.
(h) What property of water is the "most important property of water to the existence to life?" (note, this answer consists, essentially, of two answers)
(i) Define "hydrophobicity".
(j) A solution is buffered around a pH of 3. Is the solution acidic, neutral, basic, non-polar, or enzymatic?
(k) On average, how many hydrogen bonds is a water molecule participating in at any given time in liquid water? (note that you do not know the exact answer to this question plus I have not supplied sufficient information for anybody to answer this question exactly, but nevertheless the question does have a relatively straightforward, not terrible difficult answer--hint: the answer is a very simple algebraic expression rather than a whole or even a fractional number)
(l) The interaction of water with what substance (other than itself) explains surface tension?
(m) What must be "broken" in order for water molecules to transition from the liquid phase to the gaseous phase (gas state)?
(n) Why does ice float?
(o) Within water, a lowering of surface-to-volume ratios drives forward what biologically important phenomenon?
(p) What do organisms employ to maintain hydrogen ion concentrations in aqueous solutions within well-defined ranges? (looking for simple, general term)
(q) The most important substance necessary to the existence of life is __________.
(r) Indicate the placement and magnitude (or degree) of the partial charges.
(s) What is cohesion?
(t) How does sweating cool your body (assume <100% relative humidity and please describe the process rather than answering the question by stating "evaporative cooling")?
(u) Why do oil droplets suspended in water tend to clump together over time-in a process termed hydrophobic exclusion-given at least some thermal or physical agitation of the suspension?
(v) Organisms keep their fluids within well-defined pH ranges by producing and employing pH __________.
(w) Explain why life is liquid-phase based rather than solid-phase or gas-phase based.
(x) Contrast cohesion and adhesion. As you answer this, make sure that you unambiguously let me know which one (cohesion vs. adhesion) you are defining/writing about as you consider both terms.
(y) True or False, the forming of hydrogen bonds gives off heat.
(z) What do CO2, O2, CO, N2, CH4, and H2 have in common that contrasts them with H2O?
(aa) The array of water surrounding a dissolved chlorine ion is called a(n) __________.
(bb) The tendency of oils to clump when introduced into liquid water is a consequence of hydrophobic _________.
(cc) At a pH of greater than seven there is an excess of
(i) Chloride ions
(ii) Hydrogen ions
(iii) Hydronium ions
(iv) Hydroxyl ions
(v) Sodium ions
(dd) What is this a representation of, please be specific:
(ee) Each water molecule can hydrogen bond with a maximum of __________ neighboring water molecules.
(ff) Which of the following does not describe hydrogen bonds as they occur in liquid water:
(i) 1/20th as strong as covalent bonds
(ii) Brief in duration
(iii) Gives rise to some structure in liquid water
(v) Occurring between adjacent water molecules
(gg) True or False, the breaking of hydrogen bonds absorbs heat (i.e., requires an input of energy).
(hh) From what property of water would make you predict that water would be a gas at room temperature rather than a liquid (but, in fact, water is liquid as a consequence of hydrogen bonding between water molecules).
(ii) Evaporative cooling occurs because the most __________ water molecules are the ones mostly likely to escape from liquid water into the gaseous (water vapor) phase.
(jj) Adhesion refers to the ability of water to adhere to certain substances, typically forming a hydration shell. These hydrophilic substances are ones to which water readily __________ bonds.
(kk) Describe the principle of hydrophobic exclusion. Why does oil tend to clump together when suspended in a water solution?
(23) Practice question answers [index]
(a) Much of the unusual "emergent behavior" of water can be attributed to water's propensity to form hydrogen bonds; or water being a highly polar molecule
(b) The technical term for the bulk property of water, its tendency to strongly adhere to itself, is cohesion
(c) The aspect of water that makes it unexpectedly a liquid at room temperature is water's low molecular weight
(d) hydrogen bond
(e) because ice has more hydrogen bonds than liquid water, and water molecules can pack together more tightly when hydrogen bonds are lacking
(f) solvent for polar molecules, excluder of non-polar molecules
(g) Hotter molecules escape (evaporate) thus lowering the average temperature (a.k.a., velocity) associated with the individual molecules remaining in the liquid state. Thus, evaporative cooling works via a lowering of the average temperature of water molecules because on average it is hotter than-average-water molecules that evaporate
(h) The property of water that is most important to the existence of life is water's propensity to dissolve especially polar substances, i.e., water is a highly effective polar solvent, while not dissolving (i.e., excluding) non-polar substances
(i) Hydrophobicity is the tendency, by a substance, of not dissolving in water. The term literally means water-hating
(j) A solution that is buffered around a pH of 3 is termed acidic A pH of 7 is neutral while a pH of greater than 7 is termed basic
(k) Less than four or, algebraically, x < 4 where x is the average number of hydrogen bonds a water molecule is participating in at any given instant in liquid water; 4, of course, is the maximum (not average) number of hydrogen bonds a water molecule can form with water neighbors (a number better approximated by ice than by liquid water which, almost by definition, is a substance in which the maximum number of hydrogen bonds is not continuously maintained)
(l) Air: Surface tension is an emergent property of water which results from the tendency of water molecules to stick to each other better than they adhere to air molecules
(m) Hydrogen bonds must be broken
(n) Ice floats because it contains more hydrogen bonds, on average, than liquid water and (and this is a very important and) hydrogen bonding gives water structure and more unoccupied space between molecules; more space means lower density thus ice has a lower density than water so consequently floats upon water rather than sinking beneath it
(o) Hydrophobic exclusion; the surface-to-volume ratio of the hydrophobic substance is important--the greater this ratio, the more water molecules must interact with this substance rather than with themselves, and therefore the less energetically favorable the interactions; the most-stable situation is one in which water can maximally hydrogen bond with itself/minimally interact with the hydrophobic substance, hence oil clumps in water into spherical droplets (ignoring such complications as gravity and water's surface), spherical clumps having much lower surface-to-volume ratios than either less-clumped oil or clumps of any other shape
(p) pH buffers
(r) H-O-H, but bent such that the atoms form a triangle with partial positive charges associated with the Hs and twice as large a partial negative charge associated with the Oxygen
(s) Cohesion is the tendency of water molecules to stick to each other
(t) Water has a high heat of evaporation because it requires the breaking of hydrogen bonds to convert liquid water to water vapor and thus not only do only do the hottest water molecules escape to the vapor phase, but it is difficult for them to escape; your body employs this evaporative cooling effect to cool off by secreting water to the surface of the body in sweat which then evaporates, thereby cooling, thereby cooling the body; because of water's high heat of evaporation, the amount of water lost in the process of cooling the body evaporatively is comparably less
(u) This clumping is a product of hydrophobic exclusion which is driven by energetic favorability of reducing the surface area interface between the droplets of oil and the water they are suspended in; any water-oil interface represents an absence of water-to-water hydrogen bonds and the disruption of these bonds is energetically not favored (i.e., takes energy to break hydrogen bonds); since oil does not form strong bonds with water to replace the broken hydrogen bonds, oil has a tendency to clump thereby reducing the surface area of its interface with water
(w) Solids are sufficiently complex but are insufficiently dynamic. Gasses are sufficiently dynamic, but insufficiently complex. Liquids can be a good compromise in that they can be both more complex than gasses as well as more dynamic than solids. The liquid phase in living things generally is much more complex than your standard liquid, but life, still, exists more or less in the liquid phase
(x) Cohesion is the tendency of water molecules to stick to each other while in the liquid phase while adhesion is the tendency of liquid water to adhere to other, non-water substances
(z) All are gasses at typical ambient temperatures while water is a liquid
(aa) Hydration shell
(bb) (iii) Exclusion
(cc) (iv) Hydroxyl ions
(dd) The molecular structure of ice
(ff) (iv) Infrequent
(hh) Water's low molecular weight would make you think that it is a gas
(kk) Water "wants" to hydrogen bond with either other water molecules or with other substances possessing full or partial charges. Hydrophobic substances, such as oils, do not hydrogen bond with water. Consequence, the displacement of water-to-water hydrogen bonds via the intervention of an oil is energetically unfavorable. Instead, the energy minimum is represented by water maximally in contact with itself. This occurs if the contact area with the oil is minimized, which occurs by forcing the oil into a shape and arrangement approximating a single sphere.
(#) Solutions containing excessive __________ are termed acidic and have a pH that is less than 7.
A: Hydrogen ions; hydronium ions.
(#) Hydrophobic exclusion occurs because water arranges itself so as to maximize the formation of what? Be specific!
A: Hydrogen bonds between water molecules
(#) Hydrophobic substances, within biological systems, are collectively described as what?
(#) Adhesion is the tendency of water to do what?
A: Stick to substances other than water
(#) Arrange in terms of average number of hydrogen bonds between water molecules, going from highest number of bonds to lowest number of bonds: Gaseous water, Ice, Liquid water. To play things safe, please indicate which has the highest and which has the lowest number of hydrogen bonds (on average).
A: Ice, Liquid, Gaseous (highest to lowest).
(#) True or false, the formation of a hydrogen bond between water molecules releases heat.
A: True, this is the basis behind water's high specific heat
(#) Name a type of bond that is 1/20th as strong as a covalent bond.
A: Hydrogen bonds
(bonus) Why would solid-phase material make a pretty poor basis for the chemistry of life?