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    The word allotrope just means different type or alternate type. A number of elements appear naturally in more than one form at room temperature. The allotropes are usually due to the crystal shape, or lack of crystal shape, or the attachments of more than one atom of an element.

    One of the easiest materials to see and play with in this respect is sulfur. Most pharmacies will have containers of flowers of sulfur. This dull yellow powdery element is not only non- poisonous, but it is ingestible. (You can EAT it!). Is there any good reason for eating it? Yes! A spoonful eaten an hour before going out into a pine forest will keep off the no-see-ums, or chiggers, as they are called in the southern US. The sulfur supposedly comes out of your body in your sweat and discourages the insects. It would likely not be a good idea to eat sulfur before a big date, unless your date ate some also. It doesn't smell very good. Don't eat any of the sulfur from your chemistry lab. No telling what other laboratory chemicals have contaminated it.

    Sulfur also has an allotrope that is easily seen. Into a test tube put an amount of flowers of sulfur that is a depth of about twice the diameter of the test tube. Heat this over a flame until the sulfur is all melted. The melting point of sulfur is just a few degrees above the boiling point of water. Pour out the melted sulfur onto a clean disposable piece of paper. (Careful. The liquid is hot and sticky. It will burn you if it gets on you.) The sulfur quickly cools into a rubbery mass. It is still sulfur. Very slowly as it sits, the rubbery mass will turn back into a crystalline material. There are two crystalline shapes of sulfur, a monoclinic and a rhombic along with this amorphous (without crystalline shape) form.

    Oxygen has two common allotropes. The familiar O2 is the common oxygen in the air. O3 is ozone, an allotrope of oxygen. The most common way for ozone to be made is from natural lightning. Ozone accounts for the "smell of rain" that occurs around lightning strikes. The difference between them is that the O2 molecule has two atoms of oxygen attached to each other and the O3 molecule has three atoms of oxygen attached to each other. Ozone is a higher energy form of free elemental oxygen. Generally after a while the ozone gradually changes back to O2 in the air.

    Carbon also has allotropes, but it is not so easy to change the carbon from one allotrope to the other. In fact, if you know of a good easy way to change carbon black into gem quality diamonds, tell me about it. The three allotropes of carbon are carbon black or lampblack, diamond, and graphite. Carbon black is unattached carbon atoms. The black coloring material in automobile tires, the blackening in shoe polish, etc. are all carbon black. The soot from an oxygen-starved flame has a lot of carbon black in it. Coal is mostly carbon black.

    Pencil lead at one time was made with the element lead, but no more. Graphite mixed with a very fine clay is molded into a thin cylinder and placed in a wooden splint to make most common pencils. You can see purer graphite in some locksmith's lubricant. As a naturally occurring mineral, graphite is a dark gray, greasy-feeling material. This greasiness comes from the shape of the carbon. In graphite the carbon is attached in large sheets of hexagonally bonded atoms. The sheets can easily move over one another, giving it that greasy feeling.

    The bonds of diamond are tetrahedral. Each atom in a diamond is linked to four other carbon atoms tetrahedrally in three-space. Since the bonds are strong and the atoms of carbon are small, diamond is very hard. It is a crystal, though. If you tap a diamond in just exactly the right place, it will cleave along the crystal face.

    Now that you know something about allotropes, is the change from one allotrope to the other a chemical change? Most chemists would say, "No," but there is a material made that has new properties. In the case of carbon or oxygen, there are even some new chemical bonds made, even if it is between all atoms of the same type.


    Ionic compounds, those attached by an ionic bond, come apart in water into the two ions. Sodium chloride is a good example of an ionic material. Most ionic materials, as sodium chloride, are in the form of crystals in solid form and in the form of ions in the dissolved form in aqueous solution. You have seen dissolving of sodium chloride before. A pinch of table salt in a cooking pot half full of water very quickly changes to the ions. How do we know the ions are in the solution? The solution of table salt (or any other dissolved ionic compound) can conduct an electric current by the movement of the ions. The equation for this process is:

    \[NaCl \rightarrow Na^+ + Cl^-\]

    The sodium ions are attracted to the negative end (the oxygen end) of the water molecules, and the chloride ions are attracted to the positive end (the hydrogen end) of the water molecule.

    Is this a genuine reaction or not? We say that a physical change is one in which there is no change in the chemical makeup of the reactant and the original reactant(s) may be reclaimed by a physical process. A chemical change is one in which there is a new material made.

    Solid sodium chloride is definitely chemically different from sodium ions-plus-chloride ions. The ions conduct electricity and can react in ionic reactions. On the other hand, the material can completely reclaimed by the process of evaporation of the water. This is commonly done with sea water to get the salt. Have bonds been broken? Yes, if you were to tag specific sodium ions (radioactive tagging) to go with other tagged chloride ions and dissolve them with non-tagged ions, you would find that upon reclaiming the solid material the tagged ions mixed with the untagged ions. The bonds of the crystal have detached. The ions are loose in solution and make bonds again as the crystal reappears. The ions do not return to any previously determined position. The crystal itself is only a conglomerate of ions in relative positions and not a solid made of discrete molecules.

    Some ionic materials, such as acetic acid, are only partly ionized in water. We know this from the way the acetate ion frees itself from the hydrogen ion. Acetic acid is a weak acid, that is, the material may be completely dissolved, but there is only evidence of a certain distinctive portion of the hydrogen ion.


    Biochemistry is the chemistry of living things. The number of fascinating chemical changes that go on inside any living thing is enormous. The variety of large, complex, extremely intricate systems in a living thing are mind-boggling. Death of a living thing is both a cessation of the biochemical processes and a large number of biochemical reactions itself. Even before the attack of the micro-organisms on a tree, the very stopping of the everyday reactions cause a new set of chemical changes to happen.

    Blue Vitriol

    Blue vitriol is an old common name for cupric sulfate pentahydrate. That name comes from the color, of course, which is a strikingly beautiful blue, and 'vitriol' from the glassy appearance of the crystals rather than any vituperative. (Run for that Webster's!) Blue vitriol is one of a number of hydrated crystals. These crystals, often sulfates or carbonates, attract and semi-attach a definite number of water molecules to each of the non-water parts to form a crystal lattice. A dot is usually used in the formula to represent the association of the water to the ionic part of the formula.

    As with most hydrated crystals, gentle heating will free the water from the copper sulfate. Upon heating, blue vitriol will change from the glassy blue (about the color of the title of this page) to a powdery light robin's egg blue, releasing water in the form of vapor*. The equation for this process is:

    \[ CuSO_4 \cdot 5H_2O \rightarrow CuSO_4 + 5H_2O\]

    The association of the water to the copper sulfate is in exactly one to five mole ratio. The dehydrated crystal is hygroscopic, that is, it fiercely grabs up water from the atmosphere to replace into the crystal. If you place a drop of liquid water onto the dehydrated crystal, it grabs up the water with a sizzling sound, produces a bit of heat, and changes to a color closer to the hydrated crystal.

    Is this a genuine reaction or not? The exchange of heat, the exact proportions of water and copper sulfate, and the change of properties make good arguments for it being a genuine reaction. On the other hand, the water stays water and the copper sulfate formula does not change. The process is reversible by simple physical process, either adding water or heating to drive off the water.

    *Copper sulfate is poisonous due to the copper ion. Copper and other "heavy metals" such as lead and mercury are a problem in cells of living things. Blue vitriol can present a problem in doing this little dehydrating experiment because larger crystals can explode from the heat and spray copper sulfate all around. The powdery dehydrated crystal is also dangerous. It is possible to breathe in enough of the powder to poison yourself.

    Carbon Dioxide

    Carbon dioxide is a gas at room temperatures. It is an important part of the respiration of living things and the carbon cycle. Carbon dioxide in our atmosphere is a colorless, odorless gas that is heavier than air. Put a piece of dry ice into a deflated balloon and tie the open end of the balloon. The dry ice will change into gaseous CO2 and fill the balloon. This balloon will drop quickly in the air. Carbon dioxide is a product of complete combustion of any material that contains carbon.

    Dry ice is solid carbon dioxide. It goes directly from a solid to a gas at atmospheric pressures. (It takes higher pressure for the liquid to appear.) This process, along with the reverse process of the gas going directly to become a solid, is called sublimation. Sublimation of CO2 at atmospheric pressure occurs at -78.5 °C. Any wispy bit of white haze around dry ice as it sublimes is not carbon dioxide, but water. The crystals of water come about from the solidification of the water in the atmosphere. Stage or movie productions needing a gentle haze on the ground have long relied on dry ice in water. The dry ice sends up ice crystals into the air as the dry ice bubbles under the water The ice crystals melt in the air to make a wonderful little cloud. The carbon dioxide gas dissolved in the water and around the water droplets tends to make the cloud cling to the ground.

    I know of someone who entered a High School Science Club float in the Fourth of July parade with a large paper cylinder on the float in the shape of a beaker. Under the beaker was a pan of water with some pellets of dry ice to put into it to form an evil-looking mist. The individual in question became over enthusiastic about creating mist and put a large amount of dry ice into the pan. By the time the float passed the reviewing stand, the water had frozen.

    One of the best ways to get a good refreshing drink in front of a chemistry class is to place a pellet of dry ice into an Erlenmeyer flask with about a third of the flask filled with water. It is best to lean on the top of the flask with the palm of one hand. This increases the pressure on the carbon dioxide to push it into the water. Stirring helps, so every so often mix the flask around. As the liquid cools from the dry ice, even more of the gas can be dissolved into the water. The result, after a dramatic presentation and a calm swig of the liquid, is called seltzer water. This is the same material that gives any carbonated drink its fizz. Oh, one warning. Don't drink the dry ice pellet down with the water. The result is even worse than swallowing a whole undissolved Alka Seltzer tablet. Swallowing dry ice is dangerous. You can freeze a part of your stomach doing that. Just try to explain that to an emergency room doctor without laughing.

    As carbon dioxide dissolves in water, there is another change. The water becomes very slightly acid. Carbonated drinks have a bit of a sour taste or 'bite' that comes from the carbon dioxide. Once the 'fizz' has gone from such a drink, the taste is 'flat.' The solution of carbon dioxide in water can neutralize a base, so it is appropriate to call it 'carbonic acid,' but there is no compound that can be isolated. The equation for this process is:

    \[CO_2 +H_2O \rightleftharpoons H_2CO_3\]

    The reaction is written with a double-sided arrow because it is reversible. This has all the qualifications of a genuine chemical reaction, except there is no way to isolate the product. Carbonic acid only exists as ions in a water solution of carbon dioxide.


    Ammonia, NH3, is a gas with a distinctive odor. There is no way to describe the smell to you any more than there is a way to describe the smell of a pear. But you have likely smelled ammonia. Many household cleaners contain ammonia. Cats eat a large amout of protein. The protein has nitrogen in it that must be excreted from the body. The contents of a cat pan will give off a strong odor of ammonia as the deterioration process of the waste goes on.

    Ammonia has a number of uses. Some of the first refrigeration units used ammonia as a coolant. It makes a fine coolant material, but in the event of leakage from a cooling system, ammonia is dangerous. In the amounts one would get from a leaking cooling system ammonia can be a noxious gas. Ammonia can be made directly from the nitrogen in the air and hydrogen gas by the Haber process. It is used to make fertilizers and explosives.

    Due to the amazing properties of water to dissolve materials and the added properties of ammonia as a polar covalent material with a very notable separation of charge, ammonia water makes a wonderful cleaning material for glass and other materials.

    In a reaction that has some similarities to the reaction of carbon dioxide and water, ammonia dissolves in water to produce ammonium hydroxide. We know there is some ammonium hydroxide in the solution because we can make ammonium salts from it and the solution is alkaline, but there is no such compound as ammonium hydroxide. As the water evaporates from the solution, the ammonium hydroxide escapes as water vapor and ammonia gas. The name 'ammonia water' is a good way to describe a solution of ammonia in water.

    Does pressing ammonia gas into water produce a chemical reaction? This is just as tough a question as the similar one with carbon dioxide. We cannot isolate pure ammonium hydroxide, but the solution is alakali and further reactions can happen with the ammonium ion in solution.


    Methane, CH4, is the smallest of the hydrocarbons, those organic materials made of only carbon and hydrogen. It has been known as 'swamp gas,' a name that comes from the most obvious place to see it. The bottom of swampy areas, where much organic material from dead plants and animals sinks, is the place where bacterial action breaks down the complex biological materials to smaller molecules. The bacteria make their living that way. Many of these bacteria are anaerobic, that is, they are either inhibited or killed by the presence of free oxygen. Methane is one of the products of the metabolism of the bacteria. If you see a release of gas from the bottom of a swamp, light it. If it burns, it is likely methane. Once swamp air was thought to be the cause of malaria (mal - bad, and aria - air in Italian) and there was a considerable effort to study methane from the swamps. As you know, methane does not cause malaria, but is a pretty good fuel.

    Swamps are not the only source of methane. Some materials that do not easily digest in people and animals may be handled by bacteria in the intestines. Legumes (beans) are notorious for having indigestible portions that react this way, somewhat idiosyncratically, with people. Some dogs have a difficult time digesting the cereals in inexpensive dog foods, and they may develop a case of the winds from it.

    Cattle have four stomachs for the fermentation and digestion of even more difficult foods. Even so, there are many things that the digestive system of cattle cannot handle, so bovine burps have a high percentage of methane in them. Methane from cow burps has been called a major pollutant of the earth's atmosphere. It seems there ought to be a way to collect that and not only have a new source of fuel and clean the atmosphere in the process. Methane is slightly lighter than air, so collection bags on the backs of cattle would stand up from the beasts, but methane is not so much lighter than air that we have to fear floating cattle. $|8-)


    The white of an egg is almost all water and protein. The water does not change significantly on cooking. The protein does. What is a protein? Here is a rather simplified answer. A protein is a very large molecule of repeating base units on which are attached various side units. A single amino acid has an amino side (-NH2 with the nitrogen attached to a carbon) and a carboxylic acid side (-COOH with the carbon attached to another carbon). The amino end of one amino acid is attached to the acid end of the next amino acid. The 'backbone' of this huge molecule is the -N-C-C-O- series of each amino acid. Each protein has a bare minimum of thirty amino acids in it, usually several hundreds, if not thousands, of amino acids. If a string of amino acids has less than thirty or so amino acids, biochemists call the string a polypeptide. There are about twenty different types of amino acid. Every protein made by an individual has exactly the same amino acids in exactly the same place. The pattern for each protein in the body of a living thing is one of the important bits of information passed down from generation to generation in genetic material.

    The main idea we need to understand is that each protein is very large, and that each protein has to be exactly right or it will not do what it needs to do. Cooking does not usually destroy the actual molecular formula for a protein, but it destroys the shape of the protein from how it was folded and re-connected and related to the environment around it. The process of destroying the useful shape of a protein is called denaturing. Cooking is only one of the ways to denature protein. Adding acid or salt or lye (base) to the protein can also denature the protein and these processes also tend to kill bacteria and other things that would attack the protein. Salting pork or fish were the best ways to preserve these foods at one time. The proteins of milk curdle as they denature in the acid of cheese. The starch is opened up and the protein, what little bit there is, is denatured in corn when it is mixed with lye for hominy. After denaturing, proteins are not useful for whatever they were made, but only as food for something else. The animal that eats the protein breaks it down (digests it) into the individual amino acids and can use the amino acids as a source of energy or as a source of raw materials (amino acids) for building its own proteins. (Most plants make their own amino acids.)

    Is cooking an egg (denaturing a protein) a chemical reaction? Possibly, but it certainly is a biochemical reaction. The same obviously goes for a steak or any other source of protein.


    Soap is made from neutral fat and lye. Sodium lye (sodium hydroxide solution) or potash lye (potassium hydroxide in solution) when boiled with neutral fats make glycerin and soap. A molecule of neutral fat is split into one molecule of glycerin and three molecules of fatty acids, twelve to twenty-two carbon chains of fat-soluble end and an organic acid end that attracts the sodium or potassium in an ionic bond.

    Soap making is an old technology from home chemistry. The fire place of a home would be the source of heat and the cooking area. The meat cooked over the flame would drip fat into a collecting pan. Ashes from the fire have a small amount of lye in them. The ashes were washed with water and the lye was concentrated by boiling most of the water out. The fat and lye were put into a large kettle and boiled to make soap. It takes some experience for a soap maker to get it right. If too much fat is added, the soap can be too oily to use. If too much lye is added, the soap will be caustic on skin and hair. The strong alkali of lye destroys protein. Now you know the origin of the famous, 'Grandma's lye soap that will eat your face off.'

    Glycerin is a by-product of the soap-making reaction. (A by-product is another material made that is not the main material desired from the reaction.) Glycerin is an edible, sweet-tasting, oily liquid that dissolves in water and many oils. Glycerin may be used as an antifreeze or as a hair dressing.


    Have you ever seen soup bones for sale at the meat counter of a market? Meat soups are made by gently simmering some of the less meaty abattoir products. Bones, sinew, cartilage, and some meat make excellent soup.

    Simmering is cooking in water by heating to just below the boiling point, usually for several hours. Hot water leaches out proteins from bones and cartilage that make a colloidal suspension in the liquid. If you take that liquid with enough suspended protein and refrigerate it, the mixture becomes a gel. The semi-solid nature of the gel suggests that at one point there has been a change in the mixture from the protein being the dispersant and the water a dispersing agent to the water being trapped within a loose protein structure. In short, the gel has set. The gel made this way is called gelatin. You can find gelatin in the drippings of meat cooked in other ways also.

    Gelatin desserts are just the leached protein, some coloring, flavoring, and some sugar. Is it true, then, that gelatin desserts are good sources of protein? Well, maybe.

    The proteins of bones and cartilage are structural proteins. In bone the protein serves as a support lattice-work for the hardening crystals of (mostly) calcium compounds that give more rigidity to bones. This type of protein has only a few of the amino acids that occur in proteins useful for such things as enzymes and muscles. The amino acids in gelatin are all produced by the human body. The essential amino acids, those that cannot be produced by the human body are not present in gelatin in any significant quantity. For this reason, we call gelatin a source of incomplete protein.

    Gels are interesting mixtures. You might say that the liquid and solid phase of a colloidal suspension undergo a change wherein the roles of the phases have reversed. The solid becomes the structural entity and the liquid becomes the dispersed portion. A true gel will have a 'ring' to it. You can see this in the 'wiggle' of gelatin. Many commercial toothpastes, hand cleaners, and shampoos are gels. Firmly slap these into the palm of your hand to feel the gel ring. The reason for offering the public these commercial preparations in the form of a gel is that there is less likelihood of separation of the materials or settling of the suspended portions.

    Alkali Flats

    Many years ago there was a large shallow salt sea in the western portion of what is now the United States. The Bonneville Salt Flats and the Great Salt Lake are almost all that remains of that sea. There are a few other places, mostly in desert areas in which a small amount of water may remain with a lot of the concentrated salts from that ancient sea. They are flat areas because they were the bottom of the sea. There is an excess of the alkaline salts in these areas called "alkali flats."

    During the last two centuries of expansion of the United States toward the west, many land travelers and explorers have found themselves attempting to cross the desert areas on foot or by animal transport, only to run across the alkali flats. The water from the pools in these areas is full of poisonous alkali salts. The caricature drawing of these alkali pools has a sun-dried skeleton or a sun-dried skull lying about near the pool, but such obvious warning was not always there. Many a thirsty traveler learned too late that the alkali pools were dangerous.

    Allotropes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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