Water: A Foundation for Life

It makes sense to start a study of biology by thinking about the nature of water, because water appears to be necessary for biogenesis, the beginnings of life. As Tom Robbins says metaphorically in Even Cowgirls Get the Blues, “People were invented by water as a means of getting from place to place” ^[1]

We (particularly "exobiologists" who are interested in extraterrestial life) look for water on other planets as the primary precondition for life. Here's a video (one of many found on YouTube by searching "magic sand") suggesting why this may be the case, and showing how you can make Magic Sand^[2][3]

Still picture from video available from YouTube

It shows an example of how Magic Sand clumps up only in water to make shapes reminiscent of biological structures. It may model how certain biomolecules, called clump up in water to form "coascervates" or cell-like structures.

The "Magic" is in the water, not in the sand. To understand this amazing behavior (no other liquid does this quite like water), we must understand the water molecule, and to do that, we need a productive "Atomic Theory". We'll start developing a molecular model of water and see how it describes ice, liquid water, and water vapor.

Two very important things that chemists (and scientists in general) do include making quantitative measurements, and communicating the results of experiments as clearly and unambiguously as possible. We will now deal with another important activity of chemists—the use of their imaginations to devise theories or models to interpret their observations and measurements. Such theories or models are useful in suggesting new observations or experiments that yield additional data. They also serve to summarize existing information and aid in its recall.

The atomic theory, first proposed in modern form by John Dalton, is one of the most important and useful ideas in chemistry. It interprets observations of the every-day world, like the behavior of Magic Sand or biomolecules in water, in terms of particles called atoms and molecules. Macroscopic events—those which humans can observe or experience with their unaided senses—are interpreted by means of microscopic objects—those so small that a special instrument or apparatus must be used to detect them. (Perhaps the term submicroscopic really ought to be used, because most atoms and molecules are much too small to be seen even under a microscope.) In any event, chemists continually try to explain the macroscopic world in microscopic terms. In the case of water, H₂O, the contrasting properties of ice (solid water), liquid water, and water vapor (gaseous water), for example, may be ascribed to differences in spacing between and speed of motion of the constituent atoms or molecules.

In the form originally proposed by John Dalton, the atomic theory distinguished elements from compounds and was used to explain the law of constant composition and predicted the law of multiple proportions. The theory also agreed with Lavoisier's law of conservation of mass.

The atomic theory amazingly also can be used to explain the precise masses of hydrogen and oxygen that go into making water. An important aspect of the atomic theory is the assignment of relative masses (atomic weights) to the elements. Atoms and molecules are extremely small. Therefore, when calculating how much of one substance is required to react with another, chemists use a unit called the mole. One mole contains 6.022 × 10²³ of whatever kind of microscopic particles one wishes to consider. Referring to 2 mol H₂ specifies a certain number of H₂ molecules in the same way that referring to 10 gross of pencils specifies a certain number of pencils. The quantity which is measured in the units called moles is known as the amount of substance. The somewhat unusual number 6.022 × 10²³, also refered to as the Avogadro Constant, which specifies how many particles are in a mole, has been chosen so that the mass of 1 mol of atoms of any element is the atomic weight of that element expressed in grams. Similarly, the mass of a mole of molecules is the molecular weight expressed in grams. The molecular weight is obtained by summing atomic weights of all atoms in the molecule. This choice for the mole makes it very convenient to obtain molar masses–simply add the units grams per mole to the atomic or molecular weight. Using molar mass and the Avogadro constant, it is possible to determine the masses of individual atoms or molecules and to find how many atoms or molecules are present in a macroscopic sample of matter. A table of atomic weights and the molar masses which can be obtained from it can also be used to obtain the empirical formula of a substance if we know the percentage by weight of each element present. The opposite calculation, determination of weight percent from a chemical formula, is also possible. Once formulas for reactants and products are known, a balanced chemical equation can be written to describe any chemical change. Balancing an equation by adjusting the coefficients applied to each formula depends on the postulate of the atomic theory which states that atoms are neither created, destroyed, nor changed into atoms of another kind during a chemical reaction.

From ChemPRIME: 2.0: Prelude to Atoms and Reactions

References

1. ↑ *Robbins, T. Even Cowgirls Get the Blues, Houghton Mifflin, Boston, 1976, p. 2.
2. ↑ Just heat ordinary sand in the oven to dry it for an hour, then spray it thoroughly with Scotch Gard(R)
3. ↑ http://en.Wikipedia.org/wiki/Magic_sand

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.