chemical paper(exact three pages)(Your grade on the fifth essay will depend, to large measure, on the improvement demonstrated relative to the first four essays.)

LECTURE 17 An IONIC COMPOUND that CHANGED the WORLD

Salt (NaCl) comes from dead, dried-up seas or living ones. It can bubble to the surface as brine or outcroppings in the form of salt licks and shallow caverns. Below the surface of the earth, it is deposited in white veins, some of them thousands of feet deep. It can be evaporated from salt “pans,” boiled down from brine, or mined from shafts extending half a mile down. Since Neolithic times, salt from upwelling brine was recovered in an area on the outskirts of present-day Kraków, Poland, and excavations began in the 13th century. The crown jewel of the Wieliczka salt mine is St. Kinga 's Chapel (above), which is sculpted entirely in salt. The mine reaches a depth of almost 1100 feet, and extends via horizontal passages and chambers for over 178 miles. Over the centuries, among the visitors of the Wieliczka salt mine were Copernicus, Goethe, Chopin and Mendeleev.

The “history of the world according to salt” is simple: animals wore paths to salt licks; men followed; trails became roads, and settlements grew beside them. When the human menu shifted from salt-rich game to cereals (see Lecture 1), more salt was needed to supplement the diet. Since underground deposits were beyond reach, and the salt sprinkled over the surface was insufficient, scarcity kept the mineral (Halite) precious. As civilization spread, salt became one of the world’s principal trading commodities.

Salt routes crisscrossed the globe. One of the most traveled led from Morocco south across the Sahara to Timbuktu. Ships bearing salt from Egypt to Greece traversed the Mediterranean and the Aegean. Herodotus (484-425 BC), “ The Father of History ,” a title first conferred on him by Cicero in the first-century BC, describes a caravan route that united the salt oases of the Libyan desert. The passage is in his book  The Histories , a detailed record of his inquiry on the origins of the  Greco-Persian Wars . Venice’s glittering wealth was attributable not so much to exotic spices as to commonplace salt, which Venetians exchanged in Constantinople for the spices of Asia. In 1295, when he first returned from Cathay, Marco Polo delighted the Doge with tales of the prodigious value of salt coins bearing the seal of the great Khan.

As early as the 6th century, in the sub-Sahara, Moorish merchants routinely traded salt for gold, ounce for ounce. In Abyssinia (Ethiopia), slabs of rock salt, called ‘amôlés, became coin of the realm. Each one was about ten inches long and two inches thick. Cakes of salt were also used as money in other areas of central Africa.

Not only did salt serve to flavor and preserve food, it made a good antiseptic, which is why the Roman word for these salubrious crystals (sal) is a first cousin to Salus, the goddess of health. Of all the roads that led to Rome, one of the busiest was the Via Salaria, the salt route (in gray on the map below), over which Roman soldiers marched and merchants drove oxcarts full of the precious crystals up the Tiber from the salt pans at Ostia.

A soldier’s pay, consisting in part of salt, came to be known as solarium argentum (“salt money”), from which we derive the word salary. A soldier’s salary was cut if he “was not worth his salt,” a phrase that came into being because the Greeks and Romans often bought slaves with salt.

“With all thine offerings thou shalt offer salt,” says Leviticus 2:13. Because of its use

as a preservative, salt became a token of permanence to the Jews of the Old Testament. Its use in Hebrew sacrifices as a meat purifier came to signify the eternal

covenant between God and Israel. In Genesis 19:1-29, two angels of the Lord

command Lot, his wife and two daughters to flee the sinful city of Sodom without

ever looking back. When Lot’s wife cast a fleeting glance backward, she was

transformed into a pillar of salt. A Roman religious ritual in which grains of salt were placed on an eight-

day-old babe’s lips, is a precedent for the Roman Catholic baptismal ceremony in

which a morsel of salt is placed in the mouth of the child to ensure its allegorical

purification. In the Christian catechism, salt is still a metaphor for the grace and

wisdom of Christ. When Matthew says, “Ye are the salt of the earth” he is

addressing the worthy sheep in the flock, not the erring goats.

During the Middle Ages, the ancient sanctity of salt slid toward superstition. The spilling of salt was considered ominous, a portent of doom. (In Leonardo da Vinci’s painting The Last Supper, the scowling Judas is shown with an overturned saltcellar in front of him.) After spilling salt, the spiller had to cast a pinch of it over his left shoulder because the left side was thought to be sinister, a place where evil spirits tended to congregate.

The social symbolism of salt continued as late as the 18th century. The rank of guests at a banquet was gauged by where they sat in relation to a silver saltcellar on the table. The host and “distinguished” guests sat at the head of the table, “above the salt.” People who sat below the salt, farthest from the host, were regarded of little consequence.

Salt taxes either solidified or helped dissolve the power of governments. For centuries the French people were forced to buy all their salt from royal depots. The gabelle, or salt tax, was so high during the reign of Louis XVI that it became a major grievance and eventually helped ignite the French Revolution. Lavoisier was a powerful member of a number of  aristocratic  councils, and an administrator of the  Ferme générale , one of the most hated organizations of the  Ancien Régime  because of the profits it took at the expense of the state, the secrecy of the terms of its contracts, and the violence of its armed agents.  All of these political and economic activities enabled Lavoisier to fund his scientific research. At the height of the French Revolution, he was charged with tax fraud and selling adulterated  tobacco , and was  guillotined on the Place de la Concord, the largest square in Paris. Today, the American Embassy is on this square. In the Ambassador’s office, there is a remarkable portrait of Benjamin Franklin (who was in Lavoisier’s inner circle of friends), which (in my view) captures the essence of Franklin far more than the one by Duplessis in the National Portrait Gallery. As late as 1930, in protest against the high British tax on salt in India, Mahatma Gandhi led a mass pilgrimage of his followers to the seaside to make their own salt.

If the importance of a food to a society can be measured by the allusions to it in language and literature, then the significance of salt is virtually unrivaled. Nearly four pages of the Oxford English Dictionary are taken up by references to salt, more than any other food. For example, taking something with a “grain of salt” is a recipe for skepticism. In this Lecture, I will give three “recipes” for producing salt. The first is geological, the second is via chemical reaction, and the third depends on another “Molecule that Shaped the World,” water.

RECIPE #1. TECHTONIC APPROACH Alfred Wegener (1880-1930) was a German meteorologist and geophysicist. During his lifetime he was known primarily for his pioneering polar research in Greenland, and for his achievements in meteorology. Today he is most remembered as the originator of the theory of  Continental Drift . In 1912 he hypothesized that the continents are slowly drifting around the Earth (German: Kontinentalverschiebung). Wegener first thought of this idea by noticing that the different large landmasses of the Earth almost fit together like a jigsaw puzzle. The  continental shelf  of the Americas fits closely to Africa and Europe. Antarctica, Australia, India and Madagascar fit next to the tip of Southern Africa. Wegener drew together evidence from various fields to advance his theory that there had once been a giant continent, which he named "Urkontinent"  (German for "primal continent", analogous to the Greek " Pangaea ", meaning "All-Lands" or "All-Earth").  In particular, he analyzed both sides of the Atlantic Ocean for rock type, geological structures and fossils. He noticed that there was a significant similarity between matching sides of the continents, especially in  fossil plants .

Shown below are the world maps created by Wegener showing Pangaea and the continents drifting apart. Its spatial and temporal classification corresponds to his conception at that time, not to the later proven positions and geological epochs.

Wegener put forward his hypothesis in 1912, the same year that Rutherford was carrying out his “gold foil” experiments and developing his “Solar System” model of the atom. Both models when first reported were controversial and, in fact, Continental Drift was widely rejected by mainstream  geology  until the 1950s, when numerous discoveries such as paleomagnetism provided strong support for Continental Drift, thereby establishing a basis for today's model of  Plate Tectonics . PlateTectonics  (from the  ancient Greek : τεκτονικός, “pertaining to building”) is a  scientific theory  describing the large-scale motions, beginning 3.3 - 3.5 billion years ago, of seven or eight large plates (and the movements of a larger number of smaller plates) in the outermost shell of the Earth. The outermost shell is called the lithosphere. The above diagram shows the internal layering of the Earth, lithosphere above the asthenosphere. The asthenosphere is the highly viscous, mechanically weak and ductile region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 50-125 miles below the surface of the Earth. The outer core is about 1,400 miles thick, and it's made of an alloy (see Lecture 2) of iron (Fe) and nickel (Ni), along with small amounts of other dense elements like gold (Au), platinum (Pt), and uranium (U). Natural background radiation (see Lecture 16) in our everyday lives comes mainly from the radioisotopes of uranium, thorium and potassium and their decay products some of which, like radium and  radon  are intensely radioactive but occur in low concentrations. Most of these sources have been decreasing, via  radioactive decay  since the formation of the Earth (there is no significant amount currently being transported to the Earth from “outer space”). The present radioactivity activity on Earth from  uranium-238  is only half as much as it originally was because of its 4.5  billion  year half-life.   Potassium-40  (half-life 1.25 billion years) is only at about 8% of original activity. But during the time that humans have existed, the amount of radiation to which we have been exposed has decreased very little.

The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Black lines in the following figure denote plate boundaries. v

Where the plates meet, their relative motion determines the type of plate boundary, or fault: convergent ,  divergent , or  transform .  See later text. The San Andreas Fault is a continental  transform fault  that extends roughly 750 miles through  California .  It forms the  tectonic  boundary between the  Pacific Plate  and the  North American Plate , and its motion is formally described as a horizontal, right-lateral, strike-slip . In the satellite image below, arrows show the relative motion of the two plates.

The length of the San Andreas Fault  can be compared with two man-made structures: Hadrian’s Wall, a defensive fortification in Britain begun in 122 AD by the Romans, running a total of 73 miles, and the Great Wall of China, a  fortification  system built across the northern borders of  China  to protect and consolidate territories of the Chinese empire against nomadic tribes of the steppe. Begun in the 7th century BC, the wall runs 3889 miles along an arc that roughly delineates the edge of Mongolian steppe. Seeing both should be on your “to do” list.

Importantly, mountain building, earthquakes ,  volcanic activity , and trench formation occur along plate boundaries. The relative movement of the plates typically ranges from zero to ~ 3.9 inches annually. Tectonic plates are comprised of oceanic and thicker continental lithospheric crust . Along convergent boundaries,  subduction , or one plate moving under another, carries the lower one down into the  mantle . The material lost is roughly balanced by the formation of new (oceanic) crust along divergent seams by seafloor spreading. In this way, the total surface of the lithosphere remains the same. This aspect of plate tectonics is referred to as the conveyor belt principle. Tectonic plates are able to move because the Earth's lithosphere has greater  mechanical strength  density than the underlying asthenosphere asthenosphere . See above. Lateral density variations in the mantle result in convection convection , the movement caused by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity. Plate movement is thought to be driven by a combination of the motion of the seafloor away from  spreading ridges  “ridges” due to variations topographyand  density changes in the crust (density increases as newly formed crust cools and moves away from the ridge). Tsubduction zoneshe relatively cold, dense oceanic crust is "pulled" down or sinks down into the mantle over the downward convecting part of the mantle.

What is a volcano? Earth's volcanoes occur because its crust is broken into rigid tectonic plates that “float” on a hotter, softer layer in its mantle.

A volcano is a rupture in the crust of the lithosphere that allows hot lava, volcanic ash, and gases to escape from a chamber below the surface of the Earth containing magma, the molten or semi-molten natural material from which all igneous rocks are formed (See Lecture 7).

Stratovolcanoes tend to form at subduction zones, or convergent plate margins, where an oceanic plate slides beneath a continental plate and contributes to the rise of magma to the surface. At rift zones, or divergent margins, shield volcanoes tend to form as two oceanic plates pull slowly apart and magma effuses upward through the gap. Volcanoes are not generally found at strike-slip zones, where two plates slide laterally past each other. “ Hot spot” volcanoes may form where plumes of lava rise from deep within the mantle to the Earth's crust, far from any plate margins.

A map showing the divergent plate boundaries (oceanic spreading ridges) and submerged volcanoes is below.

Large, explosive volcanic eruptions inject water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock and  pumice ) into the  stratosphere  to heights of 10-20 miles above the Earth's surface. The ash from volcanic eruptions, plentiful in minerals, enrich the soil after settling to the Earth, resulting in the formation of extensive grasslands in Africa and North America, and ideal conditions for growing Kona coffee beans on Hawaii's Kona Coast. A significant consequence of injections of gases into the atmosphere is the conversion of sulfur dioxide to   sulfuric acid  (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols . See later text. Significant deposits of sulfur around a volcano are common in the Earth’s crust and upper mantle. Sulfur, once known as the biblical “brimstone”, is show below being extracted from the Ljen volcano site in Indonesia. Sulfur readily comes to the surface because it is a relatively light element. The reason that it is in gaseous form is that the melting temperature of S is 115 oC and the vaporization temperature is 444 oC. So, sulfur is released as a gas by lava that is typically between 600 and 1200 oC , a temperature (much) greater than its boiling point.

What is an earthquake? An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the  Earth 's  lithosphere  that creates  seismic waves . Earthquakes can range in size from those that are so weak that they can hardly be felt to those violent enough to propel objects and people into the air, and wreak destruction across entire cities. Seismic activity of an area is a measure of the frequency, type, and size of earthquakes experienced over a period of time. The Richter scale is a numerical scale for expressing the magnitude of an earthquake on the basis of seismograph oscillations. The more destructive earthquakes typically have magnitudes between about 5.5 and 8.9. The scale is logarithmic and a difference of one represents an approximate thirtyfold difference in magnitude.

At the Earth's surface, earthquakes manifest themselves by shaking, displacing or disrupting the ground. When the  epicenter  of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a  tsunami . Earthquakes can also trigger  landslides  and occasionally, volcanic activity.

A tsunami is a series of waves (see Lecture 13) in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explosions above or below water all have the potential to generate a tsunami. 

The  Ancient Greek  historian  Thucydides  suggested in his 5th century BC  History of the Peloponnesian War  that tsunamis were related to  submarine earthquakes . Following from the above, the energy released by the sudden displacement of two tectonic plates is imparted to water and the energy is transmitted by monstrous waves. The  Fukushima Daiichi nuclear disaster  in Japan in 2011, mentioned in Lecture 16, was a consequence of both an “on land” earthquake and a “submarine” earthquake.

How are mountains formed? Mountain formation refers to geological processes that underlie the formation of of  mountains . These processes are associated with the large-scale movement of the Earth's crust ( tectonic plates ).  Folding ,  faulting ,  volcanic activity ,  igneous intrusion and intrusion  and  metamorphism  can all be parts of the  process  of mountain building, called orogeny. There are three ways in which mountains are formed, which correspond to the types of mountains in question. These are known as volcanic, fold and block mountains. All of these are the result of plate tectonics, where compressional forces, isostatic uplift and intrusion of igneous matter force surface rock upward, creating a landform higher than the surrounding features. Over the course of many millions of years, these uplifted sections are eroded by wind, rain, ice and gravity. These forces gradually wear down the surface of mountains, causing the surface to be younger than the rocks that form them, and lead to distinctive formations. Below is The East side of the Matterhorn in Switzerland, a fold mountain that measures 4,478 meters in height. Worth a trip.

Volcanic mountains are formed when a tectonic plate is pushed beneath another (or above a mid-ocean ridge or hotspot) and magma is forced to the surface. When the magma reaches the surface, it often builds a volcanic mountain, such as a shield volcano or a stratovolcano. See earlier text. Examples of this sort of mountains include Mount Fuji in Japan, Mauna Kea in Hawaii, Nyamuragira in the Democratic Republic of Congo, Skjaldbreiður in Iceland  and Mount Etna in Sicily.

At other times, the rising magma solidifies below the surface and forms dome mountains, where material is pushed up from the force of the build-up beneath it. Examples of this formation include Navajo Mountain in San Juan County, Utah; the Chaitén lava dome of Chile, Torfajökull in Iceland, and Mount St. Helens in Washington State.

Fold mountains occur when two tectonic plates collide at a convergent plate boundary, causing the crust to thicken. This process forces the less dense crust to float on top of the denser mantle rocks, with material being forced upwards to form hills, plateaus or mountains, while a greater volume of material is forced downward into the mantle.

The Jura Mountains, a series of sub-parallel mountain ridges located in the Alps, are an example of fold mountains, e.g., the Zagros mountains, which extend from northern Syria and southern Turkey to eastern Iran and the Persian Gulf, the Akwapim-Togo ranges in Ghana and the Appalachians in eastern United States.

Perhaps most famous is the Himalayan mountain chain, located between northern India and Nepal. This chain formed as a result of the collision between the Indian subcontinent and Asia some 25 million years ago, and has given rise to the tallest mountain in the world, Mt. Everest at 29,029 feet . Satellite image of the Himalayan mountain chain, as imaged by NASA’s Landsat-7 satellite. Credit: NASA

Block mountains are caused by faults in the crust, a seam where rocks can move past each other. Also known as rifting, this process occurs when rocks on one side of a fault rise relative to the other. The uplifted blocks become block mountains (also known as horsts) while the intervening dropped blocks are known as graben (depressed regions).

Examples of this type of terrain can be found in the Upper Rhine valley, the Vosges mountains in France, the Black Forest in Germany, and the Vindhya and Satpura horsts in India. The East African Rift is an active continental rift zone with several active volcanoes that extends from Eritrea to Mozambique. See below.

Satellite image of the East African Rift, taken on December 18th, 2002. Credit: NASA/GSFC/METI/Japan Space Systems/U.S.-Japan ASTER Science Team As noted above, the way in which mountains are shaped over time is by erosion. This occurs during and after an uplift, where a newly formed mountainous region is subjected to the effects of wind, water, ice, and gravity. These forces actively shape the surface of mountain ranges, wearing down the exposed surfaces, depositing sediment in alluvial flows, and lead to the formation of distinctive landforms. Owing to erosion, “younger” mountain ranges (e.g. the Rockies) are taller than “older” mountain ranges (e.g., the Appalachians).

Salt in the ocean comes from two sources: runoff from the land (hills and mountains) and openings in the seafloor.

Rocks on land are the major source of salts dissolved in seawater. Rainwater that falls on land is slightly acidic, so it erodes rocks. This releases ions that are carried away to streams and rivers that eventually feed into the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not removed, so their concentrations increases over time.

Another source of salts in the ocean is hydrothermal fluids, which come from  vents in the seafloor . Ocean water seeps into cracks in the seafloor and is heated by magma from the Earth’s core. The heat causes a series of chemical reactions. The water tends to lose oxygen, magnesium, and sulfates, and pick up metals such as iron, zinc, and copper from surrounding rocks. The heated water is released through vents in the seafloor, carrying the metals with it. Some ocean salts come from underwater volcanic eruptions, which directly release minerals into the ocean.

Two of the most prevalent ions in seawater are sodium (Na+) and chloride (Cl−). Together, they make up around 85 percent of all dissolved ions in the ocean. Magnesium ions (Mg+2) and sulfate ions (SO₄ -2 ) make up another 10 percent of the total. Other ions are found in very small concentrations.

The concentration of salt in seawater (salinity)  varies with temperature, evaporation, and precipitation . Salinity is generally low at the equator and at the poles, and high at mid-latitudes. The average salinity is about 35 parts per thousand. Stated in another way, about 3.5 percent of the weight of seawater comes from the dissolved salts.

Are humans made of salt water?

The human body contains many salts, of which NaCl is the major one, making up around 0.4 per cent of the body's weight at a concentration pretty much equivalent to that in seawater. This is not accidental. Life evolved in pools of saline water. Below is the Grand Prismatic Spring  in Yellowstone National Park in which simple microorganisms thrive and give the Spring its technicolor.

The Midway geyser basin is in the background. A geyser is a  spring  characterized by intermittent discharge of water ejected turbulently and accompanied by steam. The formation of geysers is due to particular  hydrogeological  conditions that exist only in a few places on Earth. Generally all geyser field sites are located near active  volcanic  areas, and the geyser effect is due to the proximity of  magma . Surface water works its way down to an average depth of around 6,600 feet where it contacts hot rocks. The resultant boiling of the pressurized water results in the geyser effect of hot water and steam spraying out of the geyser's surface vent (a  hydrothermal explosion ). The most famous geyser in Yellowstone [which has more geysers (~500) than any place on Earth] is “old Faithful.”

The  Yellowstone Caldera  is the largest volcanic system in North America and is only rivalled by the  Lake Toba Caldera  on  Sumatra . It has been termed a " supervolcano " because the caldera was formed by exceptionally large explosive eruptions. The  magma chamber  of the volcano that lies under Yellowstone is estimated to be a single connected chamber, about 37 miles long, 18 miles wide, and 3 to 7 miles deep. The caldera today was created by a cataclysmic eruption that occurred ≈ 640,000 years ago, which released more than 240 cubic miles of ash, rock and  pyroclastic  materials. This eruption was more than 1,000 times larger than Mount St. Helens, an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest. Mount St. Helens produced a caldera nearly 5/8 of a mile deep and 45 by 28 miles in area. The most violent known eruption of Yellowstone, which occurred 2.1 million years ago, ejected 588 cubic miles of volcanic material. A smaller eruption ejected 67 cubic miles of material 1.3 million years ago. Each of the three climactic eruptions released vast amounts of ash that blanketed much of central North America, falling many hundreds of miles away. The amount of ash and gases released into the atmosphere probably caused significant impacts to world weather patterns and led to the  extinction  of some species, primarily in North America. Geologically then, formation of the mineral Halite took millions of years of strenuous tectonic activity over which time saltwater from sea beds was subjected to the combined stresses of pressure and temperature as layer upon layer were deposited.

RECIPE #2. CLASSICAL EXOTHERMIC REACTION The “poster child” for exothermic reactions is undoubtedly the reaction of solid sodium (Na) with gaseous, diatomic chlorine gas (Cl2). Sodium is a soft metal that must be stored in mineral oil to prevent reaction with air or water (a). Chlorine is a pale, yellow-green gas (b). When combined, they form white crystals of sodium chloride (table salt, c).  See below.

The above shows the formation of sodium chloride from sodium and chloride ions. The reaction is represented with Lewis dot symbols below.

In the reaction, immense amount of energy in the form of heat and light are given off.

RECIPE #3. WATER CHEMISTRY Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white, solid, ionic compound consisting of sodium cations Na⁺ and hydroxide anions OH⁻ . Hydrochloric acid, also known as muriatic acid, is a colorless  inorganic  chemical system with the formula HCl consisting of an aquous solution of hydrogen cations (H+) and chlorine anions (Cl−). Hydrochloric acid has a distinctive  pungent  smell.  When sodium hydroxide is dissolved in an aqueous solution of hydrochloric acid, the following reaction takes place: NaOH + HCl → H2O + NaCl

Or, more precisely,

Na+(aq) + OH –(aq) + H+(aq) + Cl− (aq) → H2O(l) + NaCl(s) where aq denotes the aqueous phase, l the liquid phase and s the solid phase.

An acid–base (or neutralization) reaction is a  chemical reaction  that occurs between an  acid  and a  base .

The first scientific concept of acids and bases was provided by  Lavoisier   around 1776. Since Lavoisier's knowledge of  strong acids  was mainly restricted to  oxoacids , such as nitric acid ( HNO3  ) and sulfuric acid ( H2SO4 ), which contain central atoms in high  oxidation states  surrounded by oxygen, and since he was not aware of the composition of the acids HF ,  HCl ,  HBr , and  HI , he defined acids in terms of their containing  oxygen , which he named from Greek words meaning "acid-former" (from the  Greek  οξυς (oxys) meaning "acid" or "sharp" and γεινομαι (geinomai) meaning "engender").

The Lavoisier definition held for over 30 years (until 1810) when Humphry Davy described his experiments on the lack of oxygen in  H2S ,  H2Te , HF ,  HCl ,  HBr , and  HI . Davy did not advance a new theory, however, concluding that "acidity does not depend upon any particular elementary substance, but upon peculiar arrangement of various substances".  

One notable modification of oxygen theory was provided by  Jöns Berzelius , who stated that acids are oxides of nonmetals while bases are oxides of metals.

In 1838,  Justus von Liebig  proposed that an acid is a hydrogen-containing compound whose hydrogen can be replaced by a metal. This redefinition was based on his extensive work on the chemical composition of  organic acids , shifting the emphasis from oxygen-based acids to hydrogen-based acids. Liebig's definition, while completely empirical, remained in use for almost 50 years. The first modern definition of acids and bases in molecular terms was devised by  Svante Arrhenius (1859-1927), a  Swedish  chemist.  Recall from Lecture 4 that Arrhenius in 1884 proposed that, even in the absence of an electric current, solutions of salts contained ions. That is, he proposed that solid crystalline solids, when dissolved in water, dissociated into pairs of charged particles (ions), and that chemical reactions in solution involved reactions between ions. A hydrogen theory of acids followed from his 1884 work with  Friedrich Wilhelm Ostwald  in which they established the presence of ions in  aqueous solution . The Nobel Prize in Chemistry was awarded to Svante August Arrhenius in 1903 "in recognition of the extraordinary services he has rendered to the advancement of chemistry by his electrolytic theory of dissociation”, the same year Henri Becquerel, Pierre Curie and Marie Curie (née Sklodowska)  received the Nobel Prize in Physics.

An Arrhenius acid is a substance that dissociates in water to form hydrogen ions (H+). That is, an acid increases the concentration of H ions in aqueous solution.

This causes the  protonation  of water, or the creation of the  hydronium   ion (H3O+) ion, also called the oxonium ion.  Today, the symbol H+ is interpreted as a shorthand for H3O+, because it is now known that a bare proton does not exist as a free species in aqueous solution.

An Arrhenius base is a substance that dissociates in water to form hydroxide ions (OH−), that is, a base increases the concentration of OH− ions in an aqueous solution.

The Arrhenius definitions of  acidity  and  alkalinity  are restricted to aqueous solutions, and refer to the concentration of the solvent ions. Under this definition, H2SO4 and HCl dissolved in toluene are not acidic, and molten NaOH and solutions of calcium amide in liquid ammonia are not alkaline. This led to the development of the Bronsted-Lowry theory and subsequent Lewis theory to account for these  non-aqueous  exceptions. See text below.

Overall, to qualify as an Arrhenius acid, upon the introduction to water, the chemical must either cause, directly or otherwise:

an increase in the aqueous hydronium concentration, or a decrease in the aqueous hydroxide concentration.

Conversely, to qualify as an Arrhenius base, upon the introduction to water, the chemical must either cause, directly or otherwise:

a decrease in the aqueous hydronium concentration, or an increase in the aqueous hydroxide concentration.

The reaction of an acid with a base is called a  neutralization  reaction. The products of this reaction are always a  salt  and water, regardless of which acid or which base are involved in the neutralization reaction.

acid + base → salt + water

In this representation, an acid–base neutralization reaction is characterized as a  double-replacement reaction . For example, the reaction of  hydrochloric acid , HCl, with  sodium hydroxide , NaOH, solutions produces a solution of  sodium chloride , NaCl, and some additional water molecules.

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

Though all three substances, HCl, NaOH and NaCl, are capable of existing as pure compounds, in  aqueous solutions  they are fully dissociated into the aquated ions H+, Cl−, Na+ and OH−, hence the notation HCl (aq) and NaOH (aq). Following the discovery of the proton by Rutherford in 1919, a new, more generalized definition of acids and bases was proposed in 1923 almost simultaneously by  J.M. Brønsted  and  T.M. Lowry in order to resolve the various difficulties in the hydrogen–hydroxide ion definitions of acids and bases. The  Brønsted–Lowry definition  of acids and bases has had far-reaching consequences in understanding a wide range of phenomena and in the stimulation of much experimental work.

The definition is as follows: an  acid  is a species having a tendency to lose a  proton , and a  base  is a species having a tendency to gain a proton. The term proton means the species H+ (the nucleus of the hydrogen atom) rather than the actual hydrogen ions that occur in various solutions. The definition is thus independent of the nature of the particular solvent. The use of the word species rather than substance or molecule implies that the terms acid and base are not restricted to uncharged molecules but can also apply also to positively or negatively charged ions. This extension, one of the important features of the Brønsted–Lowry definition, can be summarized by the equation : A ⇄ B + H+ in which A and B together are a  conjugate acid–base pair . In such a pair A must have one or more positive charges (or, one or more less negative charges) than B, but there is no other restriction on the sign or magnitude of the charges.

Several examples of conjugate acid–base pairs are given in the table.

Examples of conjugate acid-base pairs
acid	base
acetic acid, CH3CO2H	acetate ion, CH3CO2−
bisulfate ion, HSO4−	sulfate ion, SO42−
ammonium ion, NH4+	ammonia, NH3
ammonia, NH3	amide ion, NH2−
water, H2O	hydroxide ion, OH−
hydronium ion, H3O+	water, H2O

A number of points about the Brønsted–Lowry definition need to be stressed:

1. As mentioned above, this definition is independent of the solvent. The ions derived from the solvent (H3 O + and OH− in water and NH4+ and NH2− in liquid ammonia) are not accorded any special status but appear as examples of acids or bases in terms of the general definition. 2. In addition to the familiar molecular acids, two classes of ionic acids emerge from the new definition. The first  comprises  anions derived from acids containing more than one acidic hydrogen, e.g., the bisulfate ion (HSO4−) and primary and secondary phosphate ions derived from  phosphoric acid  (H3PO4).

A second and more interesting class consists of positively charged ions ( cations ), such as the  ammonium ion  (NH4+), which can be derived by the addition of a proton to a molecular base, in this case  ammonia  (NH3). The hydronium ion (H3O+), which is the  hydrogen ion  in aqueous solution, also belongs to this class. The charge of these ionic acids must always be balanced by ions of opposite charges, but these oppositely charged ions usually are irrelevant to the acid–base properties of the system. For example, if sodium bisulfate (Na+HSO4−) or  ammonium chloride  (NH4+Cl−) is used as an acid, the sodium ion (Na+) and the chloride ion (Cl−) contribute nothing to the acidic properties and could equally well be replaced by other ions, such as potassium (K+) and perchlorate (ClO4−), respectively.

3. Molecules such as ammonia and organic amines are bases by virtue of their tendency to accept a proton. With metallic hydroxides such as sodium  hydroxide , on the other hand, the basic properties are due to the hydroxide ion itself, the sodium ion serving merely to preserve electrical neutrality. Moreover, not only the hydroxide ion but also the anions of other weak acids (for example, the acetate ion) must be classed as bases because of their tendency to form an acid by accepting a proton. Formally, the anion of any acid might be regarded as a base, but for the anion of a very strong acid (the chloride ion, for example) the tendency to accept a proton is so weak that its basic properties are insignificant and it is inappropriate to describe it as a base.

Similarly, all hydrogen  compounds  could formally be defined as acids, but in many of them (for example, most hydrocarbons, such as  methane (CH4 ) the tendency to lose a proton is so small that the term acid would not normally be applied to them.

4. Some species, including molecules as well as ions, possess both acidic and basic properties. Such materials are said to be amphoteric . Both water and ammonia are amphoteric, a situation that can be represented for water by the following equation: H2O + H2O ⇌ H3O+ + OH−

This equation is demonstrated in the images below:

Here, one molecule of water acts as an acid, donating an H+ and forming the conjugate base, OH−, and a second molecule of water acts as a base, accepting the H+ ion and forming the conjugate acid, the hydronium or oxonium ion, H3O+. The amphoteric properties of water are particularly important in determining its properties as a solvent for acid–base reactions.

Another example is the secondary phosphate ion, HPO42−, which can either lose or accept a proton, according to the following equations: HPO42− ⇄ PO43− + H+ and HPO42− + H+ ⇄ H2PO4−.

5. The equation A ⇄ B + H+, used in the Brønsted–Lowry definition, does not represent a reaction that can be observed in practice, since the free proton, H+, can be observed only in gaseous systems at low pressures. In solution, the proton always is attached to some other species, commonly a solvent molecule. Thus in water the ion H3O+ consists of a proton bound to a water molecule. For this reason all observable acid–base reactions in solution are combined in pairs, with the result that they are of the form A1 + B2 ⇄ B1 + A2. The fact that the process A ⇄ B + H+ cannot be observed does not imply any serious inadequacy of the definition. As we shall see in Lecture 18, a similar situation exists with the definitions of  oxidizing  and  reducing agents , which are defined respectively as species having a tendency to gain or lose electrons, even though one of these reactions never occurs alone and free electrons are never detected in solution (any more than free protons are). The dissociation of acids and bases is characterized by a defined signature, the pH. In  Chemistry , pH is a scale used to specify how  acidic  or  basic  (or alkaline) an aqueous  solution  is. Mathematically, is defined as the negative  logarithmic  value (base 10) of the Hydrogen ion concentration [H+],

pH = − log10 [H+]

Following from this definition, a lower pH indicates a higher concentration of hydrogen ions. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature (25 °C or 77 °F), amphoteric  pure water  is neither acidic nor basic and has a pH of 7. At 25 °C, solutions with a pH less than 7 are acidic, and solutions with a pH greater than 7 are basic. This neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. The pH of some common substances follows:

Given below are some representative examples of acid-base reactions.

A prime example of acid-base chemistry is stomach acid. The pH of the stomach juice generally lies within a range of pH 1.0-2.5. Stomach acid alters the natural folded shapes of protein molecules, allowing them to be broken down by digestive enzymes.

Though stomach acid is extremely useful in this manner, it can also be harmful if unregulated, since it can destroy the protein molecules in the stomach tissue itself. To prevent this from happening, the interior of the stomach is coated with a layer of cells known as gastric mucosa, which insulates the stomach wall from acidic gastric juices. Cells beneath the gastric mucosa are activated via stimuli of taste, smell and histamine (a type of signaling molecule) that results in parietal cells releasing HCl into the stomach. Conditions such as hyperacidity, where there is excessive amounts of acid secreted into the stomach, and peptic ulcers, which are sores resulting from bacterial infections, can be regulated by medications that block histamine from signaling the parietal cells. Some common ingredients used in these medications include cimetidine, famotidine, and ranitidine.

Another example of acid-base reactions is the effect of pH on DNA. The formation of DNA occurs readily at a pH of 7. Altering the pH level of a solution containing the double-helical DNA can destabilize the DNA double helix. In a solution with double-helical DNA and a concentrated base (such as OH-), the DNA will begin to dissociate into its corresponding single strands when pH approaches 9.0. This is a result of the hydroxide ions (OH− ) and their interaction with DNA base pairs, removing specific protons. Similarly, when the pH of this solution drops too low (below 5.0), the DNA double helix is destabilized. This is because some of the hydrogen bond acceptors become protonated and can no longer participate in hydrogen bonding, so the double helix separates. Both examples show how altering the pH of DNA can disrupt its double-helical structure.

In the above text I described the weathering and erosion of mountain landscapes. Weathering  is not the same as  erosion , although they are sometimes confused. Weathering processes do not involve the transport of matter. Weathering is the breaking up of rock into small pieces. There are two types of weathering

physical or mechanical

chemical

Physical weathering has taken place in limestone landscapes ,such as the Pennines of Yorkshire, England. An example is “freeze–thaw” where water soaks into small fissures and cracks and expands when it freezes in the winter, physically breaking up the limestone. BTW, this is the origin of “potholes” in some Chicago streets.

Chemical weathering involves the decomposition of rocks due to chemical reactions between minerals such as calcite with water and gases in the atmosphere [e.g. carbon dioxide (CO2) and sulfur dioxide (SO2)]. Chemical weathering is the most important way that limestones are broken down.

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3) The Mohs scale of mineral hardness, based on scratch hardness comparison, gives the value 3 for "calcite" (Diamond is 10).  The solution of soluble minerals is particularly important in limestone landscapes.

Granite chemically weathers to form china clay.

Chemical weathering can be caused by rain water.

Rain has a major impact on  karst  scenery through chemical weathering.

“Ordinary” rain is naturally acidic because it contains dissolved carbon dioxide that forms weak carbonic acid (H2CO3). When this weak acid (pH ~ 5.5) comes into contact with calcite, the limestone begins to dissolve. The sequence of events follows:

1. Droplets of rain water (H2O) in the clouds dissolve carbon dioxide (CO2) in the atmosphere.

2. When combined, these form carbonic acid (H2CO3).

3. The slightly acidic rain then falls onto the ground.

4. The rain soaks into the soil or flows over the exposed limestone (CaCO3).

It may become even more acidic if it soaks into soil where there are naturally occurring acids from plant material or minerals such as pyrite. Also known as fool's gold, pyrite is an iron sulfide with the chemical formula FeS2. See Lecture 2.

An acid-base reaction takes place when the rain (i.e. carbonic acid) interacts with limestone (i.e. calcite).

The acid H2CO3 and the CaCO3 combine to form two ions of HCO3-1 and Ca+2. The calcite is converted to calcium bicarbonate, Ca(HCO3)2, which is soluble in water and is washed away by the rain. Fifteen million years ago, the Earth entered a phase of slow and continuous cooling. A current theory for the cooling was the formation of major mountain ranges, like the Himalayas. According to this theory, when the Indian and Asian tectonic plates collided, it brought fresh rocks up to the surface. The new rocks were more vulnerable to weathering via carbonic acid (H2CO3) as described above. Soluble carbonates were washed down the mountain side to the sea and sediments began to accumulate, thus capturing and storing CO2. Carbon dioxide is a “greenhouse gas,” so its removal from the atmosphere caused a gradual cooling of the Earth. The long-term change in the temperature of the Earth was significant. See the panel in the lower right-hand corner of the figure below:

Climate change during the last 65 million years.

Chemical weathering can also be caused by acid rain.

The problem of acid rain began with the Industrial Revolution (from about 1760 to 1840) and increased in the 20th century. Pollutants were created which escaped into the atmosphere and were dissolved in rain water. The main pollutant is sulfur dioxide (SO2), but nitrogen oxides are also present.

In simple terms, acid rain is a weak sulfuric acid (H2SO4), and this is the most significant cause of chemical weathering. Following is the sequence of events:

1. Rain water (H2O) in clouds dissolves some of the SO2, a pollutant from industrial manufacturing.

2. This makes weak sulfuric acid (H2SO4).

3. The acidic rain then falls on to the ground.

4. The rain soaks into the soil and porous limestone deposits and wets rocks containing the mineral calcite (CaCO3).

5. An acid-base reaction takes place. Acid rain (H2O, with a % of H2SO4) and limestone (CaCO3) react, causing the formation of water (H2O), carbon dioxide (CO2) and soluble calcium sulfate (CaSO4).

6. The water is added to the rain and is lost when it soaks away or evaporates, the carbon dioxide is lost to the atmosphere and calcium sulfate is lost when it dissolves in water and is washed away.

Chemical weathering in limestone areas causes special topographical solution features to form, known as  karst . Karst features include limestone pavement,  sinkholes (dolines) and fissures in limestone called clints, and groves in the limestone called  karren .

Limestone pavement, Malham Cove, North Yorkshire, England. The clints, in the foreground, are formed by the solvent action of rainwater on joints in the limestone. © NERC P005457

A stream flows into a sinkhole in the limestone in Cumbria, England. Limestone rocks dissolve when attacked by rainfall or groundwater that is acidic. © NERC P005101

Karren grooves on clint surfaces between joint controlled grikes; near Orton, Penrith. © NERC P005458

Weathering and erosion cause the disfigurement of statues exposed to the elements. The statues on Easter Island,

are eroding, principally due to weathering. Statues outdoors, subject to the combined effects of weathering and chemical erosion, deteriorate significantly.

When I first visited Venice, Italy in the 1960s, this deterioration was already very striking, caused by pollutants (CO2, SO2) released into the atmosphere from nearby coal-powered plants.