Volume 1
1.12. ACIDS AND BASES

Acids. General Information • Nomenclature of Acids • Sulfuric Acid. Dehydration • Hydrochloric Acid. Mass Percentage Concentration • Nitric Acid • Phosphoric Acid • Carbonic Acid • Bases. General Information and Nomenclature • Sodium Hydroxide (Caustic Soda), NaOH, and Potassium Hydroxide (Caustic Potash), KOH • Calcium Hydroxide (Slaked Lime), Ca(OH)2 • Insoluble Bases • Is This an Acid or a Base? Acid-Base Indicators • Exercises
1.12.1. Acids. General Information. Acids have a characteristic sour taste. Vinegar is sour because it contains acetic acid. A lemon is sour because it contains citric acid. Yogurt is sour due to the presence of lactic acid. Small quantities of phosphoric acid are added to soft drinks to give them the unique slightly sour, tangy taste. Ants produce formic acid, which is also sour. If placed on an ant hill and kept there for a few minutes, a blade of grass will taste sour from the tiny quantities of formic acid produced by the ants.

An acid contains a mobile hydrogen atom that can be displaced by a metal atom. (The word "mobile" is conventionally used to emphasize the ability of a hydrogen atom on the molecule of an acid to be displaced by a metal atom.) We have already learned that sulfuric acid (H2SO4) reacts with zinc (Zn) and iron (Fe). In this reaction, the two mobile hydrogen atoms of H2SO4 are displaced by a zinc or iron atom.

Zn + H2SO4 = ZnSO4 + H2

Likewise, iron reacts with acetic acid, as described earlier (Experiment 9 in section 1.11). Note, however, that although there are four hydrogen atoms in the molecule of acetic acid (C2H4O2), only one of the four is mobile because only one of the four can be displaced by a metal atom. We will understand why later in the course.

So, a molecule of any acid consists of one or more mobile hydrogen atoms and the rest of the molecule. That rest of the molecule we will call, for the time being, the archaic name "acid remainder". A schematic representation of an acid is shown in Figure 1-40.
Figure 1-40. A simplistic general representation of an acid.


There are acids that contain two, three, and even more mobile hydrogen atoms. An atom in a molecule that can be easily displaced with another atom is often called mobile. Acids that have only one mobile hydrogen atom are called monobasic acids. Those with two and three mobile hydrogen atoms are called dibasic and tribasic acids, respectively (Figure 1-41). An acid remainder can comprise a single atom (HCl) or a group of atoms (H2SO4).
Figure 1-41. A schematic representation of mono-, di-, and tribasic acids with examples.


1.12.2. Nomenclature of Acids. A chemical nomenclature is a set of rules for naming chemical substances. The nomenclature of acids, especially of inorganic acids that we are currently studying, is rather simple: they all have common names (Table 2) that should be memorized.


Table 2. Names of selected acids and their acid remainders.
Below is a brief description of some of the most important acids that we will be studying in our course.

1.12.3. Sulfuric Acid. Dehydration. A molecule of sulfuric acid, H2SO4, has two hydrogen atoms connected to the acid remainder (SO4) and is therefore a dibasic acid. Sulfuric acid is possibly the most important commodity chemical that has numerous applications and is manufactured on a tremendous scale. It is sometimes said that the per capita volume of manufactured and consumed sulfuric acid is an indication of a country's state of industrial development. The largest consumers of sulfuric acid are the fertilizer, petrochemical, and chemical industries.

Sulfuric acid is a colorless and odorless liquid. The content of H2SO4 in commercial undiluted sulfuric acid, called concentrated sulfuric acid, ranges from 95% to nearly 100% by weight (the rest is water). Concentrated H2SO4 is viscous (has the consistency of oil), which explains its medieval name oil of vitriol. Unlike oil, however, sulfuric acid is very dense, d = 1.84 g/mL, nearly twice as dense as water.

Sulfuric acid is miscible with water in any proportion, has a very high affinity for water, and is hygroscopic. A substance is said to be hygroscopic if it takes up and retains moisture from the air. Concentrated sulfuric acid does not only that, but also avidly pulls water out of various inorganic and organic compounds and materials. The process of water removal from a substance is called dehydration. There are two types of dehydration. In one, the already available water molecules in a moist material are transferred to a dehydrating agent. In this case, the loss of water is a physical phenomenon. In the second type of dehydration, a dehydrating agent brings about a chemical reaction that generates water and absorbs the water produced. Concentrated sulfuric acid is capable of performing both types of dehydration. Watch this spectacular demonstration of the dehydration of sugar by concentrated sulfuric acid. The formula of sugar (sucrose) is C12H22O11; we will learn the structure of sucrose in the organic chemistry module of our course (Volume 4). For the time being, it suffices to say that there are no water molecules in sugar. Being "hungry" for water, sulfuric acid forces the molecules of sucrose to decompose to water and carbon among other products (Figure 1-42). The water produced is avidly absorbed by the H2SO4 used for the reaction.
Figure 1-42. Dehydration of sucrose (sugar) induced by concentrated sulfuric acid.
Digression. Sugar (sucrose) as well as fructose and glucose are carbohydrates. The old yet still broadly used name "carbohydrate" means "watered carbon" because nearly all of the compounds of this class fit the general formula Cn(H2O)m. For example, the formula of sucrose, C12H22O11, may be rewritten as C12(H2O)11 (n = 12; m = 11) and those of fructose and glucose, C6H12O6, as C6(H2O)6 (n = 6; m = 6). As mentioned above, however, there are no water molecules in the structures of carbohydrates.
The powerful dehydrating ability of concentrated sulfuric acid is one of the two key reasons for its extreme corrosiveness. The other key reason is the very high strength of H2SO4 as an acid. We will learn what the strength of acids and bases is in Volume 2.

Concentrated H2SO4 must be handled with utmost care because it burns through cloth and causes painful and slow-healing burns to the skin. Direct contact of concentrated H2SO4 with the eyes can result in permanent blindness. The necrosis of tissues caused by sulfuric acid has both chemical and thermal origins. As dehydration by concentrated sulfuric acid occurs, much heat is released, which exacerbates the chemical damage by the thermal burn of the same area.

Aqueous solutions of sulfuric acid for a broad variety of uses are prepared by dilution of concentrated H2SO4. Great care should be exercised when mixing concentrated sulfuric acid and water because the mixing generates a massive amount of heat. To dilute concentrated H2SO4, it is always the acid that should be slowly poured into the water at stirring. Doing it the other way around is likely to result in a bad accident. In this educational video, water is added to pure H2SO4 on purpose, to demonstrate the potential danger of the wrong order of addition. Note that toward the end of this very short demonstration, the temperature in the flask rises above the boiling point of water (100 oC). Imagine scalding-hot strong acid splattering around on one's clothing and skin, let alone eyes. In contrast, no splashing takes place if the acid is added to the water. A simple mnemonic for the correct order of addition, Acid To Water, has the first letters in alphabetical order, ATW. For the wrong order of addition, "Water To Acid", the first characters are in reverse alphabetical order, WTA.
Digression. Why does the order of addition (acid to water vs. water to acid) make such a big difference? The amount of heat produced on mixing a particular volume of H2SO4 with a particular volume of H2O is the same, no matter how the two are mixed. One explanation deals with the difference in density of concentrated sulfuric acid and water (see above). When water is added to much more dense H2SO4, the water stays on top of the acid for at least a short moment before the two are fully admixed. The heat instantly released on contact of the two is sufficient to make the upper layer of the small amount of water boil abruptly and violently, spitting and splashing around the hot acid solution. When poured into the water, the more dense sulfuric acid goes to the bottom and the heat released is dissipated over the large amount of water in the upper layer.
1.12.4. Hydrochloric Acid. Mass Percentage Concentration. Hydrochloric acid, also known as muriatic acid, is a solution of hydrogen chloride, HCl, in water. At ambient temperature and atmospheric pressure, HCl is a gas. Hydrogen chloride is easily soluble in water (Table 3). Note that like any gas (and unlike most solids and liquids), HCl has a higher solubility at lower temperatures.

Table 3. Solubility of HCl gas in water at 1 atmosphere.
Both hydrogen chloride and its concentrated aqueous solutions (hydrochloric acid) have a pungent smell and are highly corrosive. Concentrated hydrochloric acid is sometimes called fuming hydrochloric acid because it fumes in humid air to produce HCl mists. Such concentrated solutions of HCl can have strong damaging effects on eyes, respiratory organs, and mucous membranes. As surprising as it can be, however, HCl is the main component of stomach acid (also known as gastric juice and gastric acid), the fluid produced in our stomachs for the digestion of the food we eat. This is yet another good illustration of the famous statement that "the dose makes the poison".

Hydrochloric acid has many applications and is manufactured on a huge scale. Large quantities of HCl are used for steel pickling, to make various inorganic and organic chemicals, for rust removal, and in the food industry. The standard concentration of commercial concentrated hydrochloric acid is approximately 37%. What does this figure mean?

Percentage concentration shows the number of grams of a solute (dissolved substance) in 100 grams of solution. Therefore, to prepare 100 g of 37% solution of HCl we have to mix 37 g of pure HCl and 63 g of water. If it is unclear where the number 63 g comes from, the total amount of the solution that we need to prepare is 100 g. The required concentration of HCl is 37%, which means that 100 g of our solution should contain 37 g of HCl. The rest should be water. Consequently, 100 g (total mass of the solution) minus 37 g (mass of HCl) = 63 g, the amount of water to be mixed with 37 g of HCl gas to make 100 g of 37% HCl.

1.12.5. Nitric Acid. The formula of nitric acid is HNO3, and it is obviously a monobasic acid. Pure nitric acid is a colorless liquid. However, samples of highly concentrated HNO3 are usually yellow, orange, or even dark orange-brown in color because the acid slowly decomposes on standing. Among the decomposition products is NO2, an orange-brown gas that stays dissolved in the acid, thereby giving it the characteristic color. As the decomposition is induced by light, it is recommended that concentrated HNO3 be stored in bottles made of dark-brown glass.

Nitric acid is also a commodity chemical that is made on a very large scale and is indispensable in the production of fertilizers, dyes, explosives, and numerous useful chemicals, including pharmaceuticals and crop protection agents.

Concentrated HNO3 is highly corrosive and can cause strong chemical burns. Although diluted nitric acid is not nearly as hazardous, it often stains the skin yellow. The yellow color is due to the reaction of nitric acid with some proteins of the skin. This reaction, called the xanthoproteic reaction, belongs to the class of nitration reactions that we will consider in the organic chemistry course module (Volume 4).

1.12.6. Phosphoric Acid. There are many phosphoric acids, including ortho-phosphoric acid H3PO4, meta-phosphoric acid HPO3, and pyro-phosphoric acid H4P2O7. The most common and widely used one is H3PO4, ortho-phosphoric acid, often called just phosphoric acid. Phosphoric acid is a tribasic acid. Pure H3PO4 is a white crystalline solid. Commercial phosphoric acid, however, is conventionally 85%, which means that each 100 g of it contains 85 g of H3PO4 and 15 g of water. Such highly concentrated solutions of phosphoric acid are colorless and very viscous, like a very thick syrup.

As mentioned above, phosphoric acid is used as an additive to soft drinks. It is the tiny quantity of H3PO4 that is responsible for that unique and enjoyable tingling sensation on your tongue when you sip an ice cold soft drink on a hot day. There are many more applications of phosphoric acid, by far the most important one being in the production of fertilizers.

1.12.7. Carbonic Acid. We often consume carbonic acid, H2CO3, while not even thinking about it. The slightly sour taste of soft drinks is due to the carbonic acid that is produced upon carbonation of water, the process of saturation of water with carbon dioxide, CO2 (Figure 1-43).
Figure 1-43. Formation of carbonic acid from carbon dioxide and water.


The formation of carbonic acid from CO2 and H2O is reversible, meaning that it can proceed both forward and backward. In other words, while CO2 and H2O combine to give H2CO3, the H2CO3 produced in this reaction decomposes back to CO2 and H2O. We will discuss reversible reactions in Volume 2. For the time being, however, it suffices to say that reversible reactions may often be controlled, so that it is either the direct or the reverse process that prevails.

Carbonic acid cannot be isolated pure and exists only in dilute solutions. At room temperature and atmospheric pressure, H2CO3 in solution decomposes to CO2 and H2O quite rapidly. As the CO2 in carbonated water bubbles off into the air, the concentration of H2CO3 decreases. That is why sparkling (seltzer) water from a freshly opened bottle tastes more tangy than after standing for some time in an open glass.

1.12.8. Bases. General Information and Nomenclature. A base is a compound made up of a metal atom and one or more OH groups attached to it. The OH group is called "hydroxide". The valence of the metal sets the number of hydroxide (OH) groups on the molecule of a base (Figure 1-44). For example, since the valence of sodium (Na) is 1, the formula of the corresponding hydroxide is NaOH. As there is only one OH group attached to the Na atom, NaOH is a monobasic base. Calcium is divalent, so the formula of calcium hydroxide is Ca(OH)2, which makes it a dibasic base. The valence of iron (Fe), as we already know, can be 2 or 3. Consequently, iron can form two hydroxides, Fe(OH)2 and Fe(OH)3. Iron (II) hydroxide is a dibasic base and iron (III) hydroxide is a tribasic base. To name a base, we first name the metal and then say "hydroxide". For example, KOH is potassium hydroxide, Mg(OH)2 is magnesium hydroxide, etc.
Figure 1-44. A schematic representation of mono-, di-, and tribasic bases with examples.


Some bases are soluble in water and some are not. The ones that are soluble in water, particularly the most commonly and broadly used NaOH and KOH, are often called alkali. Water-soluble bases leave a soapy, slippery feel on the skin. Below is a brief overview of the most common bases.

1.12.9. Sodium Hydroxide (Caustic Soda or Lye), NaOH, and Potassium Hydroxide (Caustic Potash), KOH. Both NaOH and KOH, conventionally called alkali, are highly caustic, odorless white solids that are very easily soluble in water. The solubility of NaOH and KOH in 100 mL of water at room temperature is approximately 110 g and 120 g, respectively. Both NaOH and KOH are hygroscopic and should be stored in tightly closed bottle containers. Plastic (polyethylene or polypropylene) containers are preferred over glass ones for storing solid NaOH and KOH as well as their solutions because both slowly etch glass.

One should exercise care when handling NaOH, KOH, and their solutions, especially concentrated ones. Sodium and potassium hydroxides are highly corrosive to the skin and even more so to the mucous membranes and eyes. As the eye fluid is slightly acidic, NaOH and KOH may be even more damaging to the eye than strong acids.

Both NaOH and KOH are made and used on a large industrial scale. Their applications include the production of paper, soaps, detergents, cleaning agents, and various chemicals. In many applications where NaOH and KOH are interchangeable, the industry prefers the cheaper NaOH.

1.12.10. Calcium Hydroxide (Slaked Lime), Ca(OH)2. Calcium hydroxide is a white powder that is not nearly as strong a base and caustic as KOH and NaOH. The solubility of Ca(OH)2 in water is only about 0.17 g in 100 mL at ambient temperature. A saturated aqueous solution of calcium hydroxide, called limewater, has long been used as a test reagent for carbon dioxide. Passing a gas containing CO2 through limewater makes the solution cloudy due to the formation and precipitation of poorly soluble CaCO3 (Figure 1-45).
Figure 1-45. Limewater test for carbon dioxide (CO2).


Calcium hydroxide is a commodity chemical that is made on a large scale, mainly to produce cements for construction. The pulp and paper industry also uses considerable amounts of Ca(OH)2.

1.12.11. Insoluble Bases. There are a limited number of metal hydroxides that are soluble in water. In addition to NaOH, KOH, and Ca(OH)2, water-soluble bases include more exotic LiOH, RbOH, CsOH, Sr(OH)2, and Ba(OH)2. Most other metal hydroxides are insoluble in water, such as Cu(OH)2, Fe(OH)2, Fe(OH)3, Al(OH)3, and Zn(OH)2.

1.12.12. Is This an Acid or a Base? Acid-Base Indicators. It is often important, if not critical, to know whether a solution is acidic or basic. The simplest and fastest method to determine the presence of an acid or a base in solution is to use an acid-base indicator. The oldest and most popular one is litmus, a naturally occurring dye that is blue in the presence of a base but changes its color to red in the presence of an acid. Both red and blue litmus papers are commercially available. If placing a drop of a solution on a strip of blue litmus paper changes its color to red, the solution contains an acid. On the contrary, red litmus paper turns blue if dipped in an alkaline (basic) solution.

A fascinatingly sinister example of the use of litmus paper for a detective investigation is found in Arthur Conan Doyle's Sherlock Holmes story "The Adventure of the Naval Treaty", first published in 1893. At the beginning of the story, Dr. Watson steps in the home laboratory of his friend Sherlock Holmes, the fictional detective genius, to find him busy performing chemical experiments."You come at a crisis, Watson," said he. "If this paper remains blue, all is well. If it turns red, it means a man's life." He dipped it into the test-tube and it flushed at once into a dull, dirty crimson." (Figure 1-46).
igure 1-46. "You come at a crisis, Watson" (Illustration by Sidney Paget; source).

There are many acid-base indicators available, offering change of virtually any color you like to any other color you like. In this course, we should mention another one, phenolphthalein, a synthetic organic compound whose solutions are colorless in the presence of an acid but turn beautiful raspberry-red in the presence of a base.

1.12.13. Exercises.

1. Name the following acids and bases: HNO3, KOH, H3PO4, Ca(OH)2, H2SO4, HCl, NaOH, Al(OH)3, Fe(OH)2, Fe(OH)3. Answer

2. Write chemical formulas for the following acids and bases: magnesium hydroxide, hydrobromic acid, lithium hydroxide, carbonic acid, cobalt (II) hydroxide, silicic acid. Answer

3. You received a bottle of concentrated nitric acid from a supplier. The nitric acid inside the bottle is yellow in color. Knowing that pure nitric acid is colorless, should you return the bottle to the vendor for a refund? Answer

4. Your supervisor asked you to prepare 100 g of a 10% solution of carbonic acid from water and pure H2CO3. What should you do? Answer

5. A laboratory technician was asked to prepare a 20% solution of sulfuric acid in water from concentrated (100%) H2SO4. For that, he decided to mix 100 g of water and 20 g of concentrated sulfuric acid. Since it is easier to measure liquids by volume than by mass, and, knowing that the density of water is 1 g/mL, the technician placed exactly 100 mL of water in a graduated cylinder. Then he placed 20 mL of concentrated H2SO4 in another graduated cylinder. After that, he started adding the measured quantity of water to the sulfuric acid. Once the first portion of water was added, much heat release and a violent splattering occurred. Fortunately, the technician had a lab coat, rubber gloves, and goggles on, as otherwise he would have had hot acidic splashes all over his hands, face, and, possibly, in his eyes. After the hot liquid mixture cooled down, the technician decided to check if the solution was acidic. For that, he mixed a few drops of the resultant acid solution with a few drops of a phenolphthalein indicator solution. As the mixture remained colorless rather than turning raspberry red, the technician concluded that something went wrong and the acid had decomposed. Identify the mistakes made by the technician during this operation. Answer

6. To prepare a 15% solution of H2SO4 in water using 100% H2SO4, one should mix

(a) 15 mL of H2SO4 and 100 mL of water;

(b) 15 g of H2SO4 and 100 mL of water;

(c) 15 mL of H2SO4 and 100 g of water;

(d) 15 g of H2SO4 and 85 mL of water;

(e) 8.15 mL of H2SO4 and 100 g of water;

(f) 8.15 mL of H2SO4 and 85 mL of water.

Answer

7. There are two solutions, 50 g of a 10% solution of KCl and 50 g of a 10% solution of NaCl. When asked which of the two solutions contained the larger amount of water by mass, Student A said the NaCl solution, because a sodium atom is lighter than a potassium atom. Student B, however, argued that it was the KCl solution that contained more water because the number of molecules of KCl in the KCl solution was smaller than the number of molecules of NaCl in the NaCl solution. Who is right, Student A or Student B? Answer

8. Why are solid KOH and NaOH sold in the form of pellets (Figure 1-47) rather than powder? Answer
Figure 1-47. A commercial sample of NaOH (source).