E2science

Carbonic acid is a weak acid that is produced when carbon dioxide is dissolved in water.

As you probably know, our atmosphere has a lot of carbon dioxide in it. It is also thoroughly saturated with water. From this, we might deduce that we live in a rather acidic environment — and we do. Carbon dioxide is constantly dissolving into the water that surrounds us, forming natural carbonic acid. Rainwater is naturally a little acidic, and the oceans and other bodies of water are always absorbing carbon dioxide and turning it into carbonic acid. On a day-to-day basis, this doesn't really make any difference. Carbonic acid is very weak, and generally hurts no one. But...

Carbonic acid melts the Earth

You bet it does. Calcite (calcium carbonate) is soluble in acidic water (it actually undergoes a transformation into calcium bicarbonate). Calcite is present in many rocks, most notably in limestone and marble. Over the course of centuries and millennium, water and carbonic acid slowly eat away rocks, a process too slow to see, but very important on a geological timescale. The carbonic acid breaks down not only strata at the surface, but does some serious damage underground, forming caves and karst topography.

But the acid isn't just a destructive force; it also creates cave formations such as stalactites, rock flows, soda straws and many others. Much of the water that soaks into the ground passes through biodegrading plant material, resulting in particularly high levels of carbonic acid. The higher levels of acid means that it can dissolve more calcite. When the water flows into an open cave the calcium bicarbonate will offgas carbon dioxide into the air, leaving calcite to precipitate out of the water. This calcite is left behind as the water drips, filling caves with flowing rock sculptures.

This may be the reason that the most interesting cave formations are found in warmer, wetter parts of the world, or in regions that were once warm and wet. These are the areas where there is the most composting plant matter, and thus the highest concentrations of carbonic acid in the groundwater. This is also why sinkholes and other karst features appear in these areas.

Carbonic acid flows in my blood

Yours too — carbonic acid is the primary acid buffer controlling the pH balance in the blood plasma of mammals. Many chemical reactions in your body depend on a very specific pH balance, but at the same time the level of cellular metabolism changes the pH balance.

When our cells exhale carbon dioxide into the bloodstream a large percentage of it is taken up into the red blood cells — but not carried directly to the lungs for disposal. Instead, it's combined with water to form carbonic acid, and then combined with more water, breaking the carbonic acid (H₂CO₃) into hydrogen carbonate (AKA Bicarbonate, HCO₃^-) and a positive ion (a proton, H⁺). The 'natural' rates of carbonic acid formation are much too slow to provide an effective buffer system, so this reaction is tremendously sped up by an enzyme called carbonic anhydrase that lives in your red blood cells.

The H+ produced is bound to the haemoglobin in the red blood cell, but the hydrogen carbonate slips out into the bloodstream (the red blood cell will absorb a Cl- ion from the plasma to keep its electrical balance from becoming too acid). Once the red blood cells get to the lungs they pick up oxygen and drop the H+. The H+ combines with the hydrogen carbonate to form carbonic acid, which pops out of the blood stream and into the alveoli of the lungs in the form of carbon dioxide.

This fancy dance keeps enough hydrogen carbonate (a base) circulating in the bloodstream to keep any spikes of acid muted. This is important — a healthy human body needs to have a pH balance between 7.35 and 7.45. When the blood becomes too acid, it is known as acidosis; too base and you have a condition called alkalosis. The body deals with these conditions by simply adjusting your rate of respiration, which changes the carbon dioxide level of your blood. On a much slower scale, the kidneys also adjust both hydrogen carbonate and H+ levels in the blood.

Carbonic acid poisons the oceans

Nothing tricky here; water plus carbon dioxide equals carbonic acid. Carbonic acid has been doing its part to keep down the pH balance of the oceans since the beginning of time (the oceans are naturally alkaline). In recent decades carbon dioxide emissions have been increasing; this is good news for spelunkers, but bad news for the environment. Not only is carbon dioxide a greenhouse gas, leading to global warming, but it's also mixing with the oceans at an accelerated rate, loading it up with excess carbonic acid. This process is known as ocean acidification.

If you recall, calcite dissolves in carbonic acid. As it happens, many types of oceanic plankton and other small animals use calcite and other soluble minerals to build their shells. coccoliths, planktic foraminifera, red algae, brachiopoda, echinoderms, bryozoa, corals of all types, and perhaps even bivalves (oysters and such) will be affected by carbonic acid attacking their internal and/or external structures. Other species, such as squid and whales may find it harder to keep their blood acid levels down. And a change in ocean chemistry might break down molecules, setting toxic metals free and destroying important nutrients. Even if the direct damage is limited to plankton, this still means massive disruption to the oceans' food chains.

Needless to say, this is bad, but we really have no idea how bad it might be. We don't know very much about any individual species reaction to increased acidity, and we know even less about the resiliency of oceanic ecosystems. We also don't know how other factors — such as a rise in oceanic temperatures — will interact with the acidification effects. Things could suddenly collapse in the next 20 years, leaving us without any sizable fish (or sea mammal) populations. Or things could all work out very nicely, Gaia having gone through any number of high-carbon, high-temperature shifts in the past few hundred million years. It will all be one splendid surprise for us humans.

Regardless of how much faith one wishes to place in the Earth goddess, the amount of carbon being taken into the oceans is really quite worrying. In the last two centuries the average pH of the oceans has dropped by 0.1. This doesn't sound like a lot, but it represents a thirty percent increase in hydrogen ions (H+). Scientists are predicting serious losses in species' habitats by 2030, and then onward into the foreseeable future. We are pumping more carbon dioxide than ever into our environment, and even if we were to stop today it would take decades for the effects of our actions on the oceans to become apparent.

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References:
http://en.wikipedia.org/wiki/Carbonic_acid
http://www.nps.gov/cave/planyourvisit/upload/cave_geology.pdf
http://creationwiki.org/Stalactites_can_grow_very_rapidly
http://www.nps.gov/ozar/forteachers/speleothems.htm
http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/virtmap/index.html
http://scifun.chem.wisc.edu/CHEMWEEK/BioBuff/BioBuffers.html
http://en.wikipedia.org/wiki/Bicarbonate
http://en.wikipedia.org/wiki/Acidosis
http://en.wikipedia.org/wiki/Carbonic_anhydrase
Barron's How To Prepare For The MCAT, by Hugo R. Seibel, 2006
http://www.nda.ox.ac.uk/wfsa/html/u13/u1312_02.htm
Intravenous Infusion Therapy for Nurses: Principles & Practice By Dianne L. Josephson, 2004
http://www.truthout.org/cgi-bin/artman/exec/view.cgi/61/20787
http://en.wikipedia.org/wiki/Ocean_acidification
http://www.publications.parliament.uk/pa/cm200607/cmselect/cmsctech/470/470we08.htm

We divide rocks into three basic types: igneous, formed when molten rocks cool down; sedimentary, formed when sediment is deposited at the bottom of the sea; and metamorphic, made when other kinds of rock are transformed by heat and pressure.

As you probably know, the inside of the Earth is very hot, and rocks deep inside melt in the high temperatures. Molten rock is called magma. When it erupts to the surface, for example in a volcano, we call it lava. When molten rock solidifies, it becomes igneous rock such as granite, made out of interlocking crystals. If the rocks cool slowly, as they sometimes do deep within the Earth's crust, these crystals are large, because they have a lot of time to grow. If they cool quickly, like lava does when it hits the cold air or sea water, there is little time for the crystals to grow, so they are very small. Igneous rocks tend to be very hard.

Sedimentary rocks are formed from grains, deposited on the sea bed in layers, and compacted by the pressure of the sea. The grains might come from other rocks that have been eroded by the sea or the weather, or they might be hard, left-over bits of long-dead organisms. Chalk is mostly made of the shells of microscopic organisms called coccolithophores, for example. It is quite common to find fossils of larger animals deposited in sedimentary rocks such as limestone. Because they are made of little bits of things, only loosely pressed together, sedimentary rocks tend to be far weaker than other kinds of rock, and they are very susceptible to erosion.

Metamorphic rocks are formed when other kinds of rock are transfomed (metamorphosed) by heat and pressure within the Earth's crust – enough to bring about profound changes in their chemistry and structure, but not enough to melt them. The presence of water and the effects of strain are also important here. Because there are so many different factors deciding the final form of the rock, metamorphic rocks are incredibly varied. They are usually hard, having been subjected to great heat and pressure, and often stripy or banded due to shear forces causing them to change shape as they formed.

More properly called the Brønsted-Lowry Theory, this logical extention of the original postulate on the nature of acids and bases called the Arrhenius Theory was independently developed by the chemists J. N. Brønsted and T.M. Lowry in the 1920s. The title comes from the shared credit of the two chemists in furthuring the study of acids and bases.

The basic definition supplied by the Brønsted-Lowry Theory is that an acid is a proton donor (of H⁺) and a base is a proton acceptor. Thus, an acid-base reaction is the transfer of a proton from an acid to a base. This differed from previous definitions in that it wasn't concerned so much with the make-up of a given acid or base as its characteristics; how easily did it accept or give up a proton? Any molecule that's capable of releasing a hydrogen ion is an acid, and any that can accept it is a base. This flexibility was important, because previously only molecules that contained a hydroxyl group OH^- could be considered bases, which didn't comply with new discoveries of stronger acids and bases at the time.

So why the original mix-up? Part of the issue is that bases usually do contain a hydroxyl group. Having a free OH hanging around is the easiest way to snap up a proton (but not the only way!). Some absolutely vital byproducts of this realization included descriptions of autoionizaton of water, dissociation in aqueous solutions, amphoterism, acid and base strength, leveling effects, ternary acids, and salt derivations. None could have been predicted with the original Arrhenius Theory.

Another absolute vital result of Brønsted-Lowry Theory, and included intrinsically in the definition, was the concept of conjugate acids and bases. Once an acid has donated its proton, the story is not over. The molecule now has extra space in which it could feasibly receive a proton, making it a base. Likewise for bases, which once they have received a proton, could feasibly give it away. That doesn't necessarily mean it will happen, however. The strength of an acid or base is inversely proportional to the strength of its conjugate base or acid. An extremely strong acid isn't going to easily take back the proton it was so willing to give away (making it a weak conjugate base) and a base hungering madly for a proton won't relinquish it without a fight (making it a weak conjugate acid). For acids and bases which match fairly closely in their conjugate forms, however, constant reverse reactions are common. Under circumstances where the reaction is ocurring in an aqueous environment, conjugate forms help to explain the presence of both neutral pH water and salts.

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Whitten, Kenneth W. Davis, Raymond E. Peck, M. Larry. General Chemistry, 6th ed. South Melbourne: Thomson Learning, Inc., 2000.

What Ionic Bonding Is

Ionic bonding is the type of chemical bonding that binds metals with non-metals*, forming ionic compounds. An ion is just an atom (or sometimes a molecule) with an overall electric charge - many atoms and molecules have exactly as many electrons as they have protons, so the charges cancel out; when that doesn't hold true, we end up with ions.

Metals are prone to losing electrons from their outside shell, leaving them with a positive charge; non-metals often pick up additional electrons from somewhere, filling up their outside shell and leaving them with a negative charge. Opposite charges attract, so electric forces tend to cause these positive and negative ions to stick together. Since those forces radiate out in all directions, you don't just get one positive ion (or cation) bonding with one negative ion (or anion) - any more ions that happen by get pulled in, too. There's always a sweet spot where the pushing and pulling of the ions balances out, allowing new ions to slot neatly into any existing structure. That neatness gives a very regular lattice-like pattern to the solid - in other words, ionic compounds form crystals.

What Ionic Bonding Isn't

It's worth saying something about some common misconceptions about ionic bonding. If you have learned about it before, you may have been told that an ionic bond is what you get when a metal ion donates an electron to a non-metal. This description has a pleasing simplicity to it, but it is really very misleading. For one thing, ionic bonding typically holds together many atoms at once. This is in contrast to the covalent bonds** that hold non-metals together, where the bonding is down to each atom sharing electrons with its neighbours, which leads to the formation of well-defined molecules. Ionic compounds are not really made of molecules at all, just big crystalline structures.

The other thing wrong with the electron-donation picture is that the ions have usually gained or lost electrons long before they ever meet - for many elements, like sodium and the other alkali metals, it is rare to find them any other way on Earth. Less reactive metals may have been exposed to ionising radiation, or lost an electron or two in a collision. Reactive non-metals have a tendency to pick up any free electrons they bump into, whatever the source, because they fit nicely into the geometry of their outside shells.

Ionic Compounds

Ionic compounds are characteristically hard, usually with high melting points, and very brittle. The hardness and high melting points are down to their crystal structure; as long as the lattice holds, they are solid and quite strongly bonded. However, since the crystal is made of alternating positive and negative ions, a knock that causes one layer to get out of alignment with the next will often lead to cations lining up with cations, and anions with anions, producing a repulsive force that tears the crystal apart - hence the brittleness. Metals, which also have a crystalline structure, don't suffer from this problem, which is why they are much more malleable.

Many ionic compounds are soluble in water. This is because water molecules are polar, in the sense that they have more positive charge on one side than the other. A negative ion will attract the positive ends of water molecules, and when it collects enough water molecules that way, their collective attraction can overcome its bonding with its ionic neighbours and carry the ion away. The positive ions dissolve much the same way. All these positive and negative ions allow a solution to conduct electricity - distilled water is actually an electrical insulator, whereas salt water conducts extremely well. There is a useful complication to the way ion solutions conduct electricity - because the charge is carried by two kinds of ions travelling through space, not just free-floating electrons like you get in a metal, they tend to separate over time - cations are attracted to cathodes, and anions to anodes. This process, known as electrolysis, makes it possible to extract the constituent elements of a salt; sodium, potassium, calcium and various other elements were first isolated in this way.

*Strictly speaking, some ionic compounds are formed entirely from non-metals, with polyatomic cations taking the place of the metals, but usually you can assume that ionic bonding joins metals with non-metals.

**We should note here that there is not really a sharp distinction between covalent and ionic bonds. Many covalent bonds are polar, meaning that the electrons are shared unevenly between the atoms, so that one of the atoms acquires a positive charge, and the other a negative one - these bonds can be considered to be a bit ionic. Similarly, ionic bonds can be considered mildly covalent when electrons get shared between atoms, which they inevitably do. Metallic bonding is sometimes considered a form of covalent bonding, but sometimes not - the shared electrons are more like a sea than a set of pairs. Chemistry gets pretty messy when you look close enough.

References:

• The Royal Society of Chemistry's Chemical Misconceptions
• P. W. Atkins, The Elements of Physical Chemistry
• Richard Parsons for CGP, GCSE Chemistry

This writeup also appears on my blog with an 'understand-o-gram' by Sonya Hallett.

Reel Physics is a weekly web series of 7-12 minute videos about analyzing the plausibility of extraordinary stunts in movies. Created and hosted by Jason Dean and Colby Dane, each week they stand in front of a green screen and ask questions like “Can you ‘fly’ a tank?”, “Could Indy have survived that nuclear blast inside a fridge?”, “Could that helicarrier actually fly?”, “Are cop cars appropriate projectiles for taking down helicopters?”.

After looking at the scene, they gather as much data as they can (using the specs from real objects or the closest appropriate analogues they can find) and then calculate if the stunt was actually possible as it was depicted in the scene. It’s like Mythbusters but with more math. When data isn’t available, such as the make-up of cables used in a scene or the age and wear on materials, they err on the side of the scene as long as the question being asked doesn’t depend on that specific data; such as in the case of knowing the friction force of super sticky tires versus standard commercial ones. Then they go through the scene, showing the formulas they used, and how they plugged in their data before coming to the conclusion of whether the stunt was either real physics or “reel physics”.

While the data and math comes at you fast, so you can’t calculate it yourself without stopping the video, they do supply a good amount of information. The hosts keep the pacing up, and though some of the jokes are a bit labored, they have very good energy as presenters and they are obviously having a lot of fun doing this. As an added bonus, during the credits they show outtake clips of themselves screwing up their lines and barely containing their infectious laughter.

The series is available to view on The Escapist website, and The Escapist Magazine YouTube channel.