Moles in animal world, skin and chemistry
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Avogadro's number has a remarkable history that goes back to the study of gases
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— photo: R. Ragu
SKIN DEEP: The mole in dermatology refers to a small, dark spot on the skin.
THERE ARE many moles in science. Perhaps, the most ancient of them all is the rodent called the mole.
It has a tube-like body, pin-sized eyes and almost invisible ears. Its most sensitive organ is the nose or the snout, with many tiny pimples and whiskers that detect tiny movements, temperature changes, prey and other moles.
Its tail can move right through a circle. All these come in very useful for the mole, which lives in burrows underground.
A voracious eater, it prefers insects and worms of all types in the ground and beneath. Hence the burrowing which, of course, turns the soil well and thus helps plants with soil nutrition.
A 150 g weight mole eats up about 20 kg of worms in a year. It better do so, since it has to exert enormous force while digging burrows and underground tunnels. When a bunch of moles go about digging, the tunnels they make can cover an entire field or a golf course.
As they dig, the soil they raise can grow into a mound. Hence the saying: `Making a mountain out of a molehill.' A particularly busy set of moles can dig about 25 metres per minute.
A disservice
Moles contain twice as much blood and haemoglobin as other mammals of similar size, helping them in breathing easily underground, where oxygen is low and carbon dioxide high.
It is probably this underground habitat and nosey sense of the mole that led Francis Bacon, in 1626, to use the word mole for a spy. As a biologist, I feel that the description of an antinational spy as a mole is doing disservice to the busy helpful animal.
There is another mole in biology, in dermatology. Here, it refers to a small, dark spot on the skin. It can occur at a layer below the skin or as a pigmented growth on the skin. The colour comes from the high amounts of the skin pigment melanin (hence the derived name `mole'; the animal gets its name from a variation of the Latin term).
Usually moles are congenital, and perfectly harmless. When a mole occurs in the cheek or near the lips, it is thought of as a beauty mark, but one on the nose! While most moles are seen since birth, some stain marks might also be acquired, and perhaps due to overexposure to ultraviolet rays.
When such pigmentation is acquired, it is worth consulting a doctor just to be sure all is well.
Mole to eat
While in biology, let us not forget food and nutrition — and the Mexican culinary delight called molé, pronounced with the accent on the `e' (molay). This is the general name for some types of sauces.
The more common ones are guacamole, which is made with avocado and mole poblano, made with chilli peppers, spices, unsweetened chocolate and a variety of choice ingredients. These moles are generally poured over rice, potatoes or meat, or filled in enchiladas or tortillas made of corn.
The word mole here apparently comes from the American Indian Nahuati word mulli, meaning sauce.
Meaning mass
For chemists, the mole is not an animal, nor a skin mark or a sauce but a unit of measurement. It was coined by the German physical chemist Wilhelm Ostwald in 1902, as an abbreviation for molecule, which in turn comes from the Latin moles meaning mass.
Ostwald used it to express the gram molecular weight, and denoted it to count the number of molecules present in this weight. For example, a molecule of water weighs 18 times that of a hydrogen atom.
But if we take 18 grams of water, that is, one gram molecular weight of water, it is called a mole of water and contains 6.022 x10{+2}{+3} molecules of water. Likewise, 2 grams of hydrogen contains 6.022 x 10{+2}{+3} molecules of hydrogen (molecular weight of hydrogen being two).
This enormous number (6.022 x 10{+2}{+3}) is referred to as Avogadro's number and has a remarkable history that goes back to the study of gases.
The French chemist Joseph Louis Gay-Lussac, after a detailed study of gas reactions propounded a law. This law states that when gases react to produce products, they combine in volumes that are all whole numbers.
Combining volumes
Two volumes of hydrogen gas react with one volume of oxygen gas (at the same temperature and pressure) to produce two volumes of water vapour. Note the whole numbers 2, 1 and 2.
Likewise, when nitrogen gas reacts with hydrogen gas to produce ammonia, the volume ratios are 1, 3 and 2 respectively. This came to be known as the Law of Combining Volumes.
This seemingly prosaic law was interpreted by a lawyer-turned-chemist from Turin, Italy called Amedeo Avogadro in a brilliant manner, which gave rise to the understanding of molecules.
Avogadro understood and appreciated Dalton's ideas about atoms, their indivisibility during reactions, their relative masses and their combining proportions.
Proof of existence
By the time he turned his attention and research into chemistry during the early 1800s, the idea of molecules was being discussed but no consensus nor any experimental proof of their existence had emerged.
It was at this time that Avogadro read the Gay-Lussac paper. He was puzzled as to why when two volumes of hydrogen combine with one volume of oxygen, they produce two volumes of water and not just one volume.
Professor John Blamire of CUNY Brooklyn writes in his 2001 article in `Science @ a distance' as follows: "Avogadro's answer was pure genius; since the volume of water vapour formed was twice the volume of oxygen used to create it, the reacting `particles' of oxygen had to consist of two atoms of the element oxygen combined together into a single, compound structure, which he called a `molecule.'
"It explained everything. Oxygen (and hydrogen) gas consisted of molecules that each in turn consisted of two atoms of the element!
Molecular weights
From this effective reasoning Avogadro was able to see that the relative molecular weights of any set of gases are the same as the ratio of the densities of these gases under the same conditions of temperature and pressure.
Avogadro's principle
"The power of his ideas comes in his conclusion; equal volumes of all gases, simple or complex, contain the same number of molecules under the same conditions of temperature and pressure.
"Today, this is called the Avogadro's Principle and is clearly seen as one of the corner stones of chemistry."
On a lighter, and educative vein, Auburn University in America has initiated the `Mole Day', to foster interest in chemistry.
Schools in many parts of the country celebrate Mole Day with various activities related to chemistry and/or moles.
Children come dressed as moles (the animal), and present skits and riddles on the concept of the mole in chemistry.
The event is celebrated annually on October 23 between 6.02 AM and 6.02 PM. No prize for guessing this choice of the date and time!
D. BALASUBRAMANIAN
dbala@lvpei.org
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