It’s something of an enigma: a compound composed of just two chemical elements, one of which is the simplest in the known universe. A pair of hydrogen atoms, holding firmly to a single oxygen atom, create a deceptively minimalist arrangement that belies its enormous versatility and importance.
The value of water, as a resource, is commonly understood. Without a steady supply, civilization as we know it wouldn’t exist.
But as a substance — as a molecule — water tends to elude most people’s sense of wonder. Which is too bad, really, because it’s quite remarkable.
Shaped and bonded
The basic molecular formula for water, H2O, suggests a simple structure of three atoms in a straight line. But the physical properties of the three atoms force another arrangement — a “V” shape with oxygen at the point. This nonlinear shape transforms water into a remarkable substance with astounding abilities.
The “V” shape arises from the arrangement of electrons in the molecule, which causes an imbalance in electrical charge, with the oxygen point of the “V” slightly more negative than the opposite end near the hydrogen atoms. This slight separation means the water molecules are polarized — one pole positive and the other negative.
Polarity of electrical charge is at the root of water’s fascinating properties. The slightly negative end of one water molecule attracts the slightly positive end of another and vice versa in what scientists call hydrogen bonding.
Hydrogen bonds between water molecules are exceptionally strong, giving them a propensity to cling to each other, a behavior that manifests most commonly as surface tension. For instance, water in a container that is filled to the brim appears to bulge out of the vessel in a convex shape when viewed from the side due to surface tension. And some creatures, such as water striders, can take advantage of surface tension to skim across the surface of ponds.
Hydrogen bonds also enable water to adhere to foreign substances. This adhesive quality enables plants to draw water from the ground, through their roots and up to the tips of their leaves, defying gravity’s pull.
The polar nature of water and its resulting shape cause it to be lighter as a solid than as a liquid. That’s because the molecular Vs form airy crystal structures as they freeze, making ice less dense than its fluid form. So, ice floats on rivers and lakes, forming a shield against the cold air above and keeping the water below from freezing, which allows fish and other aquatic species to survive in colder climes.
Water’s hydrogen bonding also results in another critical characteristic — a higher than expected boiling point, explains Jessica Parr, professor (teaching) of chemistry. Parr earned her PhD in chemistry at USC Dornsife in 2007 and has taught general chemistry to undergraduates ever since. Her dissertation research centered on understanding how hydrogen bonds react under exposure to intense light.
“If water wasn’t capable of such strong hydrogen bonding, it would boil at minus 200 degrees Celsius,” Parr explains, far below its actual freezing point of 0 degrees Celsius. That means it would exist on Earth by and large as a gas, making life as we know it impossible. Instead, our planet sloshes with water, about 366 million-billion gallons in all.
A universal solvent
Water’s polar nature also makes it an exceptional solvent, capable of dissolving a wide variety of substances.
“We refer to it as the ‘universal solvent’ because it can dissolve, not everything, but so much stuff,” Parr says. “A lot of other molecules are choosy about what they interact with and how they work together, but water will interact with just about anything.”
For example, its positive and negative centers attract and easily coax apart charged atoms, called ions, that make up salts such as sodium chloride, commonly used in cooking. The positively charged sodium and negatively charged chloride atoms find a comfortable home drifting among water’s polarized molecules.
But water can also dissolve substances that aren’t composed of ions, such as sugars. Rather than separating individual atoms of a sugar molecule, however, water molecules work their way between each sugar molecule, finding lightly charged parts to hydrogen bond with. This loosens the connections between the sugar molecules, drawing them away from one another and eventually into a solution.
“As long as there’s one atom present that makes the other molecule want to interact with water, water will do that,” Parr says.
It gets weird
Water doesn’t always behave as expected. While it most often transitions from solid (ice) to liquid to gas (steam or vapor) and vice versa as its temperature rises and falls, it can make the leap straight from ice to vapor under the right conditions.
“If you’ve ever noticed that your ice cubes get smaller over time, it’s because they’re sublimating in your freezer — the ice turns straight into a gas,” explains Parr. This sublimation is due to the low humidity within the freezer, which allows a few water molecules to escape from the ice into the air withoutmelting first.
In the reverse process, called deposition, gaseous water suddenly freezes without ever becoming liquid. This is how snow forms. And when conditions are right, snow may skip the melting stage and sublimate right back into the atmosphere, a particular conundrum for drought-prone areas such as California, which rely on melting snowpack as a water source.
But water can be even weirder.
“Ice has lots of different crystal forms, but it can also exist in a form that resembles glass — an amorphous solid that is somewhere between liquid and solid and which can still flow,” Parr says. When water molecules coalesce at very low temperatures and pressures — think outer space — the resulting ice also can behave like glass, Parr says. Scientists suspect this may be among the most common forms of water in the universe.
The mystery and wonder continue
Despite its prevalence in the universe and on Earth, and humans’ long familiarity with it, water continues to surprise.
Scientists recently discovered a form called “superionic ice.” Existing at extremely high pressure, such as in the core of planets, it appears to play a role in maintaining Earth’s magnetic fields.
And though water in pure form is not an electrical conductor, it behaves unexpectedly when exposed to an electric field. USC Dornsife’s Alexander Benderskii, associate professor of chemistry, and Stephen Cronin of the USC Viterbi School of Engineering recently found that water molecules near an electrode line up differently than those farther away.
“We were able to see how the molecules interacted with the electric field in a way no one had previously understood,” Benderskii said. The finding could change how chemists control reactions, including processes for making medicines and for purifying water for drinking.
As researchers continue to explore this versatile, unexpected substance, potentially revealing more strange characteristics, water may just prove to be the most complex simple molecule in the universe.